How GeneratorCalc sizes your generator

How our free generator calculator works: running watts, the largest motor start, 20% headroom, 240 V checks and the sources behind every value.

Running and starting power

We sum the running watts of enabled appliances, multiplied by quantity. Recommended running capability is that total divided by 0.80, rounded up to a whole watt. This is a planning assumption, not a guarantee about a particular generator.

Starting demand is the running total plus the largest single appliance’s extra starting demand (starting watts minus running watts). We assume one motor starts at a time. Quantity scales running demand; it does not multiply the startup increment.

A worked example

A 200 W refrigerator with 1,200 W starting power and three 100 W lights total 500 W running. The estimate is 625 W running capability and 1,500 W starting capability. Check both capabilities against generator specifications.

Voltage and exact specifications

A selected 240 V appliance adds a 120/240 V output requirement. Voltage does not change calculated watts. Exact specifications replace typical values for your selection, without changing the shared dataset.

Limitations

We do not convert VA, locked-rotor amps or DC loads into input watts. Motors missing startup data remain incomplete. Heat pumps and whole-home systems need separate assessment. Fuel, altitude, temperature, outlets, phase balance, wiring and model compatibility need additional checks.

Soft-start answers affect advisory information only; they do not reduce startup watts in this version. In an RV, do not count downstream DC appliances again if their demand is already represented by the converter.

Assumptions: v1. Dataset: 1.2.2. Review sources or report a correction.