Batteries that meet this spec
Affiliate links — we may earn a commission at no cost to you.
Assumptions used
Nominal Wh = daily Wh × days ÷ depth of discharge ÷ round-trip efficiency ÷ inverter efficiency.
LiFePO4 85% DoD, 96% round trip. AGM and flooded 50% DoD, 85% and 82% round trip.
Inverter taken as 90% efficient, applied only to the AC share of the load.
How this calculator sizes a battery bank
The figure you need is not your daily consumption. It is your daily consumption divided by everything that stands between the cells and the appliance — depth of discharge, round-trip efficiency, and inverter conversion loss. Skip those and you will buy a bank roughly 40% smaller than the one you needed.
The formula
Nominal capacity (Wh) = daily consumption × days of autonomy ÷ depth of discharge ÷ round-trip efficiency ÷ inverter efficiency. Divide by system voltage for amp-hours.
A worked example
Take a van drawing 1,450 Wh per day — lights, a compressor fridge, a laptop, a roof fan and phone charging. Two days of autonomy, LiFePO4, 12 V, with about 70% of the load passing through an inverter.
| Step | Working | Result |
|---|---|---|
| Daily consumption | Sum of watts × hours | 1,450 Wh |
| Over two days | 1,450 × 2 | 2,900 Wh |
| Depth of discharge | ÷ 0.85 for LiFePO4 | 3,412 Wh |
| Round-trip efficiency | ÷ 0.96 | 3,554 Wh |
| Inverter loss on the AC share | ÷ 0.93 | 3,821 Wh |
| At 12 V | 3,821 ÷ 12 | 318 Ah |
So a 300 Ah or 400 Ah bank, not the 200 Ah that a naive 1,450 × 2 ÷ 12 = 242 Ah calculation suggests. The same loads on AGM, which you should not take below 50%, need roughly 570 Ah — which is where the weight and the floor loading become the real constraint rather than the price.
Where the numbers come from
- Depth of discharge. LiFePO4 is taken at 85% usable. Lead acid, both AGM and flooded, at 50% — going deeper repeatedly costs a large share of rated cycle life.
- Round-trip efficiency. Energy lost putting charge in and taking it out again. 96% for LiFePO4, 85% for AGM, 82% for flooded.
- Inverter efficiency. Around 90%, applied only to the portion of your load that runs as AC. DC loads bypass it entirely, which is why a DC fridge beats a mains one on an off-grid system.
Common questions
How many batteries do I need for off-grid living?
It depends entirely on daily consumption, not on the size of your house or van. A small van drawing 1,450 Wh a day needs roughly 300–400 Ah at 12 V in LiFePO4 for two days of autonomy. A cabin at 4,200 Wh a day needs around 230 Ah at 48 V, which is about 11 kWh nominal. Work out your watt-hours first; every other figure follows from it.
How do I convert watt-hours to amp-hours?
Divide watt-hours by system voltage. 3,821 Wh at 12 V is 318 Ah; the same energy at 48 V is only 80 Ah. This is why amp-hour figures are meaningless without stating the voltage, and why comparing a 100 Ah 12 V battery with a 100 Ah 48 V battery is comparing 1.2 kWh with 4.8 kWh.
Is LiFePO4 worth it compared to AGM?
Usually, once you account for usable capacity rather than rated capacity. A 100 Ah LiFePO4 gives you about 85 Ah of use; a 100 Ah AGM gives about 50 Ah. Add several times the cycle life and roughly a third of the weight, and the price gap closes faster than the sticker suggests. AGM still wins where the bank is rarely cycled, where it will sit below freezing while charging, or where budget is fixed and hard.
How many days of autonomy should I plan for?
Two days suits most builds. One day works in a consistently sunny climate or where you have a generator. Three or more is for cloudy regions with no backup charging. Beyond two days, adding another solar panel is nearly always cheaper than adding battery.
Does cold weather affect battery capacity?
Yes, significantly. Lead acid loses a substantial share of usable capacity near freezing. LiFePO4 holds capacity better in the cold but must not be charged below 0°C without a heating circuit or a BMS that blocks it — charging frozen lithium cells causes permanent plating damage. If your build will be cold, buy cells with integrated heating.
These figures are planning estimates. Electrical installation carries real risk of fire and injury — have your design checked against the regulations that apply where you are, by someone qualified.