"How many batteries do I need for off-grid?" depends on how much energy you use per day, how many days of autonomy you want, and your battery depth of discharge (DoD). This guide gives you the formula and the reasoning so you can size your bank correctly.
The Three Inputs
- Daily energy use (Wh). From your load list. Example: 2,000 Wh/day.
- Days of autonomy. How many cloudy days you want to cover without sun. Common: 1–3 for sunny areas, 3–5 for unreliable weather. See days of autonomy explained.
- Depth of discharge (DoD). How much of the battery you use before recharging. LiFePO4 often 80–90%; lead-acid 50%. See DoD for solar batteries.
The Formula
Usable capacity needed (Wh) = Daily use (Wh) × Days of autonomy
Battery capacity needed (Wh) = Usable capacity ÷ DoD
Example: 2,000 Wh/day, 2 days autonomy, 80% DoD (0.8):
- Usable = 2,000 × 2 = 4,000 Wh
- Battery capacity = 4,000 ÷ 0.8 = 5,000 Wh (5 kWh)
So you need 5 kWh of battery capacity (at the system voltage). If each battery is 12 V 100 Ah = 1,200 Wh, you’d need 5,000 ÷ 1,200 ≈ 5 batteries (round up).
Converting to Amp-Hours (Ah)
If your battery is rated in Ah: Capacity (Wh) = Voltage × Ah. So 5,000 Wh at 12 V = 5,000 ÷ 12 ≈ 417 Ah total. Then divide by the Ah per battery to get how many batteries.
LiFePO4 vs Lead-Acid: Why It Changes the Count
- LiFePO4: High DoD (80–90%), so you need less total capacity for the same usable energy. Fewer, smaller batteries.
- Lead-acid: Low DoD (50% recommended), so you need more total capacity for the same usable energy. More or bigger batteries.
For the same 4,000 Wh usable:
- LiFePO4 at 90% DoD: 4,000 ÷ 0.9 ≈ 4,444 Wh
- Lead-acid at 50% DoD: 4,000 ÷ 0.5 = 8,000 Wh
So lead-acid often needs roughly twice the capacity (and more physical batteries) than LiFePO4. See LiFePO4 vs lead-acid for solar for a full comparison.
Round Up and Add Margin
- Round up battery count so you don’t undersize.
- Adding 10–20% capacity gives headroom for aging and cold. Use the WattSizing calculator to enter daily use, days of autonomy, and chemistry—it will recommend capacity and you can match it to real battery models.
