How battery runtime is calculated
Nominal energy is battery-bank voltage multiplied by bank amp-hours. A 48 V, 100 Ah bank therefore has 4,800 Wh, or 4.8 kWh, of nominal energy. If the battery has lost capacity, the present-capacity percentage reduces that starting value.
The energy available above the reserve is adjusted capacity × (starting charge − reserve). The calculator then accounts separately for battery discharge losses, inverter losses on AC appliances, DC appliances and inverter standby.
Runtime = energy above reserve ÷ stored-energy draw. Stored-energy draw = (AC watts ÷ inverter efficiency + DC watts + standby watts) ÷ battery discharge efficiency. Percentages are used as decimal fractions.
Worked example: a 48 V, 100 Ah battery and a 300 W load
Assume a full battery at its original capacity, a 20% reserve, 90% inverter efficiency, 95% battery discharge efficiency and 10 W inverter standby. Energy above reserve is 4,800 × 0.8 = 3,840 Wh. Stored-energy draw is (300 ÷ 0.90 + 10) ÷ 0.95 = approximately 361.4 W.
The resulting continuous runtime is approximately 10.6 hours. This energy estimate is usable only within the equipment’s power limits. Adding solar belongs in the recharge planner, because the timing of generation and consumption matters.
Use your existing appliance list
- Add appliances in the solar load calculator.
- Choose “Use these loads for battery runtime” beneath that calculator.
- Enter the capacity and ratings of the battery and inverter you actually own.
- Turn off nonessential loads using the Include checkboxes and compare the result.
Common battery-runtime questions
Why is 100 Ah not enough information on its own?
Amp-hours do not specify energy without voltage. A nominal 12 V, 100 Ah battery stores 1.2 kWh, while a nominal 48 V, 100 Ah bank stores 4.8 kWh. Use the total bank values after its series/parallel configuration.
Can I use this for LiFePO₄, lead-acid or AGM?
Yes, as an energy estimate with suitable settings. Chemistry presets are starting assumptions. Lead-acid capacity depends particularly on discharge rate; the model does not calculate Peukert effects. Use tested capacity for the intended load where possible.
How do I estimate refrigerator runtime?
Use measured average power over a representative operating period for an energy estimate. Starting power must be checked separately. A refrigerator’s rated compressor watts running continuously do not describe its normal cycling energy consumption.
Does enough battery energy mean an appliance will start?
No. The inverter’s starting power and duration, battery/BMS current limits, wiring and voltage drop can prevent a start even when the battery stores enough energy. The calculator reports the entered AC starting-power comparison separately.
Do my inputs leave my browser?
The battery and recharge calculations run in your browser. Scenario saving uses browser storage; no account or server submission is required for these calculations. See the site’s privacy policy for other site services.
Method and sources
These tools use an energy-balance model with editable assumptions. The sources below explain battery monitoring, solar-and-storage behaviour and backup-system limitations; they do not certify this calculator or your installation.
- Victron SmartShunt manual: state of charge, efficiency and remaining-time estimates
- European Commission PVGIS: solar production and off-grid energy balance. This release does not request PVGIS data; enter your own local peak sun hours.
- Australian Government Solar Consumer Guide: batteries and backup capability
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