How the calculation works
A battery bank is sized from three numbers: how much energy you draw per day, how many sunless days it must bridge, and how much of the battery's rated capacity you can actually use. That last factor, depth of discharge, is where most sizing goes wrong, and it depends entirely on chemistry.
Lead-acid batteries (flooded and AGM) age rapidly if drained past half; treating a 100 Ah lead-acid battery as 50 Ah of real storage is the honest arithmetic. LiFePO4 lithium comfortably delivers 80% or more of its rating for thousands of cycles, which is why a lithium bank can be roughly half the nameplate size of a lead-acid bank doing the same job.
Worked example
The same 2,000 Wh/day cabin from our solar panel calculator, with 2 days of autonomy on a 12 V LiFePO4 bank:
Practical build: five 100 Ah LiFePO4 batteries in parallel (500 Ah installed) gives margin, or four (400 Ah) if a generator can cover rare long stretches. The same requirement in AGM would demand roughly 667 Ah, nearly seven 100 Ah units. That gap is the cost story behind lithium's takeover of off-grid storage.
Why days of autonomy matters more than any other input
Autonomy is the multiplier that scales the whole bank. One day of autonomy assumes the sun returns tomorrow or a generator picks up the slack; three days assumes neither. Every added day adds a full daily load to the bank, so be honest about your weather and your backup. Oversizing autonomy "to be safe" is the most expensive form of caution in an off-grid build, panels are cheap, storage is not. A common balanced approach: 2 days of battery autonomy plus a modestly oversized solar array.
Common mistakes
- Using nameplate amp-hours as usable amp-hours. A "200 Ah" lead-acid bank is a 100 Ah bank in service. Chemistry sets the usable fraction; the calculator applies it for you.
- Mixing old and new batteries, or different capacities, in one bank. The bank performs at the level of its weakest unit and ages unevenly. Build banks from identical batteries of the same age.
- Sizing the bank without sizing the charger. A large bank fed by a small array never fully charges, and chronic undercharge kills lead-acid quickly. Check the pairing with our solar panel calculator.
- Forgetting temperature. Lead-acid loses meaningful capacity below freezing, and LiFePO4 must not be charged below 0 °C without low-temperature protection. Cold-climate builds need insulated or heated battery boxes.
Frequently asked questions
Is LiFePO4 worth the higher upfront price?
In most builds, yes. Per usable amp-hour and per charge cycle, lithium now works out cheaper than lead-acid: you buy roughly half the nameplate capacity and it lasts several times as many cycles. Lead-acid still makes sense for tight budgets, rarely used systems, or as a known quantity in very cold installations.
Should I choose 12 V, 24 V or 48 V?
Higher voltage means lower current for the same power, which means thinner cables and smaller losses. Rough guide: up to ~1,500 W of load, 12 V is fine; 1,500–3,000 W favors 24 V; above 3,000 W or whole-home systems, 48 V is standard. Changing voltage later means replacing the inverter and charge controller, so decide early.
Can I add more batteries to the bank later?
With lead-acid, avoid it, new cells get dragged down by old ones. With LiFePO4 it is more forgiving, but best practice is still to expand within a year or two with the same model. If you expect to grow, buy the bank slightly oversized now or plan a second independent bank.