Home backup, calculated.
Add your devices and choose how long you need backup power.
Required nominal capacity
- Average power use
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- Sum of device power ratings
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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.
For each device, average power = running power × quantity × on-time fraction. We add the average powers and use the formula above. Startup currents are not calculated in this energy model.
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 should I enter for a refrigerator?
Use its running power and compressor duty cycle. The defaults of 150 W and 35% are examples, not universal specifications. Measuring energy use with a power meter in your conditions is preferable.
Does the sum of power ratings determine inverter size?
It is only the sum of the entered running power ratings. A refrigerator or pump can require much higher startup power. This calculator estimates energy, not a safe inverter model.
What does 50% on-time mean?
The device uses the entered power for half of the selected backup period. For 6 hours of backup, this means 3 hours running. Use 100% for a router that stays on continuously.