Everyday
Generator Size Calculator
Build your appliance list and get the generator size you actually need: running watts plus the biggest starting surge, with headroom and a kVA to kW converter.
How it works
Behind the numbers
Every appliance has two numbers: the running watts it draws continuously, and the starting watts it briefly needs when its motor kicks in. The calculator sums all the running watts, then adds only the largest single starting surge, because appliances start one at a time. That sum is the minimum size that will not stall when the fridge compressor kicks on.
A 20% headroom margin is added on top, the usual recommendation, and you can adjust it from 0 to 50%. The result is shown in watts, kilowatts, and kVA, using a power factor of 0.8 that you can edit. The kVA to kW converter below works in both directions for comparing generator spec sheets.
Scenario buttons preload typical lists: a home outage with fridge, sump pump, and furnace blower; a camping setup; and a job site with saws and a compressor. Preset wattages come from published generator sizing worksheets and are labeled as typical; your nameplate is the final authority.
FAQ
Questions, answered
What size generator do I need for my house?
It depends on what you run at once. A fridge, sump pump, furnace blower, microwave, and lights total around 5,800 watts with headroom, while a full house with central air can need 18,000 or more. Build your list above for your number.
What are running watts vs starting watts?
Running watts are the continuous draw; starting watts are the brief peak when a motor starts, often two to three times the running figure. Only the largest single surge counts, since motors start one at a time.
How do I convert kVA to kW?
Multiply kVA by the power factor, typically 0.8: a 7.5 kVA generator delivers about 6 kW. The converter on this page does it in both directions using your power factor.
Is 20% headroom enough?
It is the usual recommendation: it keeps the generator out of its peak range during motor starts and leaves room for one more appliance. For well pumps or large air conditioners with big surges, consider 25%.