The formula
Panel watts = battery watt-hours ÷ sun hours ÷ 0.77
- Battery watt-hours: a 12V 100Ah lithium (LiFePO4) battery holds about 12.8 V × 100 Ah = 1,280 Wh, and you can use nearly all of it. A lead-acid one holds about the same but you should only use about half.
- Sun hours: the peak sun hours where you are. Most of the US gets 4–6 on an average day, but only about 2–5 in the darkest month on a tilted panel. Look yours up by ZIP.
- 0.77: about 23% is lost to heat, dust, wiring and the charge controller. The site's guides use the same number.
Worked numbers (12V 100Ah lithium)
| You want to refill… | Summer (5 sun hours) | Winter (3 sun hours) | Dark winter (2 sun hours) |
|---|---|---|---|
| Half the battery (640 Wh) in a day | about 170 W | about 280 W | about 420 W |
| All of it (1,280 Wh) in a day | about 330 W | about 550 W | about 830 W |
So: 200 W covers a battery you use half of each day in summer. Plan on about 400 W for winter, or to refill from empty, and more in a dark-winter state.
On a van or RV roof
Panels lying flat on a roof catch about 40% less winter sun than tilted ones. Half a battery a day in a 3-sun-hour winter then takes about 460 W instead of 280 W. That's why the van guide sizes for the roof you have, and the van and RV panel page shows what fits.
Match the charge controller
For a 12V battery, controller amps ≈ panel watts ÷ 14:
- 200 W → about 15 A: a 20 A MPPT
- 400 W → about 30 A: a 30 A MPPT
- 700 W → about 50 A: the most a Battle Born 100Ah should take (its datasheet says 50 A max), so this is also the point to add a second battery
Why MPPT: MPPT vs PWM. If it isn't charging at all: 7 things to check.
Size from what you use, not from the battery
The battery is the tank; the panels refill what you drain. If you use 600 Wh a day, size the panels for 600 Wh, whatever the battery. How much electricity do you use? shows how to find that number, and the guides do the whole sum for your place.
Questions people ask
Will a 100 watt solar panel charge a 100Ah battery?
Slowly. On a 5-sun-hour day, 100 W puts back about 385 Wh, roughly 30% of a 12V 100Ah lithium battery. It's fine for keeping a battery topped up, not for refilling one you use every day.
How long does a 200 watt panel take to charge a 100Ah battery?
About 8 sun hours from empty (1,280 Wh ÷ 200 W ÷ 0.77). That's one long summer day, or three or four winter days.
What size charge controller for a 100Ah battery?
Divide the panel watts by about 14 for a 12V battery: 200 W needs about 15 A (a 20 A controller), 400 W about 30 A (a 30 A controller). Use MPPT, not PWM, for anything beyond a small panel.
Is a lead-acid 100Ah battery different?
Yes. You should only use about half of a lead-acid battery, so a 100Ah lead-acid holds about 600 Wh you can use, and needs about half the panel of a lithium one to refill. That's also why it runs things half as long.
Can I put too much solar on one 100Ah battery?
The battery limits the charge, not the panel: a Battle Born 100Ah takes up to 50 A. At 12 volts that's roughly 700 W of panel at full sun, so past that, set the controller's charge limit to 50 A or add a second battery.
Sources
- Battle Born BB10012 datasheet (100Ah, 100% usable, 50 A max charge) Battle Born Batteries
- NASA POWER: solar resource data NASA
- SmartSolar MPPT manual, troubleshooting Victron Energy
Every number on this page comes from these sources. Spotted something out of date? Tell me.
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