Inputs and units
Each checkbox shows an assumed running wattage and, where applicable, a starting wattage. These presets are examples. Check equipment nameplates and manufacturer starting requirements before using the estimate to select a generator. The battery fields belong to the companion runtime calculation and do not affect the suggested generator wattage.
How the calculation works
Running watts = sum of selected running loads
Peak watts = running watts + largest (starting watts − running watts)
Suggested watts = peak × 1.20, rounded up to the next 250 WAdding only the largest incremental surge assumes one appliance starts while the other selected appliances are already running. It avoids adding every start event together, but it does not cover simultaneous starts. The fixed 20% allowance is a planning margin; it is not a manufacturer-specific temperature or altitude derating calculation.
Worked example
Select only the refrigerator and Wi-Fi router. Their preset running loads total 150 + 15 = 165 W. The refrigerator adds 1,200 − 150 = 1,050 W during its assumed start, giving a 1,215 W peak. With 20% headroom the result is 1,458 W; rounding to the next 250 W gives a 1,500 W suggestion.
Assumptions and limits
Check continuous output, surge duration, voltage, frequency, outlet ratings and load balance separately. This arithmetic cannot predict how a specific generator responds to a motor start or nonlinear load. Operate portable generators outdoors, at least 20 feet from doors, windows and vents, following manufacturer instructions. Never backfeed house wiring through an outlet; use a suitable transfer arrangement installed for the application.
References and calculation details
Page prepared September 18, 2026. Examples describe this calculator’s implementation; reference links do not imply independent certification.