How many solar panels to charge a battery?
The recharge math for 100–400 Ah banks at 12/24/48 V — sun hours, charge efficiency, and panel counts explained.
The recharge math
Energy stored = Ah × voltage. A 100 Ah 12 V battery holds 1,200 Wh; 200 Ah at 12 V holds 2,400 Wh. To recharge in one day, your array must deliver that energy within the available sun hours, divided by ~85% charge efficiency (controller plus battery round-trip losses):
Required array watts = (Ah × V) ÷ sun hours ÷ 0.85
A refrigerator worked from a published 500 Wh/day example is on what size solar system for a refrigerator.
Worked examples at 4.5 sun hours
| Battery bank | Stored Wh | Array needed (one-day recharge) | Typical panel count |
|---|---|---|---|
| 100 Ah @ 12 V | 1,200 | ~315 W | 1×300 W or 3×100 W |
| 200 Ah @ 12 V | 2,400 | ~630 W | 2×300 W |
| 200 Ah @ 24 V | 4,800 | ~1,260 W | 4–5×300 W |
| 400 Ah @ 12 V | 4,800 | ~1,260 W | 4–5×300 W |
| 400 Ah @ 48 V | 19,200 | ~5,000 W | 16–17×300 W |
Seasonal adjustment
The table uses 4.5 sun hours, the default on the panels-per-battery tab. Type your worst-month hours into that field. PVWatts prints a monthly figure for an address you enter.
Charge rate limits
Match the charge controller to the array: watts ÷ battery volts ÷ 0.85, rounded up to the next standard rating. The charge-current limit for one LiFePO4 model is on how to charge LiFePO4 batteries with solar. PWM versus MPPT, and the 700 W worked example, are on how to size a solar charge controller.
The series fuse, which is a different part from the charge controller, is on how to fuse a solar panel array. Charge voltages for one LiFePO4 model are on how to charge LiFePO4 batteries with solar. Matching a module's nameplate watts to the array figure is on how to buy solar panels.
Compiled with AI assistance from the sources below. Spot an error? Email [email protected].
Sources
- SolarSizer calculator, assets/app.js —
panelsForBatteryis (amp-hours × volts) ÷ sun hours ÷ 0.85. At 4.5 sun hours that is 1,200 ÷ 4.5 ÷ 0.85 ≈ 314 W (printed ~315 W), 2,400 ÷ 4.5 ÷ 0.85 ≈ 627 W (printed ~630 W), 4,800 ÷ 4.5 ÷ 0.85 ≈ 1,255 W (printed ~1,260 W), and 19,200 ÷ 4.5 ÷ 0.85 ≈ 5,020 W (printed ~5,000 W).sizeControlleris array watts ÷ bank volts ÷ 0.85, rounded up to 10, 20, 30, 40, 60, 80, or 100 A. Accessed Oct 3, 2026.
The watt column is panelsForBattery at 4.5 sun hours. The BB10012i charge-current limit is quoted on the LiFePO4 page linked above, not repeated here as a rule for every battery.