What size battery do you need?
Enter your power use and required runtime to find the battery capacity.
Required nominal capacity
- Average power use
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- Battery power draw
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An estimate for your inputs. Actual performance depends on temperature, battery condition and changing loads.
Capacity alone does not establish compatibility. Check battery voltage, battery and BMS current limits, startup loads and inverter specifications.
How we calculate
We calculate energy for the required runtime and include conversion losses. Nominal capacity must exceed energy used if some charge remains in reserve.
Capacity (Wh) = (load (W) ÷ efficiency + inverter idle power (W)) × time (h) ÷ usable fractionExample: 200 W for 6 hours, 90% efficiency, 90% usable capacity and 10 W idle power. About 1548 Wh, or 1.55 kWh, of nominal capacity is required. At 51.2 V, this is about 30.2 Ah.
Percentages in the formulas are decimal fractions: 90% = 0.9. Energy (Wh) = capacity (Ah) × nominal voltage (V).
Sources & assumptions
These formulas estimate energy. Efficiency, usable capacity and power consumption are user inputs, not specifications of a particular model.
- Victron Energy — battery system design and BMS selection
- Victron Energy — battery monitoring and energy measurement
Methodology updated October 6, 2026
Common questions
What battery do I need for a 3 kW inverter?
Start with actual power consumption and required runtime. 3 kW is the inverter output capability, not constant consumption. Also check battery and BMS current limits and voltage compatibility.
What does usable capacity mean?
The fraction of nominal energy between your starting and planned ending charge. From 90% to 20% charge means 70% usable energy. Reduce it further if the battery has lost capacity.
Can I round this result to a battery size?
The result is an energy estimate. Allow for reserve, operating conditions and manufacturer limits when buying. Capacity sizing does not replace current, power and system safety checks.