Solar Battery Size Calculator
How much battery do you need to ride out an outage — or a night off-grid? Enter the load you want to keep running and for how long, and get the required capacity in kWh and amp-hours, losses included.
Fridge + lights + internet + TV ≈ 400–700 W.
Battery capacity needed
0 kWh
for 500 W over 12 hours
Estimates only. Cold temperatures, battery age, and surge loads reduce real capacity — installers typically add 10–20% margin.
How this solar battery calculator works
The core formula: battery kWh = (load W × hours ÷ 1,000) ÷ (DoD × inverter efficiency). The energy your loads consume is only part of the story — you can't drain a battery to zero (depth of discharge), and the inverter loses ~10% turning DC into AC. Both are built into the result.
The amp-hour figure converts capacity to the units battery banks are sold in: Ah = kWh × 1,000 ÷ system voltage. Pair the result with the off-grid calculator to size panels that can recharge it each day.
Solar battery FAQ
What size solar battery do I need?
Multiply your average backup load by the hours you need, then divide by DoD and inverter efficiency. 500 W for 12 hours at 80% DoD and 90% efficiency needs ≈ 8.3 kWh — a typical 10 kWh home battery covers it.
What is depth of discharge (DoD)?
How much of a battery's capacity you can use without damaging it. Lithium (LiFePO4) handles 80–90%; lead-acid should stay at 50% or less. A 10 kWh battery at 80% DoD gives 8 kWh usable.
How do I convert battery kWh to amp-hours?
Ah = kWh × 1,000 ÷ voltage. An 8 kWh bank is ~167 Ah at 48 V, 333 Ah at 24 V, or 667 Ah at 12 V. Higher voltage means less current, thinner cables, and lower losses.
Can a solar battery power my whole house?
Essential circuits, yes — whole-house backup with AC and electric heat usually needs 20–40 kWh plus a big inverter. Most installs back up a critical-loads panel: fridge, lights, internet, a few outlets.