How the bank size is calculated
Start from what the battery must actually deliver: daily consumption divided by inverter efficiency. Multiply by the days of autonomy the bank must bridge without charging, then divide by the usable depth of discharge to get the nominal bank size in watt-hours. Dividing by the system voltage converts it to amp-hours.
For example, 1,200 Wh of AC consumption per day through a 90%-efficient inverter draws about 1,333 Wh from the battery. Two days of autonomy need 2,667 Wh of usable energy, which at 90% LiFePO4 depth of discharge means a 2,963 Wh nominal bank — about 247 Ah at 12 V, or three 100 Ah batteries.
Why real needs can differ
The v1 model assumes consumption stays constant and no charge arrives during the autonomy period. Cold weather reduces usable capacity, batteries age, and loads tend to grow over time — which is why the planning range adds headroom on top of the calculated minimum. Lead-acid Peukert effects are not yet modelled.
Same job, different chemistry
| Chemistry | Planning DoD | Bank for 2,667 Wh usable |
|---|---|---|
| LiFePO4 (LFP) | 90% | 2,963 Wh |
| Li-ion (NMC) | 80% | 3,333 Wh |
| Gel | 60% | 4,444 Wh |
| AGM | 50% | 5,333 Wh |
| Flooded lead-acid | 50% | 5,333 Wh |
Methodology
The calculator UI, the autonomy comparison table and the displayed formula all call the same calculation function. To turn a load list into daily consumption, or to check how long a chosen bank lasts, see the Battery Runtime calculator; to plan recharging, see Solar Charge Time.
- Chemistry depth-of-discharge defaults (shared with Battery Runtime) — LiFePO4 manufacturer datasheets; Trojan Battery user's guide; Victron Energy battery documentation
- Days-of-autonomy sizing method — Standard off-grid PV system sizing practice, e.g. IEC 62124 stand-alone PV system guidance and off-grid design handbooks
- Inverter efficiency 85–95% (90% default) — Typical pure-sine inverter datasheets (e.g. Victron Phoenix, Renogy)
- Cold weather and ageing headroom (+20% planning range) — Battery manufacturer derating guidance; capacity falls at low temperature and with cycle count
Assumptions last reviewed on 26 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
What are days of autonomy?
The number of days the bank must carry your consumption with no charging at all — for example cloudy days in a solar system, or the outage length a backup must bridge.
How do I estimate my daily consumption?
Sum each appliance's power in watts times its hours of use per day. Measuring with an energy meter for a few typical days is even better.
Why does battery chemistry change the required size?
Chemistries differ in usable depth of discharge. A lead-acid bank planning on 50% DoD must be roughly twice the nominal size of a LiFePO4 bank using 90%.