To fully recharge an empty 12V 100Ah lithium (LiFePO4) battery in one sunny day, plan on roughly 350 to 400 watts of solar panels. A single 100W panel will get there too, but it takes three to four days of good sun. That’s the conclusion from simple energy math: a 12.8V 100Ah LiFePO4 battery stores about 1,280 watt-hours, and LiTime and Redodo both list a 20A everyday charge current for their 100Ah batteries, which a 300–400W array roughly matches. Below are the formula, tables for one-day and multi-day charging in different sun conditions, charge controller sizing and how lead-acid changes the answer. I’ve also included my own 100W panel and MPPT setup for reference.
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The battery I reference for specs is the LiTime 12V 100Ah LiFePO4. All charge times below are estimates from the formulas shown, not measurements. Here’s my off-grid test of a 100W panel charging a 12V lithium battery through an MPPT controller:
The short answer
| Goal | Panel size (LiFePO4, 5 peak sun hours) |
|---|---|
| Full charge from empty in one day | About 370W (use 400W) |
| Full charge from 20% in one day | About 290W (use 300W) |
| Top up half the battery each day | About 180W (use 200W) |
| Slow charge or maintenance only | 100W |
These are estimates using the formula below. Cloudy days, winter sun and shading push the numbers up.
The formula
Three numbers decide it: how much energy the battery needs, how many hours of strong sun you get, and how much of a panel’s rating actually reaches the battery.
- Battery energy: amp-hours × nominal volts. For LiFePO4 that’s 100Ah × 12.8V = 1,280Wh. Multiply by the share you need to refill; from 20% state of charge, that’s 1,280 × 0.8 = 1,024Wh.
- Peak sun hours (PSH): the equivalent hours of full-strength sunlight per day. It varies by location and season; 4–6 is common in sunny US summers, with less in winter.
- Real-world factor: I assume a panel delivers about 70% of its rating to the battery once heat, sun angle, wiring and controller losses are counted.
Panel watts ≈ Wh needed ÷ (PSH × 0.7)
Example: 1,280Wh ÷ (5 × 0.7) = 1,280 ÷ 3.5 ≈ 366W. Rounded up to a standard size, that’s 400W.
One-day charging: panel size by sun hours
| Peak sun hours | Empty to full (1,280Wh) | 20% to full (1,024Wh) | 50% to full (640Wh) |
|---|---|---|---|
| 3 (winter, northern US) | About 610W | About 490W | About 305W |
| 4 | About 460W | About 370W | About 230W |
| 5 | About 370W | About 290W | About 185W |
| 6 (sunny summer Southwest) | About 305W | About 245W | About 150W |
Already have panels? Days to a full charge
Here’s the same formula run the other way, for an empty 1,280Wh LiFePO4 battery: days ≈ 1,280 ÷ (panel W × 0.7 × PSH).
| Panel array | 3 PSH | 4 PSH | 5 PSH | 6 PSH |
|---|---|---|---|---|
| 100W | About 6.1 days | About 4.6 days | About 3.7 days | About 3 days |
| 200W | About 3 days | About 2.3 days | About 1.8 days | About 1.5 days |
| 300W | About 2 days | About 1.5 days | About 1.2 days | About 1 day |
| 400W | About 1.5 days | About 1.1 days | About 0.9 day | About 0.8 day |
These assume nothing is drawing from the battery while it charges. If a fridge or lights run during the day, add their daily watt-hours to the energy you need.
Worked example: a weekend van setup
Batteries rarely start completely empty, and loads keep running while the sun is up. So it’s often better to size panels from what you use each day. Here’s an example with typical figures. These are estimates; measure your own loads where you can.
| Load | Average watts × hours | Daily Wh |
|---|---|---|
| 12V compressor fridge (about 16W average) | 16W × 24 h | About 384Wh |
| LED lights | 20W × 4 h | 80Wh |
| Laptop charging | 60W × 3 h | 180Wh |
| Starlink Mini (I measured about 22W on my setup) | 22W × 4 h | 88Wh |
| Total | About 732Wh |
Panel size ≈ 732 ÷ (5 × 0.7) ≈ 209W, so a 300W array gives a comfortable cushion for hazy days. The 100Ah LiFePO4 battery’s 1,280Wh then covers about 1.7 days of these loads with no sun at all (1,280 ÷ 732). Halve the loads and a 200W array is enough; add a second laptop or a CPAP and 400W makes more sense.
Match the panels to the battery’s charge current
Charge current = panel watts reaching the battery ÷ charge voltage. At a 14.4V LiFePO4 charge voltage:
| Array | Max current at 14.4V (rated watts) | Typical real current (about 70%) | Suggested MPPT rating |
|---|---|---|---|
| 100W | About 6.9A | About 5A | 10A |
| 200W | About 13.9A | About 10A | 20A |
| 300W | About 20.8A | About 15A | 30A |
| 400W | About 27.8A | About 20A | 30–40A |
LiTime and Redodo both list 20A as the recommended charge current for their 12V 100Ah batteries. A 400W array delivering about 20A in real sun lines up with that figure; much more solar than this mostly adds speed on cloudy days. For deeper controller sizing, including voltage limits, see my guide to what size MPPT you need.
MPPT or PWM?
With a PWM controller, the panel is pulled down to the battery’s voltage, so any extra panel voltage is wasted. That works acceptably with “12V” panels, which typically run around 18V. An MPPT controller converts the extra voltage into extra current, which lets you wire panels in series and usually harvests more energy, especially in cold weather. My MPPT vs PWM explainer covers the difference in detail. For a 300–400W array on a 100Ah battery, I’d choose MPPT.
Check panel voltage against the controller
My HQST 100W panel has an open-circuit voltage of about 20.4V. Two in series make about 41V, and four in series about 82V. Panel voltage rises in cold weather, so keep the series string’s cold open-circuit voltage under the controller’s PV input limit with margin.
What about a lead-acid 100Ah battery?
A 12V 100Ah AGM or flooded battery is normally used to about 50% depth of discharge to protect its life. That makes the usable energy roughly 12V × 100Ah × 0.5 = 600Wh, so it needs smaller panels:
| Battery | Energy to refill daily | Panel size at 5 PSH (estimate) |
|---|---|---|
| LiFePO4 12.8V 100Ah, empty to full | 1,280Wh | About 370W |
| LiFePO4 12.8V 100Ah, 20% to full | 1,024Wh | About 290W |
| Lead-acid 12V 100Ah, 50% to full | About 600Wh | About 170W, plus extra time to finish absorption |
Lead-acid also slows down near full as it enters absorption, so the last stretch takes longer than the simple math suggests. LiFePO4 accepts a steady current almost all the way to full, which is why the formula works better for it. My 100Ah battery runtime guide compares how far each type goes once charged.
Setup steps for a 100Ah solar system
- Pick a target: one-day recharge (300–400W) or slow top-ups (100–200W).
- Pick an MPPT controller rated for the array’s current and its cold open-circuit voltage.
- Connect the battery to the controller first. That’s the order I followed in my SunSwift video, so the controller detects system voltage before the panels are connected.
- Set the battery profile: LiFePO4, at a charge voltage inside the battery maker’s range. LiTime specifies 14.4V ± 0.2V.
- Connect the panels, using a fuse or breaker on the battery side as the controller manual specifies.
- Watch the controller display at midday to see your real-world watts and amps, then compare them with the tables above.
My SunSwift 60A MPPT review walks through that wiring order with a 100W panel and a 12V lithium battery.
FAQ
Will a 100W solar panel charge a 100Ah battery?
Yes, but slowly. In 5 peak sun hours, a 100W panel delivers about 350Wh by my estimate, so an empty 1,280Wh LiFePO4 battery takes about 3.7 days.
How many solar panels do I need to charge a 100Ah battery in a day?
About 370W at 5 peak sun hours for a LiFePO4 battery from empty. Four 100W panels or two 200W panels cover it.
Is 200W enough for a 100Ah lithium battery?
For daily top-ups of about half the battery, yes. A full charge from empty takes about two sunny days.
What size charge controller do I need for 400W on 12V?
About 30A minimum, since 400W ÷ 14.4V ≈ 28A at full rating. A 40A MPPT gives extra headroom.
Can I connect a solar panel directly to the battery?
Not without a charge controller. The controller regulates voltage and current so the battery isn’t overcharged.
Does winter change the panel size?
Yes. At 3 peak sun hours, a one-day full recharge needs about 610W, compared with about 370W at 5 hours. LiFePO4 batteries also shouldn’t be charged below freezing; the LiTime and Redodo standard models list a 32°F minimum charge temperature.
How long does a 300W panel take to charge a 100Ah battery?
About 1.2 days at 5 peak sun hours from empty, or within one day if you start from about 20%.
Bottom line
For a 12V 100Ah LiFePO4 battery, 300–400W of solar with a 30–40A MPPT controller gives a one-day recharge in good sun. It also lines up with the 20A charge current LiTime and Redodo recommend. Go smaller if you only need daily top-ups, and larger if you camp in winter or run loads during the day.