Solar battery charge time calculator
Enter the battery, how far down it is, and the panel. The calculator counts the energy needed (with charging losses) against what the panel makes on a day of your peak sun hours.
Enter your measurements
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
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)
Quick answers for common sizes
| Example | Answer |
|---|---|
| How long a 100W solar panel takes to charge a 100Ah battery | 2.9 sunny days (12.9 hours of full sun) |
| How long to charge a 200Ah battery with 200W of solar | 2.9 sunny days — 1 day with about 575 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 |
Worked examples: how long a 100W solar panel takes to charge a 100Ah battery and how long to charge a 200Ah battery with 200W of solar.
How charge time is worked out
energy to put back = Ah × volts × share to refill ÷ charging efficiency panel energy per day = panel watts × (1 − system losses) × peak sun hours days = energy to put back ÷ panel energy per day
A peak sun hour is an hour of sunlight at 1,000 W per square metre — the condition panels are rated at. A day with 4.5 peak sun hours delivers the same energy as 4.5 hours of that full sun, even though it is spread from dawn to dusk. Your number depends on place and season: NREL’s PVWatts gives about 6.6 for Phoenix and 3.8 for Seattle as a yearly average on a tilted panel, with Seattle’s December closer to 1.6. See peak sun hours by city.
System losses cover dirt, heat, wiring and conversion; 14% is PVWatts’ default. Charging efficiency covers what the battery itself loses: Victron suggests starting at 85% for lead-acid, and lists 92% round-trip efficiency for its LiFePO4 batteries.
Peak current: what the controller sees
An MPPT controller turns panel watts into battery amps: current = watts ÷ battery volts. Victron’s worked example is a 100 W panel giving 7.7 A into a 13 V battery. That peak only happens in full midday sun on a cool panel; the daily total is what refills the battery.
Why it takes longer in real life
- The battery is also powering things. Anything running during the day comes out of the panel’s output first. Subtract your daytime load, or size with the solar panel calculator.
- Winter. Peak sun hours in a northern winter can be a third of summer’s. Size year-round systems on the worst month.
- Shade. A shadow across part of a panel can cut its output by far more than the shaded area.
- Lead-acid absorption. The last stretch of a lead-acid charge runs at reducing current, which no panel size fixes.
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
Can a 100W solar panel charge a 100Ah battery?
Yes, slowly. Refilling 80% of a 12.8 V 100 Ah LiFePO4 battery takes about 1,113 Wh with charging losses, and a 100 W panel makes about 387 Wh on a 4.5-sun-hour day after 14% losses — about 2.9 sunny days, with nothing else running. See how long a 100W solar panel takes to charge a 100Ah battery.
How many hours of sun to charge a 12V battery?
Divide the watt-hours to put back by the panel’s watts after losses. A 100 W panel (86 W after 14% losses) needs about 13 peak sun hours to put back 1,113 Wh.
Does a bigger panel charge faster?
Yes, proportionally, until the charge controller or the battery’s maximum charge current limits it. Two 100 W panels halve the time; check the controller’s amp rating.
Should I fully charge a LiFePO4 battery every day?
Follow the battery manufacturer’s charge settings rather than a generic lead-acid profile. For its 12.8 V LiFePO4 batteries Victron gives a charge voltage of 14.2 V (14.0–14.4 V allowed) and a float of 13.5 V.
Related guides
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.