Measuring the Power of a Solar Panel
When measuring the power of a solar panel the use of a digital multimeter is required to measure the voltage and amperes being generated by a panel under different light conditions. Knowing the power output of a particular photovoltaic panel is an important requirement of any solar system.
As we have seen throughout this website, solar power is a renewable form of electrical energy generation that is commonly created using photovoltaic solar panels, either individually or connected together in strings to form larger solar array’s.
Understanding the way that photovoltaic (PV) solar panels work is a basic requirement as most people assume, rightly or wrongly, that just because they have purchased a 100 watt solar panel, it will deliver 100 watts of electrical power continuously all day. However, in reality this is not always the case as the electrical power delivered at a certain instant in time during the day is a direct function of positioning and the weather conditions.
Sunlight is an intermittent energy source constantly changing throughout the day so photovoltaic solar panels have to be able to operate under these varying conditions. As the efficiency of a solar panel is the ratio of electrical power output to the amount of sunlight, that is solar irradiance absorbed by the panel.
Therefore it is important that the solar panel orientation is correct to receive the maximum amount of sunlight throughout the day. Larger panels have the capability to produce more electrical power than smaller PV panels for a given solar irradiance.
The performance of photovoltaic solar panels can be determined by measuring the relationship between the panels voltage, current, and therefore power output under different meteorological conditions, such as total solar irradiance.
Inclination of the panel, ambient air temperature as well as panel temperature all play an important role in the power output of a solar panel.
Manufacturers rate their photovoltaic panels based on the DC output power at an irradiance of 1000 W/m 2 (full sun) and a panel temperature of 25 o C in order to get you to buy their product.
A standard 12volt PV panel will generate a maximum terminal voltage of about 20 volts in full sunlight with no connected load. However in the real world, photovoltaic solar panels operate below these ideal settings resulting in an output power much less than the PV panels possible maximum output power rating.
So how do we measure the output power of a photovoltaic solar panel. Well in its most basic of terms, the output power (P) of the solar cell is obtained by multiplying the output voltage (V) by the output current (I) at maximum power conditions, giving P = V x I which itself is the basis of Ohm’s Law
Solar Panel Power Output
The output power of a typical solar panel depends very much on the electrical load connected to it. Maximum power is transferred when the panel’s (or array’s) dynamic resistance equals that of the connected load. The simplest load for a pv panel to supply current too is that of an electrical resistance such as a DC water heating element. As Ohm’s Law describes the relationship between the voltage across and the current flowing through a resistor, we can use a simple resistance to measure the power of a solar panel.
Solar panel IV Characteristic Curves are used to give a visual representation of the current and voltage ( IV ) characteristics of a particular photovoltaic panel (cell or array) giving a detailed description of its solar energy conversion ability. Knowing the electrical IV characteristics of a solar panel is critical in determining the output performance and therefore its efficiency. Measuring the output power of a solar panel, for example the 100W, 12V Renogy Solar Panel is not that difficult if we apply some simple steps.
The product label of the Renogy 100 Watt 12 Volt Polycrystalline Solar Panel gives us the electrical characteristics of the panel, according to the manufacture Renogy when it is exposed to an irradiance of 1000 W/m 2. But what does it all mean.
Our electrical data label states that the panels Open Circuit Voltage, (VOC) is 22.4 volts and that its Short Circuit Current, (ISC) is 5.92 amps.
So we can use Ohm’s Law then to find the output power of the solar panel, right! But V times I gives us 22.4 x 5.92 = 132.6 watts, which is a lot more than the 100 watts quoted by Renogy, so what’s going on?
The opencircuited voltage, VOC means that the PV panel is not connected to any load, so its terminals are therefore open (infinite resistance) resulting in maximum voltage, in this case 22.4 volts, at its terminals. As its terminals are open there will be no current flow (I = 0) because there is no electrical circuit or load for the current to circulate through. Then the output power of the solar panel in this instance is P = V x I = 22.4 x 0 = 0 watts. In other words no generated electrical power.
Likewise, the shortcircuited current, ISC means that the PV panels terminals are shorted or connected together (zero resistance) creating a fully closed electrical circuit allowing maximum panel current, in this case 5.92 amps, to flow. However, as the terminals are shorted together there will be no output voltage drop (V = 0), so the output power of the solar panel will be P = V x I = 0 x 5.92 = 0 watts. Again no generated electrical power.
Then we may think that this Renogy solar panel can generate 132.6 watts of solar electricity, in reality it can not. Photovoltaic panels provide usable electricity when connected to an electric load and by measuring the output of a solar panel, we can use Ohm’s Law to determine the maximum output power point, or MPP.
Measuring the Power of a Solar Panel
We said previously that the output power of a solar panel mainly depends on the electrical load connected to it. This load can vary from an infinite resistance, (∞Ω) to a zero resistance, (0Ω) value thus producing an opencircuit voltage, VOC at one end and a shortcircuit current, ISC respectively, at the other. Then we need to be able to find an external resistive value somewhere inbetween these two extremes.
As the theoretical maximum power, Pmax was shown to be 132 watts, and the maximum opencircuit voltage, VOC as 22.4 volts. If we assumed the panel has a maximum wattage of 150 watts and a maximum terminal voltage of 30 volts, this would give us the panels dynamic resistive value of:
There are many ways to achieve this 6.0 Ohm value using fixed value resistors, to variable resistors, to rheostats. The option used would depend on what’s available and budget, as large power resistors can be expensive. In the example given, the load resistance will be achieved using a Rheostat, thus giving us a fully variable resistance between a minimum value of zero Ohms (0Ω) and the maximum value of 6Ω. We know that our solar panel has a manufacturers rating of 100 watts, so this would be the minimum power rating of our rheostat.
To measure the voltage across the terminals of the PV panel we would require a voltmeter. This could be a digital or analogue multimeter or a simple voltmeter but must have a scale high enough to read the panels opencircuit voltage (VOC). The type used would depend on what’s available and budget.
Measuring the current generated by the PV panel would require an ammeter to measure the short circuit current (ISC) into a dead short. Again this second meter could be digital, analogue or a multimeter depending on what’s available and budget.
Although we have calculated the maximum resistive value required above as being 6Ω, chances are we would not be able to purchase one at the exact 6Ω resistive value. So let’s assume that we bought a 10Ω rheostat which is fully variable from between 0 and 10 Ohms.
This would allow us to increase the externally connected load resistance in 10 steps of 1 Ohm each, while at the same time measuring the solar panels output voltage and current. Then we can complete the following table of our results to determine the electrical power supplied by the PV solar panel for different values of load resistance.
Table of Measured Results for our Solar Panel
Resistance(in Ohms)Voltage(in Volts)Current(in Amperes)Power(in Watts)3Open Circuited  22.4  0.00  0.00 
10  22.4  2.24  50.2 
9  22.3  2.47  55.1 
8  22.1  2.76  61.0 
7  21.6  3.08  65.9 
6  20.80  3.50  72.8 
5  20.05  4.01  80.4 
4  18.80  4.70  88.4 
16.86  5.62  94.7  
2  11.60  5.80  67.3 
1  6.0  6.00  36.0 
Short Circuited  0  5.92  0.00 
Having taken our readings and tabulated the results in the above table, we can clearly see that the maximum power occurs when the load resistance RL has a resistive value around the around the 3 Ohms value. At 3Ω’s the panel produces a current of 5.6 Amperes at about 16.9 volts, thus giving a calculated output power of 94.7 watts.
This value closely matches the manufactures data label for an operating voltage (Vmp) and operating current (Imp) of 17.8V and 5.62A respectively giving a dynamic panel resistance at maximum power of Vmp/Imp = 17.8/5.62 = 3.17 ohms. We could if so wished fine tune our measurements to get voltage and current values even closer to the 100 watt, 3.17 ohms target.
It therefore shows for our simple example that the maximum power is generated by the panel around this 3.17 ohms point and matching panel resistance to load resistance results in maximum power transfer from panel (or array) to load and therefore, increased efficiency.
Using the data from the table above, we can plot a graph of the measured voltage against the panels current as shown.
IV Characteristics Curves
Above is the IV characteristics graph for our PV solar panel example and is typical for all PV solar panels. Measuring the power of other types and ratings of solar panel will produce similar results, just the voltages and current values will be different. Also notice that power is zero for an opencircuit (zero current condition) and also for a shortcircuit (zero voltage condition).
The maximum power output of a PV panel can be defined as its peak DC output given by multiplying the voltage and the current. Here the optimum operating point for our solar panel is shown at the midpoint in the bend (or knee) of the characteristics curve. In other words, this is the point at which the solar panel generates its maximum power, known commonly as the maximum power point or MPP.
The IV characteristics curve presents an important property of a photovoltaic solar panel, or cell in that it shows it to be a current source device rather than a voltage source device, like a battery. However, unlike a battery which has a constant terminal voltage, (12V, 24V, etc.) and provides variable amounts of current to a connected load, the photovoltaic cell or panel provides a constant supply of current over a wide range of voltages for a given amount of solar insolation.
Measuring the Power of a Solar Panel Summary
Measuring the power of a solar panel is not too dificult but requires an assortment of multimeters, power resistors, or a single rheostat capable of handling the generated power. We need to remember that the greater the power output of your panel. For example, 200W, 285W, 330W, etc. the greater will be the voltages and currents and therefore safety is important.
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Great detailed explanation of PV parameters. In your example of a 100w panel would charging a 12 volt battery be more efficient with a direct connection to such a solar panel compared to PWM or MPPT? Thinking about this, and a little bit of guessing PWM is just a direct connection with hysteresis that allows connect and disconnect at the required voltages and because it is working at
Pulse Width Modulation, or PWM for short, is a technique of sending very short ON/OFF cycles (pulses) several hundred times per minute to control the magnitude of the output voltage. It is called “pulse width” because the width of the pulses may vary from a few microseconds to several seconds. Photovoltaic (PV) panels produce an almost constant DC output voltage and not a PWM output. Then you are incorrect in your assumption that PWM is just a direct connection with hysteresis. Battery charge controllers commonly use PWM to regulate a batteries charging current and float charge current keeping the batteries fully charged without overcharging them. Since both the battery terminal voltage and the PV output voltage can vary during use due to the batteries stateofcharge during charging, temperature and PV insolation variations throughout the day. Direct connection of a PV panel to a battery leads to mismatch and therefore energy losses. The algorithm of MPP trackers measure the currents, voltages or the power of the PV panel/array to establish the optimum operating voltage. Therefore, it is able to react to changes in the photovoltaic panels/array performance.
Thank you for this information. What is the relationship between short circuit current and solar energy? Is this linear? Can a small solar panel be used as a sensor to measure solar energy using short circuit current?
Shortcircuit current, Isc is the current through the solar cell when the positive and negative terminals of a photovoltaic solar cell are shorted, thus resulting in zero voltage output from the cell. Isc is the largest current value which can be delivered by the pv cell. Clearly, Isc is directly dependent on the light intensity as no sunlight, no current. Then Isc is not linear.
It would make for an interseting school project, but not very accurate at low light levels. As explained previously, shortcircuit current, Isc represents the maximum cell output current when the output leads are shorted and the voltage developed by the cell almost vanishes to zero (V = 0). It is assumed Isc is directly proportional to the radiant energy intensity and yes, increasing the irradiance increases the magnitude of the short circuit current, but not in a linear fashion as suggested. This is because at V = 0 Isc = the cells lightgenerated current depends on the solar cell structure, material properties, spectral response and the operating temperature. Commercial light sensors use photodiodes which linearly measure irradiance.
I have seen many IV curves for solar panels and these all seem to show a linear relationship between I and solar irradiance up to about 70% of maximum current, at which point the Isolar energy plot begins to curve a lot. I do wonder if there is a mathematical relationship for this curve that could be used to produce an Isolar energy result for a small solar panel, a relationship that could be applied to all solar panels of the same type (e.g. monocrystaline, etc). That would allow a small solar panel of a particular type to be used as an effective solar energy sensor. However if this curve is particular to every solar panel, then such a sensor would not be possible without ‘calibrating’ each small panel.
Thank you for the article. I need to point out one flaw, however, so that people can make the proper calculations on their own. Based on Joule’s Law (Power = Current Voltage), power divided by voltage equals current, NOT resistance. Using Ohm’s Law and Joule’s Law, resistance equals voltagesquared divided by power (R=V²/P). Therefore, in your assumption of 150W and 25V, the resistance would be calculated as 4.17 ohms (25²V / 150W), not 6 ohms. Fortunately, your incorrectly calculated value of resistance was close enough to the proper value to allow you to identify the proper resistance range of 010 ohms, so your results did not suffer from the miscalculation. If the assumed values for power and voltage were much higher, say for an array of panels connected in series, then the resulting resistance would have be way off. Like I said, I do appreciate your article. I just want your readers to have the correct formulas so that their results are as accurate as possible.
Thanks. The error was we wrote 25 volts by mistake and not the 30 volts as per our original notes, which would give a dynamic resistance of 6 ohms
Hello. Very valuable analysis, thank you. We want to power a 230v 2.5 Kw mains immersion heater (straight resistive load of about 10 ohms at the bottom of a thermal store) with approx 240v DC from a string of panels. The latitude is 59 north roof slope 20 degrees facing due south. Total output will be poor in winter because of the latitude but summer days are long and that is when the hot water is needed. I see that output voltage is fairly constant and current varies with sun power. The DC switching will use a dedicated high voltage and power DC relay. Itself controlled by a much smaller current through the immersion heater’s thermostat. Do you see a problem with this?
Yes, many. Firstly, ordinary resistive heating elements will work with DC power as: 230 Vrms = 230 VDC. Relay contacts switching DC power will arc more than switching AC power. Element size (resistance) needs to have an approximate match to the PV array (Vmp/Imp) for maximum power transfer, for example. Let’s assume you are using standard 250 watt photovoltaic panels: Maximum power per panel at full sun (1000W/m^2) of solar insolation is: 250 watts Typical voltage at Maximum Power (Vmpp) for a 250W PV panel is about: 30.45 V Typical current at Maximum Power (Impp) for a 250W PV panel is about: 8.21 A Therefore panel resistance at full sun = Vmp / Imp = 30.45V / 8.21A = 3.71 ohms minimum per panel. Changes in solar irradiance will vary this value. Heating element rated wattage = 2.5kW = 2500W Heating element resistance: R = V^2 / P = 230 230 / 2500 = 21.2 ohms (not 10 ohms) Ideal number of panels per single string assuming 1000W/m^2 of full solar insolation is: 21.2 Ohms / 3.71 Ohms = 5.71 panels, that is maximum power transfer will occur with 5.71 pv panels, but we will round this down to 5 whole panels per series string, (better down than up). Thus a 1250 watt (5250W) system, 5 panels times 1 series string will have a resistance of: R = V / I = (30.45 5) / 8.21 = 18.54 ohms. String voltage = 5 30.45 = 152.25 volts. 1250 watts of installed panels in bright sunlight for maybe 3.5 hours to 4 hours per day translates to about 4.4 kWh per day of DC power into the water heaters resistive element at full sun. Or upgrade to 20 panels, 10 panels per string times 2 strings, gives a resistance of: (30.45 10) / (8.21 2) = 18.54 ohms (same dynamic resistance as above). This will give a string voltage of about: 10 30.45 = 304.5 volts and 2500 watts of panel wattage (equivalent to your heating element wattage) which equates to about 8.8 kWh of DC current per day of solar power. Other PV panel wattages will have various Vmpp and Impp specifications (and thus different dynamic impedancies) obtaining different heating results when matching series panels with the resistance of your 2500W heating element. Option 2), Use a low voltage 24 or 48 volt heating element to reduce wattage and number of panels, or accept lower temperature water. Option 3), Use a solar thermal hot water panel to heat, or at least preheat the water. This could give you about 4 times the heat for the same square footage of panel area used than for a PV panel. Solar thermal is a more cost effective way to heat water. Option 4). Use a Hot Air Heat Pump to give hot water 24 hours a day, rain or shine.
Quick answer, kind of. Long answer, it will affect it along with an other parameter, the efficiency rate. The efficiency rate is determined by the type of cells (mono vs polycristaline), the technology used to create ce cells and the quality of the materials. So a low quality 100w polycristaline cheap solar panel at 17% efficiency will be bigger than a 100W quality monocrystaline solar panel at 22% efficiency (i believe the surface would need to be about 5% smaller, but there’s more elements to consider…) So theres more than the watts.
Decoding Solar Panel Output: Voltages, Acronyms, and Jargon
For those that are new to solar power and photovoltaics (PV), unlocking the mysteries behind the jargon and acronyms is one of the most difficult early tasks. Solar panels have many different voltage figures associated with them. There is a good amount to learn when it comes to solar panel output.
Types of solar panel voltage:
 Voltage at Open Circuit (VOC)
 Voltage at Maximum Power (VMP or VPM)
 Nominal Voltage
 Temperature Corrected VOC
 Temperature Coefficient of Voltage
 Measuring Voltage and Solar Panel Testing
Voltage at Open Circuit (VOC)
What is the open circuit voltage of a solar panel? Voltage at open circuit is the voltage that is read with a voltmeter or multimeter when the module is not connected to any load. You would expect to see this number listed on a PV module’s specification sheet and sticker. This voltage is used when testing modules fresh out of the box and used later when doing temperaturecorrected VOC calculations in system design. You can reference the chart below to find typical VOC values for different types of crystalline PV modules.
Nominal Voltage VOC – typical VMP – typical # of cells in series12  21  17  36 
18  30  24  48 
18  33  26  54 
20  36  29  60 
24  42  35  72 
Voltage at Maximum Power (VMP or VPM)
What is the Max Power Voltage of a solar panel? Voltage at maximum power is the voltage that occurs when the module is connected to a load and is operating at its peak performance output under standard test conditions (STC). You would expect to see this number listed on a modules specification sheet and sticker. VMP is at the place of the bend on an IV curve; where the greatest power output of the module is. It is important to note that this voltage is not easily measured, and is also not related to system performance per se. It is not uncommon for a load or a battery bank to draw down the VMP of a module or array to a few volts lower than VMP while the system is in operation. The rated wattage of a PV module can be confirmed in calculations by multiplying the VMP of the module by the current at max power (IMP). The result should give you [email protected] or power at the maximum power point, the same as the module’s nameplate wattage. The VMP of a module generally works out to be 0.5 volts per cell connected in series within the module. You can reference the chart to find typical VMP values for different types of crystalline modules.
Nominal Voltage
What is the voltage of a solar panel? Nominal voltage is the voltage that is used as a classification method, as a carryover from the days when battery systems were the only things going. You would NOT expect to see this number listed on a PV module’s specification sheet and sticker. This nomenclature worked really well because most systems had 12V or 24V battery banks. When you had a 12V battery to charge you would use a 12V module, end of story. The same held true with 24V systems. Because charging was the only game in town, the needs of the batteries dictated how many cells inside the PV should be wired in series and or parallel, so that under most weather conditions the solar modules would work to charge the battery(s). If you reference the chart, you can see that 12V modules generally had 36 cells wired in series, which over the years was found to be the optimum number for reliable charging of 12V batteries. It stands to reason that a 24V system would see the numbers double, and it holds true in the chart. Everything worked really well in this off grid solar system as the and evolved along the same nomenclature so that when you had a 12V battery and you wanted solar power, you knew you had to get a “12V” module and a “12V” controller. Even though the voltage from the solar module could be at 17VDC, and the charge controller would be charging at 14V, while the inverter was running happily at 13VDC input, the whole system was made up of 12V “nominal” components so that it would all work together. This worked well for a good while until maximum power point technology (MPPT) became available and started popping up. This meant that not all PV was necessarily charging batteries and that as MPPT technology evolved, even when PV was used in charging batteries, you were no longer required to use the same nominal voltage as your battery bank. String inverters changed the game for modules, as they were no longer forced in their design to be beholden to the voltage needs of deep cycle batteries. This shift allowed manufacturers to make modules based on physical size, wattage characteristics, and use other materials that produced module voltages completely unrelated to batteries. The first and most popular change occurred in what are now generally called 18V “nominal” modules. There are no 18V battery banks for RE systems. The modules acquired this name because their cell count and functional voltage ratings put them right in between the two existing categories of 12V and 24V “nominal” PV modules. Many modules followed with 48 to 60 cells, that produced voltages that were not a direct match for 12V or 24V nominal system components. To avoid bad system design and confusion, the 18V moniker was adopted by many in the industry but ultimately may have created more confusion among novices that did not understand the relationship between cells in series, VOC, VMP, and nominal voltage. With this understanding, things get a lot easier, and the chart should help to unlock some of the mystery.
TemperatureCorrected VOC
The temperaturecorrected VOC value is required to ensure that when cold temperatures raise the VOC of an array, other connected equipment like MPPT controllers or grid tie inverters are not damaged. This calculation is done in one of two ways. The first way involves using the chart in NEC 690.7. The second way involves doing calculations with the Temperature Coefficient of Voltage and the coldest local temperature.
Temperature Coefficient of Voltage
What is a solar panel temperature coefficient? The temperature coefficient of a solar panel is the value represents the change in voltage based on temperature. Generally, it is used to calculate Cold Temp/Higher Voltage situations for array and component selection in cooler climates. This value may be presented as a percentage change from STC voltages per degree or as a voltage value change per degree temp change. This information was not easily found in the past, but is now more commonly seen on spec pages and sometimes module stickers.
Measuring Voltage and Solar Panel Testing
How do I measure voltage on a solar panel? Voltages can be read on a solar panel with the use of a voltmeter or multimeter. What you’ll see below is an example of a voltmeter measuring VOC with a junction box. This would be the view from the back of the PV module. Using a multimeter is the best way to measure solar panel output.
When researching solar panel output, it can be overwhelming to understand the different voltage figures and acronyms used. For those new to solar power and photovoltaics (PV), decoding the terminology can be a challenge. In this blog post, we will break down the basics of solar panel output, including voltage, acronyms, and jargon, to help you get up to speed.
What are solar amps and watts?
Solar amps and watts are two measurements of the amount of electrical energy that a solar panel produces. Solar amps (A) measure the rate of electric current produced by a photovoltaic cell, while solar watts (W) measure the amount of power delivered to an electrical load. Both solar amps and watts are related to the efficiency rating of residential solar panels. The higher the efficiency rating, the higher the number of solar amps and watts produced.
There are many types of 60cell solar panels on the market for home solar applications, each with varying efficiency ratings and amp/watt outputs. High efficiency panels are capable of producing more solar watts than lowefficiency panels, although they tend to cost more upfront. By choosing the right panel, homeowners can ensure that their solar array is producing enough power to meet their electricity needs.
Why do solar panels have so many voltages associated with them?
Solar panels have a variety of voltage figures associated with them due to the different types of solar panels, their placement in a solar panel system, and their power production. The most common type of rooftop solar panel uses a direct current (DC) and produces a low voltage. This low voltage is typically between 20 and 40 volts, depending on the specific type of panel. To increase the voltage output, multiple solar panels can be wired together in a series or parallel connection, or both, depending on the specific solar energy system.
When solar panels are connected in a series, the voltages are added together. This means that connecting two 20volt solar panels in series would yield a total voltage output of 40 volts. Connecting three panels in series would result in a 60volt output, and so on. This method is often used when the total voltage needs to be higher than what a single panel can provide.
In contrast, when solar panels are connected in parallel, the wattage is added together. This means that connecting two 10watt solar panels in parallel would yield a total wattage output of 20 watts. Connecting three panels in parallel would result in a 30watt output, and so on. This method is often used when the total wattage needs to be higher than what a single panel can provide.
The voltage output of a solar panel also depends on its power production, which is measured by the manufacturer at Standard Test Conditions (STC).
What does STC mean?
STC is defined as an irradiance of 1,000 W/m2 and cell temperature of 25 degrees Celsius. Because realworld conditions are rarely equal to STC, the actual power output of a solar panel may differ from its rated output. This is why it’s important to understand the various voltages associated with your particular solar energy system to ensure it meets your needs. To determine solar panels rated output, you need to know two figures: the solar panel wattage (measured in watts) and solar panel efficiency (measured in percent). Solar installation involves connecting solar panels to a photovoltaic system that can use or store the generated electricity. The efficiency rating of solar panels varies depending on factors such as environment, angle, and geographic location, but typically ranges between 15–20%. Knowing what wattage solar panels generate helps determine their overall performance in terms of power production for any given solar installation project. Understanding the various voltages associated with solar energy systems can be challenging for those new to the technology but once you’ve grasped this knowledge, you’ll have the knowledge you need to make informed decisions about your own solar energy installation.
How many size should my solar panel be?
When choosing a solar panel size, you must consider your energy needs and the hours of sunlight available in your area. The size of the solar panel will determine how much electricity it can produce, measured in kilowatt hours (kWh). Your energy needs will determine the type of solar panel that you need.
If you’re looking to produce a specific amount of electricity, the total number of solar panels that you need will depend on their wattage rating. Generally, the higher the wattage rating, the more electricity it will generate. You can calculate how many solar panels you need to meet your energy requirements by dividing your kWh requirement by the wattage of each panel.
For example, if you have an energy requirement of 10 kWh per day and you are using solar panels with a rating of 250 watts, then you would need 40 solar panels.
When choosing the size of your solar panel, make sure to consider the hours of sunlight available in your area as well. The more sunlight available, the fewer solar panels you’ll need to meet your energy requirements.
In summary, the size of the solar panel that you need depends on your energy needs and the hours of sunlight available in your area. You can calculate how many panels you need to meet your energy requirements by dividing your kWh requirement by the wattage of each panel.
Mathematical modeling of photovoltaic cell/module/arrays with tags in Matlab/Simulink
Photovoltaic (PV) array which is composed of modules is considered as the fundamental power conversion unit of a PV generator system. The PV array has nonlinear characteristics and it is quite expensive and takes much time to get the operating curves of PV array under varying operating conditions. In order to overcome these obstacles, common and simple models of solar panel have been developed and integrated to many engineering software including Matlab/Simulink. However, these models are not adequate for application involving hybrid energy system since they need a flexible tuning of some parameters in the system and not easily understandable for readers to use by themselves. Therefore, this paper presents a stepbystep procedure for the simulation of PV cells/modules/arrays with Tag tools in Matlab/Simulink. A DS100M solar panel is used as reference model. The operation characteristics of PV array are also investigated at a wide range of operating conditions and physical parameters.
Result
The output characteristics curves of the model match the characteristics of DS100M solar panel. The output power, current and voltage decreases when the solar irradiation reduces from 1000 to 100 W/m 2. When the temperature decreases, the output power and voltage increases marginally whereas the output current almost keeps constant. Shunt resistance has significant effect on the operating curves of solar PV array as low power output is recorded if the value of shunt resistance varies from 1000 ohms to 0.1 ohms.
Conclusion
The proposed procedure provides an accurate, reliable and easytotune model of photovoltaic array. Furthermore, it also robust advantageous in investigating the solar PV array operation from different physical parameters (series, shunt resistance, ideality factor, etc.) and working condition ( varying temperature, irradiation and especially partial shadow effect) aspects.
Background
Mathematical modeling of PV module is being continuously updated to enable researchers to have a better understanding of its working. The models differ depending on the types of software researchers used such as Cprogramming, Excel, Matlab, Simulink or the toolboxes they developed.
A function in Matlab environment has been developed to calculate the current output from data of voltage, solar irradiation and temperature in the study of (Walker 2001) and (GonzalezLongatt 2005). Here, the effect of temperature, solar irradiation, and diode quality factor and series resistance is evaluated. A difficulty of this method is to require readers programming skills so it is not easy to follow. Another method which is the combination between Matlab mfile and Clanguage programming is even more difficult to clarify (Gow and Manning 1999).
Among other authors, a proposed model is based on solar cell and array’s mathematical equations and built with common blocks in Simulink environment in (Salmi et al. 2012), (Panwar and Saini 2012), (Savita Nema and Agnihotri 2010), and (Sudeepika and Khan 2014). In these studies, the effect of environmental conditions (solar insolation and temperature), and physical parameters (diode’s quality factor, series resistance Rs, shunt resistance Rsh, and saturation current, etc.) is investigated. One disadvantage of these papers is lack of presenting simulation procedure so it causes difficulties for readers to follow and simulate by themselves later. This disadvantage is filled in by (Jena et al. 2014), (Pandiarajan and Muthu 2011). A stepbystep procedure for simulating PV module with subsystem blocks with userfriendly icons and dialog in the same approach with Tarak Salmi and Savita Nema is developed by Jena, Pandiarajan and Muthu et al. However, the biggest gap of the studies mentioned above is shortage of considering the effect of partially shading condition on solar PV panel’s operation.
In other researches, authors used empirical data and Lookup Table or Curve Fitting Tool (CFtool) to build P–V and I–V characteristics of solar module (Banu and Istrate 2012). The disadvantage of this method is that it is quite challenging or even unable to collect sufficient data if no experimental system be available so that modeling curves cannot be built and modeled.
From the work of (Ibbini et al. 2014) and (Venkateswarlu and Raju 2013), a solar cell block which has already been built in Simscape/Simulink environment is employed. With this block, the input parameters such as short circuit current, open circuit voltage, etc. is provided by manufacturers. The negative point of this approach is that some parameters including saturation current, temperature, and so on cannot be evaluated.
Solar model developed with Tag tools in Simulink environment is recorded in the research of (Varshney and Tariq 2014), (Mohammed 2011), etc. In these papers, only two aspects (solar irradiation and temperature) are investigated without providing stepbystep simulation procedure.
In overall, although having advantages and disadvantages, different methods have similar gaps as follows:
 The proposed models are not totally sufficient to study all parameters which can significantly affect to I–V and P–V characteristics of solar PV array, including physical parameters such as saturation current, ideality factor, series and shunt resistance, etc. and environmental working conditions (solar insolation, temperature and especially shading effect).
 Lack of presenting stepbystep simulation procedure and this causes difficulties for readers and researchers to follow and do simulation by themselves.
Therefore, the study proposes a robust model built with Tag tools in Simulink environment. The proposed model shows strength in investigating all parameters’ influence on solar PV array’s operation. In addition, a unique stepbystep modeling procedure shown allows readers to follow and simulate by themselves to do research.
Methods
Mathematical equivalent circuit for photovoltaic array
The equivalent circuit of a PV cell is shown in Fig. 1. The current source Iph represents the cell photocurrent. Rsh and Rs are the intrinsic shunt and series resistances of the cell, respectively. Usually the value of Rsh is very large and that of Rs is very small, hence they may be neglected to simplify the analysis (Pandiarajan and Muthu 2011). Practically, PV cells are grouped in larger units called PV modules and these modules are connected in series or parallel to create PV arrays which are used to generate electricity in PV generation systems. The equivalent circuit for PV array is shown in Fig. 2.
The voltage–current characteristic equation of a solar cell is provided as (Tu and Su 2008; Salmi et al. 2012): Module photocurrent Iph:
I_ = [I_ K_ (T. 298)] \times Ir/1000
Here, Iph: photocurrent (A); Isc: short circuit current (A) ; Ki: shortcircuit current of cell at 25 °C and 1000 W/m 2 ; T: operating temperature (K); Ir: solar irradiation (W/m 2 ).
Module reverse saturation current Irs:
Here, q: electron charge, = 1.6 × 10 −19 C; Voc: open circuit voltage (V); Ns: number of cells connected in series; n: the ideality factor of the diode; k: Boltzmann’s constant, = 1.3805 × 10 −23 J/K.
The module saturation current I0 varies with the cell temperature, which is given by:
Here, Tr: nominal temperature = 298.15 K; Eg0: Band gap energy of the semiconductor, = 1.1 eV; The current output of PV module is:
Here: Np: number of PV modules connected in parallel; Rs: series resistance (Ω); Rsh: shunt resistance (Ω); Vt: diode thermal voltage (V).
Reference model
The 100 W solar power module is taken as the reference module for simulation and the detailed parameters of module is given in Table 1.
Module photoncurrent is given in Eq. (1) and modeled as Fig. 4 (Ir0 = 1000 W/m 2 ).
I_ = [I_ K_ (T. 298)] \times Ir/1000
Module reverse saturation current is given in Eq. (2) and modeled as Fig. 5.
Module saturation current I0 is given in Eq. (3) and modeled as Fig. 6.
Modeled circuit for Eq. (6) (Fig. 7).
Modeled circuit for Eq. (4) (Fig. 8).
The solar module simulation procedure is shown from Fig. 3 to Fig. 8b. The solar PV array includes six modules and each module has six solar cells connected in series. Therefore, the proposed model of solar PV array is given in Fig. 9.
Experimental test
In order to validate the Matlab/Simulink model, the PV test system of Fig. 10 is installed. It consists of a rheostat, a solar irradiation meter, two digital multimeters and a solar system of two DS100M panels connected in series, each panel has the key specifications listed in Table 1.
Result and discussion
Simulation scenario
With the developed model, the PV array characteristics are estimated as follows.

(i) I–V and P–V characteristics under varying irradiation with constant temperature are given in Fig. 11a and b. Here, the solar irradiation changes with values of 100, 500 and 1000 W/m 2 while temperature keeps constant at 25 °C.
 With two modules connected in series (Ns = 72, Np = 1), the value of current output is similar to that of it in case of one module (Ns = 36, Np = 1) but the voltage output doubles so the power output doubles.
 In term of two modules connected in parallel (Ns = 36, Np = 2), the value of voltage output is similar to that of it in case of one module (Ns = 36, Np = 1) but the current output doubles so the power output doubles. Similar value of power output is experienced in both cases of two modules despite different ways in module connection (parallel or series).
The proposed model has advantages not only in studying effect of physical parameters such as series resistance Rs, shunt resistance Rsh, etc. but also in investigating impact of environmental condition like varying temperature, solar irradiation and especially shading effect. In this study, the evaluation of shading effect on solar PV array’s operation is carried out through following cases. The simulation results are given in Fig. 15a and b.
No shaded PV module (full irradiation on solar PV array): 1000 W/m 2
One shaded module (receives irradiation of 500 W/m 2 ), others receive full irradiation of 1000 W/m 2
Two shaded modules (receive irradiation of 500 W/m 2 ), others receive full irradiation of 1000 W/m 2
Two shaded modules (receive irradiation of 500 and 250 W/m 2 ), others receive irradiation of 1000 W/m 2
 The power output of PV array reduces noticeably when it works under partial shading condition.
 The I–V curve experiences multiple steps whereas the P–V curve gives many local peaks along with the maximum power point (the global peak). In addition, more shaded modules are higher number of power output peaks is shown.
Experimental results and validation
The Matlab/Simulink model is evaluated for the experimental test system (two DS100M panels are connected in series). The results are shown in Fig. 16. On the other hand, the empirical results with a solar irradiation of 520 W/m 2 and operating temperature of 40 °C are given in Fig. 17. The I–V and P–V simulation and experimental results show a good agreement in terms of short circuit current, open circuit voltage and maximum power output.
Conclusion
A stepbystep procedure for simulating a PV array with Tag tools, with userfriendly icons and dialogs in Matlab/Simulink block libraries is shown. This modeling procedure serves as an aid to help people to closer understand of I–V and P–V operating curves of PV module. In addition, it can be considered as a robust tool to predict the behavior of any solar PV cells, modules and arrays under varying environmental conditions (temperature, irradiation and partially shading condition) and physical parameters (series resistance, shunt resistance, ideality factor and so on). This research is the first step to study a hybrid system where a PV power generation connecting to other renewable energy production sources like wind or biomass energy systems.
Authors’ contributions
XHN initiated, proposed model developed in Matlab/Simulink and analyzed. He also prepared a draft manuscript for publication. MPN assisted in designing, data collection, analysis and reviewed the manuscript and edited many times and added his inputs. Finally, XHN decided finally the content of the research revised final manuscript. All authors read and approved the final manuscript.
Authors’ information
Xuan Hieu Nguyen is a lecturer in Department of Electric Power System, Faculty of Engineering, Vietnam National University of Agriculture, Hanoi, Vietnam. He had bachelor degree in electric power system, Hanoi University of Science and Technology, Vietnam in 2008. He received master degree in electrical engineering in University of Wollongong, Australia in 2012. Minh Phuong Nguyen is graduate student in Faculty of Engineering, Vietnam National University of Agriculture.
Acknowledgements
The authors are grateful to the support by this work through the project “Study, design and manufacture a solar PV system using SPV technology served for chicken farms in Faculty of Animal Science, Vietnam National University of Agriculture”, Vietnam (2014–2017).
Competing interests
The authors declare that they have no competing interests.
Author information
Authors and Affiliations
 Department of Electric Power System, Faculty of Engineering, Vietnam National University of Agriculture, Trau Quy town, Gia Lam district, Hanoi, 10000, Vietnam Xuan Hieu Nguyen
 Faculty of Engineering, Vietnam National University of Agriculture, Trau Quy town, Gia Lam district, Hanoi, 10000, Vietnam Minh Phuong Nguyen
Solar Panel Wattage and Output Explained (2023)
Learn about the typical solar panel wattages used in rooftop installations and how to estimate the ideal system capacity for your home.
The rated wattage of a solar panel indicates its electricity output when tested under ideal laboratory conditions. In reallife installations, actual solar panel wattage depends on external factors such as sunshine and ambient temperature.
The rated wattage specified by the panel manufacturer can give you an idea of how much electricity you can expect to generate. In this article, we at the Guides Home Team explain solar panel wattage, how to figure out the number of solar panels you’d need to power your home and what you’re likely to pay.
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How Much Energy Does A Solar Panel Produce?
As you can see on the atlas, western states like California and Arizona enjoy many more annual sun hours than northeast states like New York and Massachusetts. However, favorable conditions for solar panels exist throughout the U.S.
Assuming you have a site with decent sunshine and no shade, each kilowatt of solar capacity can generate more than 1,400 kWh per year. The following table summarizes how much you can expect to generate with different system capacities:
Solar System SizeExpected OutputWith a higher PVOUT value, the figures in the table above increase accordingly. For example, if you live in a sunny location where the Global Solar Atlas shows 1,700 kWh/kWp, a 6kW system would yield 10,200 kWh/year.
Factors That Affect Solar Panel Output
The electricity output of photovoltaic modules depends on the direct sunlight reaching their surface. That’s why solar panel productivity is higher in sunny locations and lower in cloudy weather conditions or in places where excessive shade comes from surrounding buildings. Under identical sunlight and temperature conditions, the energy output of solar panels depends on their efficiency.
Let’s discuss the primary factors that determine the amount of electricity generated by solar panels.
Efficiency
Solar panel efficiency can range from less than 10% to more than 20%. The efficiency rating is simply the amount of sunlight that gets converted into electricity, when the panel is tested under ideal conditions in a laboratory. As of 2023, the most efficient solar panels available in the market range from 20.60% to 22.80%, with SunPower panels at the top of the efficiency ranking.
In actual installations, the efficiency of solar panels is affected by factors like dust accumulation and high temperatures. You can prevent dust buildup by having your solar panels cleaned one or two times a year. There’s nothing you can do about high temperatures, and panels lose from 0.30% to 0.40% of their productivity for every Celsius degree of temperature rise. Fortunately, this is a temporary effect, and the lost efficiency gets recovered when panels cool.
Type of Panel
There are three main types of solar panels: monocrystalline, polycrystalline and thinfilm. Here’s what you need to know about them.
 Monocrystalline panels are the most efficient.Each of their photovoltaic cells is a single crystal of highpurity silicon, which has a sophisticated production process.
 Polycrystalline panels have intermediate efficiency ratings. Their solar cells are made of multiple silicon crystals, as opposed to a single piece. This has a negative effect on panel efficiency, but production costs are less.
 Thinfilm panels are the least efficient. This type of solar panel uses a layer of photovoltaic material, without crystalline structure, applied on a rigid or flexible substrate. However, there are now thinfilm panels of the same efficiency as polycrystalline cells.
Thanks to their high efficiency, monocrystalline panels have the highest kilowatthour output per square foot covered. Industry experts consider them the best solar panels for homes, especially if roof space is limited.
You can classify solar panels based on the number of their photovoltaic cells. Most panels have either a 60cell design in a 6×10 arrangement or a 72cell design in a 6×12 layout.
 Traditionally, 60cell panels are more common in home solar panel installations, while the larger 72cell panels are used in commercial and industrial roofs.
 A 72cell panel will be 20% more productive than a 60cell panel because it has 12 more cells. Solar panels with a capacity of more than 400W normally have a 72cell design.
Some solar manufacturers offer an inbetween size and design with 66 cells. Some solar brands use halfcells with a higher efficiency, but the overall solar panel size does not change. They have 120, 132 or 144 halfcells in the same space (instead of 60, 66 or 72 fullsized cells).
Position
To increase the energy produced by solar panels, make sure they face the sun for as much time as possible throughout the year. The sun’s position in the sky constantly changes, and the ideal tilt angle for solar panels depends on your geographic location. The sun is lower in the sky as you reside farther north, and this means solar panels must be tilted more to increase the hours of direct sunlight.
Other than the optimal tilt angle, you must also consider the orientation of solar panels. In northern hemisphere countries like the U.S., it makes sense to have your panels face south because there is more sunlight coming from that half of the sky. However, westfacing and eastfacing panels are useful in some applications:
 Eastfacing panels have a higher power production during the morning because the sun rises in that direction. They work well in schools and other buildings with a high energy consumption during the morning.
 Westfacing panels are more productive in the afternoon. They make sense for buildings with a low morning consumption and a high afternoon consumption.
Location
As you can see in the Global Solar Atlas, annual sunshine depends on your geographic location. If two 6kW solar power systems are installed in different states and one of them gets 30% more sunshine during the year, energy production also increases by around 30%.
How Many Solar Panels Do I Need?
The number of solar panels a home needs depends on three things: sunshine, home electricity consumption and the panel wattage used. For an accurate calculation and a professional design, you should contact one of the toprated solar installation companies.
You can estimate the number of solar panels needed using the following information:
 Your annual electricity consumption in kWh.
 The specific photovoltaic power output in your location, which you can get from the Global Solar Atlas.
 The panel wattage you plan to use. You can assume 350W for residential solar panels if you don’t have a specific panel brand in mind.
U.S. homes consume an average of 10,632 kWh/year, according to the Energy Information Administration. You can search for your location in the Global Solar Atlas and click to display the PVOUT value. For example, if you get 1,400 kWh/kWp, you can divide both values to get an estimated capacity of 7.6 kW (or 7,600 W).
 At this point, you only need to divide the total system wattage (7,600 W) by the individual solar panel wattage (350 W).
 In this case, the homeowner would need 22 panels, reaching a total capacity of 7,700 W.
As of 2023, the average cost of solar panels in the U.S. is 2.85/watt. You can expect to pay around 21,945 for a 7.7kW system. However, you get a 30% federal solar tax credit. thanks to the Inflation Reduction Act. The tax credit in this case is 6,583.50, and the net cost of your system drops to 15,361.50.
What To Do With Excess Energy
Solar panels don’t produce electricity evenly throughout the day. Their output gradually increases during the morning, reaching a peak around noon before decreasing in the afternoon. Because many homes are empty during the day, solar panels often generate excess energy that doesn’t get consumed right away. Homeowners have two options to deal with this excess electricity, and each have their pros and cons:
 Storing surplus electricity in a battery system and using it later
 Exporting excess energy to the local grid and getting a credit on your next electric bill
A battery system gives you the option of storing solar electricity to be used at any time, even at night when panels are no longer productive. Solar batteries also qualify for the 30% federal tax credit, and additional incentives may be available from your local government or utility company. If you purchase a solar battery capable of operating offgrid, you can use it as a backup power system during blackouts. However, home battery systems are expensive, and they can add more than 10,000 to your solar installation costs.
Exporting surplus solar power to the grid is a convenient option because you can trade unused electricity for power bill credits. However, the kWh price you sell your electricity for is normally lower than the retail price you pay, which means you don’t save the full value of each kWh. States have different buyback rates, so your location matters. And many states don’t even have a net metering or solar buyback policy, which means you don’t get credit for sending excess energy to the grid. Battery storage becomes your only option in this case.
Frequently Asked Questions About Solar Panel Output
How much power does a 500watt solar panel produce per day?
Assuming favorable sunlight conditions, a 500watt panel will produce around 2 kWh per day, and more than 700 kWh per year.
How many solar panels are needed for a 2,000watt system?
This will depend on the individual wattage of the solar panels you choose. Simply divide the total capacity required by the panel wattage:
Solar Panel Wattage  Number Required for 2,000 W 
250W  8 panels (2,000 W) 
300W  7 panels (2,100 W) 
350W  6 panels (2,100 W) 
400W  5 panels (2,000 W) 
What does the average solar panel produce per day?
Residential solar panels have typical power ratings of around 350400 W. Under favorable sunlight conditions, a panel of this wattage can generate over 1.5 kWh of electricity per day.
What will a 2,000 watt solar system run?
According to the Energy Information Administration, U.S. homes consume 10,632 kWh/year, on average. With decent sunshine, a 2,000watt solar energy system generates more than 2,800 kWh/year, covering 26% of the electricity usage of a typical home; 2,800 kWh/year is roughly equivalent to the annual consumption of a threeton central air conditioner.
Leonardo David is an electromechanical engineer, MBA, energy consultant and technical writer. His energyefficiency and solar consulting experience covers sectors including banking, textile manufacturing, plastics processing, pharmaceutics, education, food processing, real estate and retail. He has also been writing articles about energy and engineering topics since 2015.