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
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
Winter changes everything. In northern US winters, sun hours fall to 2–2.5, roughly half the annual average — meaning a 200 Ah 12 V bank needs double the panels, or you accept 2 days instead of 1. Cold actually improves panel output slightly, but short days and cloud dominate. Verify your local worst-month figure with NREL's PVWatts or Global Solar Atlas before buying.
Charge rate limits
Don't over-panel either: most LiFePO4 banks accept up to 0.5C charge (a 100 Ah battery up to 50 A charging current), while many lead-acid banks should stay near 0.15–0.2C. Match your charge controller to the array: watts ÷ battery volts ÷ 0.85, rounded up to the next standard rating — the calculator's charge controller tab does this for you.