Solar Panel Charge Time Calculator
Note: Use our peak sun hours calculator to find out how many peak sunlight hours your location gets per day.
Tip: If you’re charging your battery with a battery charger rather than solar panels, check out our battery charge time calculator.
How to Use This Calculator
Enter your battery voltage. For instance, if you’re using a 12V battery, you’d enter the number 12.
Enter your battery capacity in amp hours. If you have a 50Ah battery, you’d enter the number 50. (If you only know your battery’s capacity in watt hours, first convert watt hours to amp hours.)
Select your battery type. Select “Lead acid” if you’re using a flooded or sealed (AGM or gel) lead acid battery. Select “Lithium (LiFePO4)” if you’re using a lithium iron phosphate battery.
Optional: Enter your battery depth of discharge as a percentage. If your battery is 80% discharged, you’d enter the number 80. (If you have a lead acid battery, keep in mind that they should usually only be discharged 50%.)
Enter the wattage of your solar panel or solar array. If you’re using a 100W solar panel, you’d enter the number 100. If you’re using a 400W solar array, you’d enter the number 400.
Select your charge controller type.
Click “Calculate” to get your results. Your estimated charge time is given in peak sun hours. You can use our peak sun hours map or calculator to find out how many peak sun hours your location gets. For example, let’s say your estimated charge time is 8 peak sun hours and your location gets on average 4 peak sun hours per day. In that case, you know it’ll take about 2 days for your solar panel(s) to charge your battery.
How to Calculate Charging Time of a Battery By Solar Panels
Besides using our calculator, here are 3 ways to estimate how long it’ll take to charge a battery with solar panels.
I’ll run through each method step by step, starting with the simplest and ending with the most complex.
Note: None of these methods is perfect. Each makes a number of assumptions that aren’t obvious to the untrained eye. I talk more about these assumptions at the end of this section.
Method #1
This is one of the more common ways you’ll see people estimate charge time. It’s simple but inaccurate. For this one, your battery and solar panel need to have the same nominal voltage.
Accuracy: Lowest
Complexity: Lowest
Steps
Divide solar panel wattage by solar panel voltage to estimate solar panel current in amps. For example, here’s what you’d do if you had a 100W 12V solar panel.
Solar panel current = 100W ÷ 12V = 8.33A
Divide battery capacity in amp hours by solar panel current to get your estimated charge time. Let’s say you’re using your 100W panel to charge a 12V 50Ah battery.
Charge time = 50Ah ÷ 8.33A = 6 hours
If using a lead acid battery, multiply charge time by 50% to factor in the recommended max depth of discharge of lead acid batteries.
Charge time for lead acid batteries = 6 hrs × 50% = 3 hours
Method #2
This way takes into account two important factors that the first method doesn’t: battery depth of discharge (DoD) and solar charge controller efficiency. Incorporating DoD adds flexibility. You can estimate charge time regardless of what state of charge your battery is at.
Accuracy: Medium
Complexity: Medium
Steps
Multiply battery voltage by battery amp hours to get battery capacity in watt hours. For example, let’s say you have a 12V 100Ah battery.
Battery capacity = 12V × 100Ah = 1200Wh
Multiply battery watt hours by battery depth of discharge to estimate how much of the battery’s capacity has been discharged. Let’s say your battery is discharged 80%.
Discharged battery capacity = 1200Wh × 80% = 960Wh
Multiply solar panel wattage by rule-of-thumb charge controller efficiency (PWM: 75%; MPPT: 95%) to estimate solar output. Let’s say you’re using a 200W solar panel and an MPPT charge controller.
Solar output = 200W × 95% = 190W
Divide discharged battery capacity by solar output to get your estimated charge time.
Charge time = 960Wh ÷ 190W = 5.1 hours
Method #3
This last method builds on the previous one. It takes into account system losses to give you an even more accurate estimate.
Accuracy: Highest
Complexity: Highest
Steps
Multiply battery voltage by battery amp hours to get battery capacity in watt hours. For example, let’s say you have a 12V 200Ah battery.
Battery capacity = 12V × 200Ah = 2400Wh
Multiply battery watt hours by battery depth of discharge to estimate how much of the battery’s capacity has been discharged. Let’s say your battery is discharged 50%.
Discharged battery capacity = 2400Wh × 50% = 1200Wh
Divide discharged battery capacity by the battery’s rule-of-thumb charge efficiency factor (lead acid: 85%; lithium: 99%) to get the amount of energy required to fully charge the battery after factoring in losses during charging. Let’s say you’re using a lead acid battery.
Energy required for full charge = 1200Wh ÷ 85% = 1412Wh
Multiply solar panel wattage by rule-of-thumb charge controller efficiency (PWM: 75%; MPPT: 95%) to estimate solar output. Let’s say you’re using a 400W solar array and an MPPT charge controller.
Solar output = 400W × 95% = 380W
Multiply solar output by 100% minus a fixed percentage to take into account system losses. The National Renewable Energy Laboratory’s PVWatts Calculator uses 14.08% as its default value for system losses, so I’ll use that number here.
Adjusted solar output = 380W × (100%. 14.08%) = 380W × 85.92% = 326W
Divide the amount of energy required to fully charge the battery (in watt hours) by the adjusted solar output (in watts) to get your estimated charge time.
Charge time = 1412Wh ÷ 326W = 4.3 hours
Assumptions Shortcomings of All These Methods
All these methods make assumptions. And they all leave out factors that affect solar charging time in the real world. Here are a handful of the main ones:
- Assumption: The solar panels are outputting their rated power. A solar panel will only output its stated wattage under ideal conditions (called Standard Test Conditions, or STC). You’ll rarely see a 100W panel output 100 watts. Environmental factors such as weather, shading, and ambient temperature all affect solar output in ways that these calculation methods fail to capture.
- Assumption: There are no loads connected to the battery. Your battery may be powering something while your solar panels are charging it. That device draws power from the battery, so your battery will need even more energy to reach full charge. Also, the solar charge controller itself is a load that will always be connected to the battery and using up a little power. The charge controller is usually a negligible load, but for some scenarios — particularly trickle charging a large battery with a small solar panel — leaving it out does have a material effect on charge time estimates.
- Shortcoming: Rules of thumb don’t always hold true. The calculations use rules of thumb for simplicity’s sake. Reality is always much more complex. PWM and MPPT charge controller efficiency varies wildly based on factors such as temperature and the difference between PV voltage and battery voltage. A lead acid battery’s charge efficiency changes based on state of charge. And each system will have different sources and sizes of losses.
- Shortcoming: Charge controllers often have a timed absorption stage. Once a battery reaches a certain voltage, charge controllers usually enter an “absorption” stage for the remainder of the charge cycle. This stage often lasts a fixed amount of time. For lead acid batteries, the absorption stage typically lasts 2 hours. Lithium batteries don’t need absorption, but charge controllers will often do a 20-30 minute absorption charge to balance the battery cells. This timed stage isn’t captured using these methods.
Our charge time calculator takes into account a couple of these variables for a more precise estimate. But, alas, it can’t predict the weather…yet.
How Do You Charge a Battery with a Solar Panel?
Never connect a solar panel directly to a battery. Doing so can damage the battery.
Instead, connect the battery then solar panel to a solar charge controller. Charge controllers regulate the current and voltage coming from solar panels to safely charge the battery.
There are two main types of charge controllers: PWM and MPPT. Check out our recommendations and reviews for each type:
Help my 900 watts of solar panels to generate at least 340 watts.
I have 6 solar panels, each rated at 150 watt at 18.5Vmp, 8.1 Imp(A). Totaling 900 watts in all. The tilt is not optimaldue to ease of installation. lets say 25% losses can be expected due to the tilt. The panels are wired in parallel, it runs through a PWM 30a charge controller (cm3024) to a gel 155ah battery. I have an inverter connected to the battery to draw power.
During solar peak, the panels manages to generate 17 amps (-255 watts) max. And that’s if the inverter has a 600 watt load on it. The remaining power difference is drawn from the battery.
Should I buy a MPPT Tracer4210A 40A to produce at least 340 watts or should I wire the panels into a 24v system.
If the panels are at 24v nominal then, it will create a greater voltage differential of 37v to 13.8v (battery voltage). I am hoping that this will allow greater amps to flow through the charge controller, 20 amps would be nice. Since the PWM charge controller would be throwing away half of the power from the solar panels in such a setup, I was also thinking to attach a buck converter to the panels to charge a secondary battery bank of 18650s (400 cells in a 4s setup) and gain 4 amps that a way. 20 amps at 13.8v and 4 amps at 16v would equal the desired 340 watts that I hope to generate.
Any advice would be greatly appreciated.
Korishan
With solar panels, I would say that an MPPT is a great help as it adjusts the incoming voltage/amps to give the best output results. If you don’t want to buy one, you can always look up Julian Illet’s MPPT arduino based controller. It’s pretty easy, simple, straight forward, expandable, and cheap. It does the job and a great job at it for being DIY’d.
That’s my.02 worth about it, though. Others may have other opinions.
Proceed with caution. Knowledge is Power! Literally! Knowledge is Power; Absolute Knowledge is Absolutely Shocking! Sub-Certified 18650 Cell Reclamation Technician
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ozz93666
I don’t think replacing your charge controller is going to make a significant difference.
You don’t say where you are locate if far away from the equator the sunlight has a lot of atmosphere to go through. starting at 900w (that’s when the panels are brand new and perfectly clean) you say discount 25% due to tilt that leaves 675W then resistive losses in wires taking power to charge controller you don’t say how far this is. or how thick the wires are. perhaps 5% loss. loss in controller 10-15%. loss in inverter.
changing to 24V will only help a little. I think the best plan is to get more panels.
I suspect you’ve underestimated the 25% loss. if you can orientate a panel directly at the sun. you can see the max possible out put at your location. you maybe surprised. chemtrailing has reduced out put from panels. even when the sky appears to be clear blue ,it isn’t
daromer
If your Vmp is at 18.5V you loose alot on a 12V system with a PWM controller during load where the battery bank perhaps are around 12.5V.
Vmp is the voltage where the panels produce the 900watts ad ideal angle towards the sun at optimal place. If you drop that down to 13V you loose alot in combination of the other factors mentioned.
So i would say yes to a proper MPPT controller. How much you loose you can check on the datasheet of your panels compare to the voltage you have on the system when you meassure above.
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brian_dean2002
New member
Yes I agree. 18.5V at 17 amps equals 314.5 watts and 12.5v at 17 amps equals 212.5 watts. That’s a loss of 32% right there.
I am using 10 AWG stranded wiring. It can handle 30 amps. However, I was thinking to shift to 24v not so much to reduce the amp load on the wire but more for creating a voltage difference.- 18.5v. 13.2v = 5.3v (little room to push energy potential) whereas, 37v.13.2 = 23.8v (lots of room to push energy potential).
Yes, I am thinking that I should get an MPPT charger and it will buck convert 18.5v at 17 amps to 13.2v at 23.8 amps. If it does not then, I will have wasted 120 on an MPPT charger.
Thank you all for the great advice. It is appreciated a lot!
Elmo
How long is your run of 10 AWG total for both the positive and negative cables?
At 17A you are losing 17mV for every foot of 10 AWG (1mOhm per foot) in both leads. If your panels are only 5 feet from your controller you’ll be losing 170mV (5 feet 2 cables 0.017), 50 feet and you’ll be losing 1700mV.
At 50 feet and 30A you would be losing 3V in the 10 AWG cables.
The 4210 is maxed out at 400W on a 12V system, even 340 consistently is going to be pushing it a bit, particularly in the heat.
ozz93666
I maybe a biased against MPPT. I bought an expensive one. Outback brand. lots of ridiculous features like security code to get it to work. it lasted about 18 months. taking it apart I found a gecko had got inside and was fried shorting wires. this device was designed for the outback. and yet has a ventilation grill so critters can crawl inside.
The savings with mppt are not easy to gauge. many factors to consider. I would guess. averaged out. perhaps 10 to 15 %.
Before you buy one you may want to measure the power your panels are delivering. to see if that’s the problem. disconnect the cables where they enter the power controller and connect them to a load (electric kettle. lights ). adjust the load so the measured voltage is Vmp (written on back of panels. mine are 17.1V) measure the current V x I gives you the power.
If you have room it’s probably more sensible to get more panels. very cheap now. they never short or die. electronic equipment does !
Elmo
But you still need electronics, even a PWM controller is electronic in nature and if you are running a higher voltage battery setup you could still fry the occasional gecko.
Your comment about the gecko reminded me of something, I’ve used Geckodrive components extensively and they are the best bang per buck in lower end CNC drives, the owner is a regular on at least one CNC forum I sometimes frequent. Very Smart guy, has forgotten more about power electronics than I will ever know.
Too bad Mariss hasn’t gotten into solar controllers, I’d buy one from him in a moment.
brian_dean2002
New member
The cable length is around 30 feet. And I don’t understand what a GeckoDrive does and its relevance here. I think it is an LED driver.
Mppt is a moving target in terms of savings. At 900 watts, I think it would be better to buy 2 more solar panels and that will deliver greater savings/output.
Korishan
The gecko drive was in reference to dead gecko in the controller that got fried. The mention of the dead gecko reminded Elmo of the other one. It was a comical reference, not a practical one.
Proceed with caution. Knowledge is Power! Literally! Knowledge is Power; Absolute Knowledge is Absolutely Shocking! Sub-Certified 18650 Cell Reclamation Technician
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Elmo
It was also that the designer of the motion control drives, Mariss Freimanis, hangs out in the forums concerned with how his products are used and participates, hence his products are more likely to meet the needs of the actual user.
Technologically his product is quite similar to an MPPT controller, I just wish Mariss did solar as well as he does servo and stepper motion drives.
The cable length is around 30 feet. And I don’t understand what a GeckoDrive does and its relevance here. I think it is an LED driver.
Mppt is a moving target in terms of savings. At 900 watts, I think it would be better to buy 2 more solar panels and that will deliver greater savings/output.
You were the one complaining that your current setup doesn’t deliver the way you think it should. We were replying to your initial request of helping the 900 watts of solar panels to produce 340 watts of power or at least I was.
The entire solar field is a moving target at the moment and the moving seems to get faster all the time. Something I tried hard to do five years ago now comes in a Crackerjack box practically, whoever is making the MPT7210 needs to make it buck or boost and make it the 7220.
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