How Many Solar Panels to Charge a 100Ah Battery
Every week someone emails us this exact question, usually from a camper van or a village house that just got its first solar panel.
“I have a 100Ah battery. How many panels do I need?”
The honest answer is: fewer than you think. And you can work it out yourself in about two minutes.
What your battery actually holds
A 12V LiFePO4 battery rated at 100Ah stores 12.8 × 100 = 1,280 watt-hours. Round it to 1.3 kWh.
That’s the target: you need to push 1.28 kWh back in to fill it from empty.
Quick note for anyone coming from lead-acid: a “100Ah” lead-acid battery only safely delivers about half that, because you shouldn’t drain it past 50%. This is one of those quiet reasons people switch to LiFePO4.
Peak sun hours, and why your panel never hits its rating
Your 100W panel will almost never make 100W. Panels only hit rated output in strong, direct sunlight, so the useful unit is “peak sun hours”: how many hours a day the sun is strong enough.
Most of Africa, Southeast Asia, the Middle East and Latin America gets 4 to 5 peak sun hours on average. Deserts push 6. Rainy seasons drop to 3. Plan on 4.5 and you’re safe.
The math, once
Panel output per day:
panel watts × 4.5 hours × 0.8
The 0.8 is the loss pile: controller, wiring, battery charging. Nothing is 100%.
A 200W panel: 200 × 4.5 × 0.8 = 720Wh per day. Your battery needs 1,280Wh, so 1,280 ÷ 720 = about 1.8 days of sun to go from empty to full.
Here’s the practical table:
| Panel | Daily energy | 100Ah from empty |
|---|---|---|
| 100W | 360 Wh | ~3.5 days |
| 200W | 720 Wh | ~2 days |
| 300W | 1,080 Wh | ~1.5 days |
| 400W (2×200W) | 1,440 Wh | ~1 day |
Real life is friendlier than the table: nobody drains a battery to zero and waits. If you use half the battery overnight, a 200W panel has you back to full by late afternoon.
MPPT vs PWM, in one paragraph
Your charge controller matters more than people think. A PWM controller is cheap but throws away 20-30% of your panel’s energy. An MPPT controller squeezes most of it out. The 0.8 above assumes MPPT; with PWM use 0.6. For a permanent install, pay for MPPT. It earns its keep within months.
Sizing for your actual life
- Weekend trips, occasional outages: 100-200W with MPPT is plenty.
- Daily use, off-grid: 200-300W, so one good day refills the battery.
- Cloudy season where you are: size up one step now. Adding a panel later is annoying; buying it now is cheap.
One more thing worth knowing: lithium batteries charge more efficiently than lead-acid and accept higher current, which makes them the better partner for solar in the first place.
In short
200W of panels with an MPPT controller recharges a 100Ah battery in about two days of normal sun. 300W gets you there in a day. That’s the whole answer.
LINEX 12V 100Ah LiFePO4 batteries are built for solar charging: 6,000+ cycle rating, full depth of discharge, BMS included.