You’ve got (or are about to buy) 400 watts of solar panels. How many batteries do you need to store that power?
Short answer: one to two 12V 100Ah LiFePO4 batteries, or two to four 12V 100Ah AGM batteries. The right number depends on how much power you use at night and how many cloudy days you want to ride through. Here’s the math in three steps.
Step 1: How much does 400W of solar make per day?
Daily energy ≈ panel watts × peak sun hours × 0.75
The 0.75 covers real-world losses: heat, dust, wiring, controller and panel angle.
| Peak sun hours | Daily energy from 400W |
|---|---|
| 3 (cloudy/northern winter) | ~900 Wh |
| 4 | ~1,200 Wh |
| 5 (typical summer) | ~1,500 Wh |
| 6 (sunny southwest) | ~1,800 Wh |
Eco-Worthy, for example, quotes an average of about 1.6 kWh per day for its 400W kit, which matches the upper part of this range.
Step 2: How much can one battery store?
| Battery (12V 100Ah) | Rated energy | Usable energy |
|---|---|---|
| LiFePO4 | 1,280 Wh | ~1,150–1,280 Wh |
| AGM / lead-acid | ~1,200 Wh | ~600 Wh (to 50%) |
LiFePO4 can be used almost completely. AGM should only be drained to about half to keep a decent lifespan, which is why you need roughly twice as many AGM batteries for the same usable energy.
Step 3: Pick your goal
Goal A: store one sunny day (most cabins, RVs, sheds)
Match the battery to one day of production, around 1,500 Wh:
- LiFePO4: 1 × 12V 100Ah (stores most of it); 2 if you’re in a very sunny area
- AGM: 2–3 × 12V 100Ah
Goal B: one day of backup for cloudy weather
Size the battery for your daily use × 2. If you use 1,200 Wh per day:
- LiFePO4: 2 × 12V 100Ah (~2,400 Wh usable)
- AGM: 4 × 12V 100Ah
Going beyond two days of storage on 400W rarely makes sense: the panels can’t refill a much bigger bank in one day anyway. If you need more autonomy, add panels first.
Quick answer table
| Daily use | Lithium (12V 100Ah) | AGM (12V 100Ah) |
|---|---|---|
| Up to 800 Wh | 1 | 2 |
| 800–1,200 Wh | 1–2 | 2–4 |
| 1,200–1,600 Wh | 2 | 4 |
| More than 1,600 Wh | Add panels first | Add panels first |
Not sure what your daily use is? Our cabin solar calculator works it out from your appliances.
Check your charge controller
400W into a 12V battery is about 25–30A of charging current at peak. That’s why most 400W kits ship with a 40A MPPT controller. A battery bank can be bigger than the controller “needs”, but the controller limits how many panels you can add. Renogy, for example, rates its Rover 40A at about 520W of panels on a 12V battery and about double on 24V.
Wiring several batteries
- Parallel (positive to positive, negative to negative) keeps 12V and adds capacity. This is the usual choice at 400W.
- Series doubles voltage (two 12V batteries = 24V). Your controller and inverter must support 24V.
- Use identical batteries, equal-length cables, and a fuse or breaker between the battery bank and everything else.
Batteries for a 400W system. 12V 100Ah LiFePO4 batteries are the simplest match for a 400W kit. Many kits are also sold as bundles with one or two batteries.
See 12V 100Ah LiFePO4 at LiTime See 400W kits with batteries at Eco-Worthy
What off-gridders say about 400W in real life
Forum users who actually run 400W systems keep repeating a few lessons:
- Expect less than 400W. Owners commonly report something like 20–24 amps into a 12V battery on a good day, or roughly 280W of real output, especially with panels mounted flat on a roof. That’s why our formula uses the 0.75 factor. (iRV2, DIY Solar Forum)
- Winter roughly halves it. One owner in Colorado noted winter production is about half of summer, which makes battery capacity matter more when the days are short. (iRV2)
- Cloudy streaks are the real test. Experienced users warn that two batteries and 400W can leave you short when bad weather lasts more than a day or two, so the system works best for sunny-season use or with a backup charging source. (iRV2)
- Don’t oversize the bank either. A very large battery can’t be refilled by 400W in one day; forum members point out it could take three or four days to recharge it from empty. (DIY Solar Forum)
The bottom line
For a 400W solar system, one 12V 100Ah LiFePO4 battery stores a typical sunny day, and two give you a day of backup. With AGM, double those numbers. Size by your nightly and cloudy-day use, not by the panels alone.
Choosing a kit? Compare Eco-Worthy vs Renogy.
Sources
- iRV2: Is 400 watts of solar enough?
- DIY Solar Forum: How many batteries for 400 watts of solar?
- iRV2: Is 400 watts of solar enough? (continued)
Frequently asked questions
- Is one 100Ah battery enough for 400W of solar?
- For a LiFePO4 battery, one 12V 100Ah (about 1,280 Wh) is enough to store most of a typical sunny day from 400W of panels. If you want a day of backup for cloudy weather, use two. With AGM, plan for at least two to three 100Ah batteries.
- How long does it take 400W of solar to charge a 100Ah battery?
- In good sun, 400W of panels feeds a 12V battery with roughly 25–30A, so an empty 100Ah LiFePO4 battery charges in about 4–5 hours of strong sun. Real-world times are longer with clouds, low sun angle or shade.
- Should I use a 12V or 24V battery bank with 400W?
- 12V is simplest and fine at 400W. A 24V bank halves the current, lets the same 40A controller handle about twice as many panels and reduces cable size, which helps if you plan to grow.
- Can I mix old and new batteries?
- Avoid it. Batteries in the same bank should be the same chemistry, model, capacity and age. Mixing them makes the weaker battery limit the whole bank and can shorten its life.