What this calculator does
Generator sizing works out generator capacity needed to cover a set of appliances, including starting surge. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaApparent power = surge watts / power factor; generator size = apparent power / target operating capacity
The inputs explained
| Field | What to enter |
|---|---|
| Total running watts (W) | A number, measured in W. Starts at 3000. |
| Peak surge watts (largest startup load included) (W) | A number, measured in W. Starts at 4500. |
| Power factor | A number. Starts at 0.8. |
| Target operating capacity | A number. Starts at 0.7. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with total running watts
Every other input is held at the calculator’s starting values while total running watts varies. Select any row to load that scenario into the calculator.