Short answer: For backup, battery capacity (kWh) ≈ critical load (kW) × backup hours ÷ usable depth of discharge ÷ inverter efficiency. A 2 kW critical load for 6 hours at 90 % depth of discharge needs about 14.5 kWh. The inverter must deliver the continuous load plus motor start-up surges.
How to use the result
The calculator gives a first estimate. Before ordering, our engineers also check:
| Factor | Why it matters |
|---|---|
| Surge loads | Motors and compressors need 3–6× running power for a moment |
| Temperature | Batteries deliver less in cold and age faster in heat |
| Recharge | The solar array must refill the battery between outages |
| Growth | Adding loads later is cheaper if the inverter is sized for it now |
Example
A shop runs lights, a fridge, a till and Wi-Fi — about 1.5 kW — and faces 4-hour evening outages. 1.5 × 4 ÷ 0.9 ÷ 0.92 ≈ 7.2 kWh, so a 51.2 V battery bank of about 7.5–10 kWh with a 3–5 kW hybrid inverter is a sensible start.
