LiFePO4 Battery Calculator: How Many Ah Do I Need?
Add your devices with their watts and daily hours. The calculator totals your daily watt-hours, adds a margin and divides by the battery voltage to give the Ah you need. As a rule of thumb, Ah = daily Wh × 1.2 ÷ 12.8 for a 12 V LiFePO4 battery.
This calculator tells you how many amp-hours of LiFePO4 battery your devices need. Add each device with its watts and hours of use per day. It totals your daily energy, adds a margin and converts it to Ah at your battery voltage.
Capacity needed: 0 Ah · 0 Wh
That is 0 100 Ah batteries or 0 200 Ah batteries (at the chosen voltage)
Formula: Ah = Wh × days ÷ (V × usable depth). Add 10-15% if you run an inverter.
How does the calculation work?
Three steps: daily Wh, a margin, then divide by voltage.
- Daily energy: Σ(watts × hours) for every device, in Wh. For devices on an inverter, divide their watts by 0.85 first.
- Margin: multiply by 1.2 or more. Use a higher factor if you want to ride out cloudy days without charging.
- Ah needed: divide by the nominal voltage: 12.8 V for a 12 V LiFePO4 battery, 25.6 V for 24 V, 51.2 V for 48 V.
Example: a van runs a 45 W fridge for 24 hours (1,080 Wh), a 60 W laptop via inverter for 3 hours (212 Wh) and 20 W of lights for 5 hours (100 Wh). The total is 1,392 Wh. With a 1.2 margin, that is 1,670 Wh, or about 130 Ah at 12.8 V.
How do I read the result?
Treat the Ah as a minimum, then check current and cold. Two more checks matter as much as capacity:
- BMS current: your largest load divided by 12.8 V must stay under the battery’s BMS amps. A 100 A BMS supports about 1,280 W.
- Cold charging: if you charge below 0 °C (32 °F), you need a battery with self-heating.
| Your result at 12 V | What it usually means | Where to look |
|---|---|---|
| Up to about 100 Ah | Weekend camper, trolling motor, small solar setup | 12V 100Ah batteries |
| 100-200 Ah | Van with a fridge; two 100Ah in parallel or one big battery | Best batteries for RVs |
| 200-320 Ah | Full-time RV or small cabin | Best LiFePO4 batteries |
| Over 320 Ah | Consider 24 V or 48 V and a bank | Off-grid system guide |
Which mistakes skew the result?
Using label maximums, forgetting the inverter and ignoring days without charging.
- Label maximums: fridges cycle on and off, so their average draw is far below the label.
- Inverter losses and idle draw: an inverter left on uses power even with little load.
- No autonomy: if you want to last two days without sun, double the daily figure before the margin.
For the full buying checklist, read how to choose a LiFePO4 battery. If you would rather have battery, inverter and charger in one box, use our solar calculator to size a portable power station instead.
Frequently asked questions
How do I calculate battery Ah from watts?
Multiply each device’s watts by its hours of use, add them up and divide by the battery voltage. For a 12 V LiFePO4 battery use 12.8 V: 1,000 Wh ÷ 12.8 ≈ 78 Ah, before any margin.
Why add a margin to the result?
To cover cloudy days, cold weather and battery aging, and to avoid running the battery completely flat. A 20 percent margin is a sensible minimum.
Should I include inverter losses?
Yes, for anything that runs on 120 V AC through an inverter. Divide those devices’ watts by about 0.85 before entering them.
What if the result is between two sizes?
Round up. A larger battery works less hard, lasts longer and leaves room for new devices.