How this instrument works
Every appliance with a motor — a refrigerator, a well pump, a sump pump, a central air compressor — draws far more current for a second or two while starting than it does once running. That's its starting or surge wattage, and it can run several times higher than the appliance's steady running wattage. A generator has to be able to supply that brief spike on top of whatever else is already running, or it stalls or trips offline the moment the motor tries to start.
The standard sizing convention doesn't add up every appliance's starting surge, though — it adds up all the running watts of everything operating simultaneously, then adds only the single highest starting surge among them. That works because starting surges are brief, and in practice loads don't all start at the exact same instant: you switch on one motor-driven appliance at a time, so the generator only ever has to absorb one surge on top of its existing running load, not several stacked together.
This convention describes the minimum generator size for that specific set of loads — it isn't a hard safety margin. Nameplates vary a lot by brand and motor type, real-world starting surges can run higher than a spec sheet's typical figure, and most guides recommend not routinely loading a generator at the very edge of its rated capacity. Treat the total here as a floor to size from, not a number to run right up against.
- Enter Running watts for up to three appliances that will run at the same time — leave any unused field at 0.
- Enter Highest starting/surge watts — the single largest starting surge among those same appliances, usually a compressor or pump motor.
- Read Total running watts — the continuous load the generator must sustain.
- Read Total generator size needed — running watts plus that one largest surge, the minimum generator rating to shop for.
- If you have more than three simultaneous appliances, add their running watts together yourself first and enter the sum as one of the three fields.
Worked example — a fridge and a sump pump on backup power
Enter 800 into Running watts — appliance 1 (a refrigerator), 400 into Running watts — appliance 2 (a sump pump), 0 into Running watts — appliance 3, and 1200 into Highest starting/surge watts, the sump pump motor's own startup draw. Total running watts reads 800 + 400 + 0 = 1,200 W, and Total generator size needed reads 1,200 + 1,200 = 2,400 W.
A generator rated at roughly 2,400 W or more would cover this pair comfortably, assuming the fridge and the sump pump aren't both trying to start at the exact same instant — which is also why many households manually stagger big loads, switching one motor-driven appliance on at a time, when running on a portable backup generator.
Questions
Why not add up every appliance's starting watts instead of just the biggest one?
Because starting surges last only a second or two, and appliances don't all start at the exact same moment in normal use — you plug things in or flip breakers on one at a time. Sizing for every surge added together would demand a needlessly larger, more expensive generator for a scenario (every motor starting simultaneously) that staggered use avoids in practice. The convention budgets for the realistic worst case: one surge on top of everything else already running.
Where do I find running and starting watts for my appliances?
Check the appliance's nameplate or owner's manual first — some list both figures directly. Where only running watts (or amps) are given, generator manufacturers publish wattage reference charts with typical running and starting figures for common household appliances and motors, which is a reasonable stand-in when a specific nameplate doesn't list a starting figure.
Should I add a safety margin on top of this total?
It's good practice. This calculator gives the minimum generator size for the specific loads entered; real starting surges can run higher than typical published figures, and most guidance suggests not routinely operating a generator right at its rated maximum. Buying some headroom above the calculated total gives room for that variance and for adding a load later.
What if I have more than three appliances running at once?
Add all their running watts together outside the calculator and enter the sum into one of the three running-watts fields (leaving the others at 0). Separately identify the single largest starting surge across that full list of appliances — it may not be the appliance with the highest running watts — and enter that as the starting-surge figure.
Does this sizing method apply to whole-house standby generators too?
The same running-plus-largest-surge logic applies to both portable and standby systems. Whole-house standby installations often add a load-management module that automatically staggers large loads like an AC compressor or well pump, which is effectively what makes the single-largest-surge assumption hold true even when several big appliances are wired into the same system.