How long does a 100W solar panel take to charge a 100Ah battery?
Refilling 80% of a 12.8 V 100Ah LiFePO4 battery takes about 2.9 days of sun from a single 100W panel at 4.5 peak sun hours a day — 12.9 hours of full sun in total, with nothing else running.
How to work it out
- 100 Ah × 12.8 V × 80% = 1,024 Wh ÷ 92% charging efficiency = 1,113 Wh
- 100 W × (1 − 14% losses) = 86 W per peak sun hour × 4.5 h = 387 Wh a day
- 1,113 Wh ÷ 387 Wh = 2.9 days (12.9 hours of full sun)
A single 100 W panel is a trickle by battery standards: at most about 7.8 A into a 12.8 V battery in perfect midday sun, and an average of a few amps across the day. It keeps a battery topped up nicely; refilling one from flat takes days.
Adjust for your project
More options: Refill · Charge controller · Losses
- Peak sun hours needed
- 12.9 hours
- Energy to put back (with charging losses)
- 1,113 Wh
- Panel output per day
- 387 Wh
- Peak charging current (MPPT, full sun)
- 7.8 A
- Panel watts to refill it in one day
- 288 W
Recharge time with other panel sizes
| Solar panel watts (total) | Sunny days to recharge | Peak sun hours needed | Peak charging current (MPPT, full sun) |
|---|---|---|---|
| 50 | 5.8 days | 25.9 hours | 3.9 A |
| 100 | 2.9 days | 12.9 hours | 7.8 A |
| 200 | 1.4 days | 6.5 hours | 15.6 A |
| 300 | 1 day | 4.3 hours | 23.4 A |
| 400 | 0.7 days | 3.2 hours | 31.3 A |
| 600 | 0.5 days | 2.2 hours | 46.9 A |
What to know about charging 100 Ah from one panel
- Lead-acid needs less energy but more time. Refilling half a 12 V 100Ah lead-acid battery is about 706 Wh with charging losses — about 1.8 days — but the slow absorption stage at the end adds hours that no panel speeds up.
- Any daytime use comes off the top. A 12 V fridge averaging 25 W uses 600 Wh a day — more than the panel makes on a 4.5-hour day. The panel would never get ahead.
- Winter can triple the time. In Chicago, PVWatts gives a lowest-month average of about 2.7 peak sun hours on a tilted panel, against 4.5 here.
- Two panels, half the time. 200 W refills the same battery in about 1.4 days; 300 W in about a day.
For panel output in your city, see how much power a 100W solar panel produces per day. To size panels to keep up with a daily load instead, use how many solar panels to charge a 100Ah battery.
Different size? Use the full Solar Battery Charge Time 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) - Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
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 long does a 100W solar panel take to charge a 100Ah lead-acid battery from 50%?
About 1.8 days at 4.5 peak sun hours, before the slow absorption stage at the end.
How long does a 200W solar panel take to charge a 100Ah battery?
About 1.4 days (6.5 hours of full sun) for 80% of a 12.8 V LiFePO4.
How long does a 100W panel take to charge a 100Ah battery in 6 peak sun hours?
About 2.2 days.
Other common sizes
- How long to charge a 200Ah battery with 200W of solar — 2.9 sunny days — 1 day with about 575 W
Sources
- 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.
- Victron Energy — Which solar charge controller: PWM or MPPT? (2020) (opens in a new tab) — MPPT output current = panel watts ÷ battery voltage (100 W ÷ 13 V = 7.7 A); a PWM controller pulls the panel down to battery voltage (81 W from a 100 W panel at 25 °C, about the same as MPPT at 75 °C); temperature-coefficient method; example panel Voc 21.6 V, −0.35 %/°C.
- 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.