Off-Grid Battery Bank Sizing: A Step-by-Step Guide
How to size the battery bank for an off-grid inverter system — daily capacity, depth of discharge, days of autonomy and a worked example for a typical home.
Battery banks are the most expensive part of an off-grid system and the easiest to get wrong. Size them too small and the system cuts out early on cloudy days; too large and you pay for capacity you rarely use.
The sizing formula
Battery capacity (Ah) = daily energy (Wh) ÷ battery voltage (V) ÷ depth of discharge (%)
Three variables drive the result:
- Daily energy use (Wh/day) — from your load audit (see our solar panel sizing guide)
- Battery voltage (V) — 12 V, 24 V or 48 V, matching your inverter
- Depth of discharge (DoD) — the usable portion of the battery
Depth of discharge matters
Different chemistries allow different discharge depth:
| Battery type | Safe DoD | Notes |
|---|---|---|
| Lead-acid (flooded) | 50% | Regular topping charge needed |
| Lead-acid (AGM / gel) | 50–60% | Maintenance-free, moderate cost |
| LiFePO4 | 80–90% | Longer life, higher upfront cost |
Worked example
Continuing the home from our sizing guide:
- Daily load: 2,160 Wh
- Inverter / system voltage: 48 V
- Battery type: LiFePO4 at 85% DoD
- Desired autonomy (no sun): 2 days
Step 1 — energy per day the battery must store: 2,160 Wh ÷ 0.85 = 2,541 Wh of usable capacity
Step 2 — with 2 days of autonomy: 2,541 × 2 = 5,082 Wh
Step 3 — capacity in amp-hours at 48 V: 5,082 Wh ÷ 48 V = 106 Ah
A practical choice: a 48 V, 100 Ah (≈ 5 kWh) LiFePO4 bank, or two parallel 100 Ah packs if you want more margin.
Matching the inverter
- Pick an inverter whose DC input voltage matches the bank (12/24/48 V).
- Keep the bank’s charge current within the MPPT controller’s rating — a 5 kWh bank usually charges at 40–60 A comfortably.
- LiFePO4 banks need a BMS with communication (CAN/RS485) where the inverter supports it, so charging stops correctly.
Our off-grid inverters support 12 V, 24 V and 48 V banks (selected models also 60 V and 72 V) — the specification table on each product page lists the DC input range.
To run the capacity, inverter and array numbers together instead of one at a time, use our off-grid system sizing calculator — it applies the depth-of-discharge figures from the table above and suggests matching catalog models for the system voltage you pick.
Battery sizing — FAQ
How many days of autonomy should I design for?
Two to three days of autonomy is typical for a home with generator backup; three to five for remote sites where no backup exists. More autonomy means more batteries and more cost.
Can I mix battery types in one bank?
No. Mixing chemistries or ages causes imbalance, uneven charging and premature failure. Replace the whole bank or expand with identical packs.
Do I need a bigger battery for a bigger inverter?
Not directly. Inverter size follows peak load; battery size follows daily energy and autonomy. Match both to the same system voltage so they work together.