How Many Solar Panels Do I Need?
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Solar panels have enjoyed a meteoric rise in popularity in recent decades. That’s thanks in part to an increase in environmentally responsible behavior and the desire to reduce energy bills by exchanging traditional sources of heating, cooling and electricity with cleaner, more natural sources. If you’re thinking of making this switch, you’re probably wondering how many solar panels it takes to power a house.
While the answer can be a bit complicated, if you hire a professional or solar panel installation company to consult with, they’ll likely handle this part of the process as well (and may give you tips on how to maintain and clean solar panels ).
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But if you’re wondering how many solar panels you may need, and you’d like to take a stab at calculating it yourself, you’ll need a few pieces of information: how much energy your household uses; how much space you have on your roof that can be used for solar panel placement, how many hours of sun your home gets and the wattage and relative efficiency of the photovoltaic (PV) solar panels you’ll be installing.
How to Determine Solar Panel Needs
In order to learn how many solar panels are needed to power a house, you’ll use a formula with three key factors, according to EnergySage: annual energy usage, panel wattage and production ratios. But what does that mean exactly?
Annual Electricity Usage
The first step is to determine your annual electricity usage; that’s the amount of electricity consumed by your entire household in a year. Measured in kilowatt-hours (kWh), this number includes all sources of electricity in your home, including small and large appliances, air conditioning units, lights, air purifiers and water heaters. The U.S. Energy Information Administration (EIA) indicates the average household consumes about 11,000 kWh of electricity per year.
Solar Panel Wattage
When choosing the best solar panels you may think they look mostly the same, but they aren’t exactly created equally, so you’ll need to know the wattage of the panels you’re hoping to install. The panel wattage is the amount of electricity emitted from the panel. Most solar panels range between 250 to 400 watts of power, so it’s safe to assume 300 is the average panel wattage you might find.
Production Ratios
According to EnergySage, a solar panel system’s production ratio is the ratio of the estimated energy output of a system over time (in kWh) to the actual system size (in W). You might think that this would be a 1:1 ratio – that you get out what goes in. But variations in the amount of sunlight that beams down on your home cause that not to be the case.
A 10 kW system that produces 16 kWh of electricity in a year will have a production ratio of 1.6 (16/10 = 1.6). In a place like Hawaii, which enjoys long days and consistent sunshine, it’s totally possible to have this type of ratio, whereas cloudy, rainy New England might see an average production ratio of only 1.2
Calculate How Many Solar Panels You Need
Here’s the actual formula, used by EnergySage, that you can use you’re hoping to determine how many solar panels you’ll need:
- Number of panels = system size / production ratio / panel wattage
- Using the numbers we’ve determined so far, we get:
- Number of panels = 11,000 kW / 1.6 / 300 W
That equates to about 20 to 25 solar panels to do the job. You can use this same formula to determine how many solar panels you’ll need to power your home. Or, you can use the easier route, which is to look at your energy bill to determine what you’ll need.
THIS IS AN ADVERTISEMENT AND NOT EDITORIAL CONTENT. Please note that we do receive compensation for any products you buy or sign up to via this advertisement, and that compensation impacts the ranking and placement of any offers listed herein. We do not present information about every offer available. The information and savings numbers depicted above are for demonstration purposes only, and your results may vary.
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An Alternative Way to Figure Out Solar Energy Usage
If you’re not interested in doing the math on your own, simply look at your utility bills to figure out how much energy you’re using. Doing this allows you to multiply your energy usage by the number of hours of strong sunlight your home gets, then dividing that result by the wattage of the panels you’re thinking of installing.
Factors that Affect How Many Solar Panels You Will Need
Is there anything else to think about outside of the above calculations? Turns out, there are a few other things factors need to consider when determining how many solar panels to power a house.
Solar Panel Output Efficiency
Your solar panels won’t draw the sun’s energy at top capacity all the time. Think of those three-day bouts of rain that come in the fall, or large snowfalls in the winter that take several days to melt. Those are times you’ll need a buffer in your energy usage, so it’s recommended to have about 25% more solar panels than you need.
Hours of Sunlight
The amount of energy you’ll get from your solar panels is directly related to how much sun your home gets. panels will be required if you live in an area without long hours of strong sunshine.
Wattage of Your Panels
Most solar panels range in wattage from 150 to 350 watts per panel. If you choose lower wattage panels, you’ll need more in order to generate enough energy for your home. Of course, that’s assuming you’d like to replace 100% of your energy usage with solar energy. If you’re only hoping for a partial conversion, the difference in solar panel wattage may not matter as much.
Cost of Solar Panels
How much do you want to or plan to spend on your solar panels ? Before you buy, make sure you know how many make sense for your budget.
Solar Panel Size
To understand how many solar panels you’ll need, you must know the standard sizes of solar panels to know how many you can place on your roof. The area of a residential 60-cell solar panel is 17.62 square feet, and the area of a commercial 72-cell solar panel is 21.13 square feet. Solar panel installation companies will measure the area of your roof to determine how many panels can be installed safely.
THIS IS AN ADVERTISEMENT AND NOT EDITORIAL CONTENT. Please note that we do receive compensation for any products you buy or sign up to via this advertisement, and that compensation impacts the ranking and placement of any offers listed herein. We do not present information about every offer available. The information and savings numbers depicted above are for demonstration purposes only, and your results may vary.
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How Much Electricity Does a Solar Panel Produce?
In one year, the sun sends an immense amount of energy to our planet, around 222,500,000 terawatt hours [1]. This is more energy than we could ever hope to use, and it represents a vast potential for the solar power. However, to harness this energy, we need to convert it into usable electricity.
The efficiency of this conversion is a critical factor that affects the amount of solar energy we are able to harvest for our use.
Solar panels as devices that convert sunlight into electricity by the photovoltaic effect are the tool that should make this potent source of energy available to us. But the most efficient solar panels on the market today have a maximum efficiency of around 22 percent, meaning they can convert up to 22 percent of the energy from the sun that falls on them into usable electricity [2].
The twist is that most solar arrays you see around reach only up to 18 percent efficiency. This means that, even with the best technology available, we are only able to convert a fraction of the sun’s energy into electricity.
Understanding how much electricity does a solar panel produce is essential to predicting the monthly output of a solar system. This information helps when designing and installing solar systems that should provide sufficient electricity to meet a household’s needs.
So even though, solar energy is free and abundant, there is still a lot of space for improvement when it comes to our ability of harnessing it, and efficiency of solar panels is just one of the limiting factors on the way.
How much electricity does a solar panel produce per day?
The energy output of a solar panel varies based on the size of the panel you have, maximum efficiency and output rating in watts.
Solar panels are commonly evaluated according to the amount of direct current power they generate under standard test conditions. This power output is measured in watts and represents the panel’s potential to produce electricity under ideal sunlight and temperature conditions. Ideal conditions mean a temperature of 25 degrees Celsius or 77 degrees Fahrenheit and unobstructed sunlight hitting the whole surface of a solar panel.
The test temperature represents the average temperature during the solar peak hours of the spring and autumn in the continental United States. The problem is that in real life you have rarely the ideal conditions, so the values will differ.
Most residential solar panels have power output ratings ranging from 250 to 400 watts. This rating affects the total cost of a solar system. Premium solar panels with higher wattage produce more energy but also cost more than the lower rated ones, since the price for solar panels is calculated per watt.
The size of solar panels affects the final power output as well. Larger solar panels (with 72 solar cells) produce more energy than the 60-cell ones, assuming they receive the same amount of sunlight.
The 72-cell panels usually carry higher rating of 300 to 400 watts. Higher energy output simply correlates with the number of solar cells. solar cells mean more energy produced. However, in most residential application, the smaller solar panels with 60 cells are used.
And similarly, solar panels with higher efficiency rating convert more of the sunlight their solar cells receive into usable electricity, resulting in higher energy output. Monocrystalline solar panels still represent the most efficient option for homeowners on the market.
How to calculate the average solar panel output?
How much energy does one solar panel produce in a day also depends on environmental factors like the amount of unobstructed sunlight in a day.
An average solar panel with efficiency of 18 percent usually produces around 250 to 300 watts of power under ideal conditions. In reality, the actual output varies depending on the amount of sunlight available.
To estimate the expected energy output of a solar panel, we can use a simple formula that takes into account the wattage of a solar panel and the amount of sunlight it receives. This formula is as follows:
For example, if we have a 250-watt solar panel and it receives 5 hours of sunlight per day, the expected energy output would be:
Energy output = 250 watts x 5 hours = 1,250 watt-hours per day or 1.250 kilowatt-hours
How many hours of sunlight a solar panel receives is a crucial factor in determining its energy output. The intensity of sunlight varies depending on the location, time of day, and season. In such a large country like the United States, this significantly differs from one region to another.
According to the National Renewable Energy Laboratory (NREL) report, the amount of sunlight received per day can range from around 2.5 to 7.5 kilowatt-hours (kWh) per square meter, depending on the location [3]. This means that a solar panel in sunny Arizona will produce on most days more energy than a panel in Seattle.
You can find a good data on the solar potential for your location in the Global Solar Atlas.
What environmental factors affect total energy output of solar panels?
Although each solar panel has slightly different technical characteristics, there are some environmental factors that affect the total electricity output of every panel. It is good to know them if you want to make sure that you are getting the best of your solar system.
#1 Direction
The position of panels on the roof is crucial. In the northern hemisphere, panels should face south, eventually south-west or south-east, to achieve the best outcome. In the southern hemisphere, it is the other way around.
#2 Roof angle and tilt
Since the most affordable method is to mount panels on the roof, the angle depends on the pitch of your roof. Although, to maximize panels’ output, you can get adjustable installation, where you are able to tilt the panel according to the season.
The basic rule to determine the proper angle is to tilt the panel by 15 degrees more vertically from the starting position in winter and by 15 degrees more horizontally in summer. For example, panels in New York City should be tilted at an angle of around 40 degrees to maximize energy output [4].
How much power does a solar panel produce for my direct consumption?
An average quality residential solar panel produces between 250 to 270 watts under ideal sunlight and temperature conditions. Such panel consists of 60 solar cells. One solar cell generates 5 watts, with the efficiency between 15 to 20 percent. The size of the panel in this case is around 65 inches in length and 40 inches in width.
But you can have also good quality solar panels generating as much as 400 watts. You should know, though, that panels with higher output are more expensive and are usually installed when free space is a limiting factor.
On the back of each panel is a number stating the maximum power rating. This number expresses the direct current produced under standard test conditions. Since solar panels produce direct current and our houses run off alternating current, the generated power needs to be converted. During the conversion, further energy losses occur. The losses make around 20 percent [7].
This means that from direct current produced by solar panels we get 80 percent of alternating current for our use. So, if you want to calculate how many solar panels are needed to cover your household energy needs, you have to take this number into consideration.
For example, you need 5 kilowatts (kW) of alternating current in total. 5 kilowatts have to be divided by 0.8 (as 80 percent efficiency during conversion). The final number you get is 6.25 kW of direct current, which equals 25 panels of 250 watts (6,250 divided by 250 watts equals 25 panels) or 20 panels with higher rating of 325 watts.
Both systems generate the same amount of power, your choice depends only on the available space and money you want to invest, because the 325-watt panels cost more.
The energy produced by a solar system in a day
The expected energy output of a whole solar system is calculated by summing the energy output of all the panels in the system.
Our example, a 5-kilowatt solar system consisting of 25 panels rated at 250 watts each, would have an expected energy output of:
Energy Output = 250 watts x 5 hours of sunlight x 25 panels = 31,250 watt-hours per day, or 31.250 kWh per day
This is enough energy to power a typical American household for a day, assuming an average energy consumption of around 30 kWh per day [5].
Now that you know how much solar electricity you can expect one solar panel to produce and how much a whole system can produce, you might be wondering about the cost savings that solar panels can bring.
If you are considering getting solar and would like a rough estimate of the cost and savings of installing a solar system, you can use this solar calculator. The calculator will provide you with an upfront cost estimate and an estimate of long-term savings based on your location. Once you have a better idea of the potential savings, you can begin to explore your solar options in more detail and get real solar quotes from local installers.
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Many factors go into calculating the best solar system size. Some of these variables include your home’s energy usage, your roof’s available square footage, the solar panel wattage, and amount of sun the solar panels will receive.
For example, in Maine where sunlight is often at a minimum, homeowners prefer higher-rated panels that generate more electricity in comparison to panels typically selected for homes in sunny California. For all residences, high efficiency panels generate more wattage which means fewer panels on your roof.
Design a Custom Solution
Each Sunrun solar system is custom designed and built for your energy needs using a proprietary solar design software platform. There’s definitely not a one-size-fits-all solution. It can be as small or large as you want or need. Sunrun doesn’t install cookie-cutter systems. Our proprietary technology, called BrightPath enables us to design a system and solar plan specifically for your home.
Large photovoltaic systems produce the most electricity and reduce your carbon footprint more than a smaller system. Yet, even if you’re not limited by cost, the size of your south-facing roof may limit your system size. In that case, maximize your solar installation’s yield by considering smaller, high efficiency panels to achieve your energy goals.
Solar Power Generation Choices. Wattage or Efficiency?
The broad selection of solar panel models available for household use are not created equal. And, they do not all generate the same amount of power.
Solar panels are rated based on the watts they generate. The higher the wattage rating, the greater amount of power your solar installation will produce. Most residential solar panels have power output ratings from 250 to 400 watts, depending on panel size and how well they convert sunlight into energy. While higher power ratings are considered preferable, power output is not the sole factor in judging a solar panel’s performance.
For instance, two solar panels may each have a 15% efficiency rating, yet the power output rating of one is 250 watts and the other is 300 watts. 4 The higher output may simply be due to the 300 watt panel’s larger physical sizerather than having a high efficiency or more advanced technology. Therefore, panel efficiency is a better indicator of solar panel performance than just power output for meeting your energy needs.
Size vs. Quantity
In application, you could use either of these panels in a solar system to build a system with a total rated capacity of 5kW. The difference is it would have either 20 250-Watt panels or 16 300-Watt panels. 4 With either panel’s wattage, the systems would generate an equal amount of power if installed in the same location.
A solar panel’s wattage represents its potential power production under ideal conditions. The table below shows the minimum, maximum, and average power outputs of the solar panels from a few leading manufacturers. Each company has a wide range because they produce multiple solar panel models. Panel wattage is important but just one factor that goes into your equation.
Variables Affecting Solar Panel Output
In addition to the amount of sunlight received per day, there are other factors that affect the output of your solar panel or system.
Dirt
Anything that builds up on the surface of your solar panel can affect the output. This can include dust, leaves, snow, or bird droppings. A clean solar panel can be 6.5% more efficient than a dirty and dusty panel.
Roof Direction and Angle
Solar panels are most efficient when directed in a south-east to south-west direction, at an angle of 30-45 degrees. Systems at other directions and angles can still work, but your outputs will be decreased.
Shade
Solar panels are very sensitive to shade, including trees, or a building next door. Minimal shading in the morning or evening is fine, but significant shading throughout the middle part of the day will significantly impact the amount of electricity a panel can produce.
Location
The amount of sunshine and Cloud cover will affect the amount of energy a solar panel can produce.
Time of year
Solar panels can produce electricity year-round, even on overcast days. Through summer, the days are longer which generates more output, but shorter days in winter mean your output will be lower over these months.
Age
As solar panels age, their efficiency decreases at around 0.5% each year. The life cycle of the system is approximately 25 years before performance has decreased to the point a new system is needed.
Temperature
The efficiency of solar panels is usually measured at 77 F, and temperatures above this can end up decreasing their efficiency. Solar panels can work well in cold weather, and can still generate power in snowy conditions, too.
How Much Electricity Does My Home Need?
One solar panel on its own isn’t going to create enough electricity to power your entire home, but a solar panel system can. To work out what size system you need, you’ll need to complete some basic calculations that we’ve covered in our article How Many Solar Panels Do You Need?
To fully power an average home using 11,000 kWh per year, a typical solar power system will need between 21-24 panels of 320 watts each. The exact number and wattage of panels, as well as the output they can produce, will depend on where you live and the setup of your specific system.
Types of Solar Panels and Output
There are three main types of solar panels used for domestic systems:
- Monocrystalline. These are the most popular type of panel, made with pure silicon. They have an efficiency of 24.4%, with a moderate cost and a long lifespan.
- Polycrystalline. These are made of silicon crystals that have been melted together. They have an efficiency of 19.9%, a low cost and a moderate lifespan.
- Thin-film. Made with a variety of materials including small amounts of silicon, thin-film panels have an efficiency of 18.9%, with a high cost and a shorter lifespan.
The output of each type of panel will vary depending on the individual manufacturer, but will always be stated as a power rating in watts. The higher the watts, the higher the output. You may also see a kilowatt peak rating, which is the maximum power the panel can produce under the standard test conditions mentioned earlier.
Cost vs. Value
The solar market is very cost competitive, but some brands will offer you a more efficient system for a slightly higher investment. These systems will generate more electricity over the life of the system, so in the long run, they will create more value with consistent increased output.
Remember to look into federal tax credits and other incentives designed to reduce the cost of solar panel installation.
Given your house gets about six hours of daily sunshine, a standard 250-watt solar panel would produce 1.5 kWh of energy in a day.
You would need about 20 250-watt solar panels to generate the amount of energy the average American home uses in a day.
You can increase solar panel efficiency by cleaning the dirt off your panels regularly, pruning any trees that could be shading the panels, optimizing the panels’ angle (ideally to a 30- to 45-degree angle facing south), or installing an automatic solar tracker that rotates the panel to keep it aligned with the sun.
- How Many Solar Panels Do I Need for My Home? EnergySage.
- How Much Electricity Does an American Home Use? U.S. Energy Information Administration, 2020.
- Do Solar Panels Work at Night or on Cloudy Days? EnergySage.
- Hussain, Athar, et al. An Experimental Study on Effect of Dust on Power Loss in Solar Photovoltaic Module. Renewables: Wind, Water, and Solar, vol. 4, no. 9, 2017., doi:10.1186/s40807-017-0043-y
- Solar Panel Performance: How Much Does Roof Orientation and Angle Matter? EnergySage.
- Jordan, Dirk and Sarah Kurtz. Overview of Field Experience—Degradation Rates and Lifetime. National Renewable Energy Laboratory, 2015.
- Mow, Benjamin. STAT FAQs Part 2: Lifetime of PV Panels. National Renewable Energy Laboratory, 2018.
- How Hot Do Solar Panels Get? Effect of Temperature on Solar Panel Performance. EnergySage.
- Belyakov, Nikolay. Chapter Seventeen—Solar Energy. Sustainable Power Generation: Current Status, Future Challenges, and Perspectives. 2019, pp. 417-438., doi:10.1016/B978-0-12-817012-0.00031-1