Solar Panel Charge Time Calculator
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
1. Enter your battery voltage. For instance, if you’re using a 12V battery, you’d enter the number 12.
2. 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.)
3. 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.
4. 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%.)
5. 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.
6. Select your charge controller type.
7. 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.
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.
1. 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
2. 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
3. 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
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.
1. 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
2. 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
3. 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
4. Divide discharged battery capacity by solar output to get your estimated charge time.
Charge time = 960Wh ÷ 190W = 5.1 hours
This last method builds on the previous one. It takes into account system losses to give you an even more accurate estimate.
1. 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
2. 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
3. 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
4. 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
5. 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
6. 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.
Tutorial: How to Connect a Battery to a Solar Panel
There are two main types of charge controllers: PWM and MPPT. Check out our recommendations and reviews for each type: