Generator Size Calculator
Add up the running watts of what must stay on - fridge, lights, phone chargers, the furnace fan - then pick the biggest motor on the list. The calculator adds the starting surge, rounds to a real generator size, and shows the headroom figure that buys a longer engine life.
Generator dealers size you up to the biggest inverter on the lot. The physics is simpler: motors ask three times their running watts for a second when they start, and everything else is arithmetic. Size for the surge, not for the sales pitch.
How to use
- List what must run in an outage and add the running watts from each label or the manual.
- Pick the single biggest motor - fridge, sump pump, well pump or AC.
- Buy the size shown, or the headroom figure if the budget allows.
Frequently asked questions
What size generator for a house fridge and lights?
A fridge (200 W running, 600+ starting) with lights, internet and phone charging lands around 2,000-2,500 W - the calculator adds your exact motor surge to your running total. That is inverter territory: quiet, clean power for electronics, sipping fuel.
Why do motors need more watts to start?
An electric motor is briefly a stalled coil at switch-on, drawing two to three times its running current for under a second. A generator that cannot deliver that surge stalls - which is why the size is running watts plus the biggest motor, not just the sum of the labels.
Should I buy a generator with headroom?
Yes if the budget allows: engines last longest around 80 percent load, headroom covers the appliance you forgot, and a lightly loaded big generator wastes fuel. The 25 percent figure shown is the sweet spot between those.
Where should a portable generator run?
Outdoors only, at least 20 feet from any window or vent, exhaust pointed away - carbon monoxide kills quietly and quickly indoors, garages included. And connect appliances by cord or a transfer switch, never by backfeeding a wall outlet, which can electrocute the utility worker restoring your street.