How many solar panels to charge a 100Ah battery?
To put back the usable 1,024 Wh of a 12.8 V 100Ah LiFePO4 battery in one day at 4.5 peak sun hours, you need about 288 W of panels — 3 panels of 100 W.
How to work it out
- Each watt of panel makes 4.5 sun hours × (1 − 14% losses) × 92% battery round trip = 3.56 Wh a day
- 1,024 Wh ÷ 3.56 = 288 W of panels
- 288 W ÷ 100 W = 3 panels (300 W)
“Charging a 100Ah battery” really means replacing what you took out of it. This page sizes the panels to put back the battery’s full usable 80% in a single day — the right target if you use the whole battery every day.
Adjust for your project
More options: Losses
- Panels
- 3 panels · 100 W
- Installed watts with that many panels
- 300 W
- Expected output per day
- 1,068 Wh
- Expected output per month (30 days)
- 32 kWh
Array size at other peak sun hours
| Peak sun hours per day | Solar array needed | Panels |
|---|---|---|
| 2 | 647 W | 7 panels |
| 3 | 431 W | 5 panels |
| 4 | 324 W | 4 panels |
| 5 | 259 W | 3 panels |
| 6 | 216 W | 3 panels |
What to know about solar for one 100 Ah battery
- One panel is a maintainer, three are a recharger. A single 100 W panel needs about three sunny days to refill the same battery — see how long a 100W solar panel takes to charge a 100Ah battery.
- Lead-acid needs about two-thirds the panels. You only use about 600 Wh of a 12 V 100Ah lead-acid battery, so about 194 W refills it (against about 288 W for lithium) — but it also needs a full charge regularly to stay healthy.
- Two batteries, double the panels. 200 Ah used fully every day needs about 580 W.
- Your sun hours change the answer. At the Seattle December figure of about 1.6 peak sun hours, the same job takes about 810 W; in Phoenix’s yearly average of 6.6, about 200 W.
The controller for 300 W on 12 V is about 25–30 A — see what size charge controller for a 400 watt solar panel for the method. The battery’s energy is on how many watt-hours in a 100Ah battery.
Different size? Use the full Solar Panel Calculator.
What you’ll need
- 100 W solar panelMid-range price
The building block of most van, boat and RV systems. Check the data-sheet Voc and Isc before you decide how to wire several of them.
See options on Amazon (opens in a new tab) - MPPT solar charge controllerMid-range price
Converts a higher panel voltage into charging current, so you can wire panels in series and lose less in the cables. Size it on output amps and maximum PV voltage.
See options on Amazon (opens in a new tab) - 12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many solar panels to charge a 100Ah lead-acid battery?
To put back 600 Wh (half of a 12 V 100Ah) in a day: about 194 W — 2 panels of 100 W.
How many solar panels to charge two 100Ah batteries?
About 575 W — 6 panels of 100 W, at 4.5 peak sun hours.
How many solar panels to charge a 24V 100Ah battery?
The same as two 12 V batteries: about 575 W (25.6 V × 100 Ah × 80% = 2,048 Wh).
Other common sizes
- How many solar panels for a small cabin — 3 × 400 W panels (948 W) for 3 kWh a day
Sources
- National Laboratory of the Rockies (formerly NREL) — PVWatts® Calculator (opens in a new tab) — Look up the solar resource (peak sun hours) and yearly output for your own address; default system losses 14%.
- NLR (formerly NREL) — PVWatts® v8 API, NSRDB PSM V3 TMY data (opens in a new tab) — City figures on this site: fixed open-rack array facing due south, tilted at the site’s latitude, 14% system losses, queried October 2026.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.