How it works
The calculator adds up the running watts of every appliance you tick, then finds the single ticked appliance with the highest starting figure. Total starting watts = (total running watts minus that appliance's own running watts) plus its surge. That total gets a flat 20% safety margin, and the answer is rounded up to the next entry on a list of common generator ratings: 2,000; 3,000; 3,500; 4,000; 5,000; 6,500; 7,500; 8,500; 10,000; 12,000; 15,000 and 20,000 watts.
The 15-row table arrives with Refrigerator / Freezer (700 W running, 2,200 W starting) and Lights (100 W) already ticked. The input that moves the answer most is whichever appliance carries the biggest surge, because only one surge is counted: a Well pump at 2,500 W or a Window AC at 3,600 W sets the peak on its own. Every running and starting figure is editable, so replace the defaults with your appliance nameplate numbers whenever you have them.
Counting only one surge assumes motors rarely start at the same instant. If your fridge compressor and well pump kick in together, the real peak is higher than shown. Nothing here models power factor, generator derating for altitude or heat, or the gap between a generator's peak and continuous ratings. The Central AC (3 ton) row carries a footnote for exactly this reason: a load that size normally needs a permanently installed whole-house standby unit.
Frequently asked questions
What size generator do I need to run a fridge and some lights? +
With the two default ticks (fridge at 700/2,200 W and 100 W of lights) the tool returns 3,000 W. That comes from 800 W running, 2,300 W starting once the fridge surge replaces its running draw, then 20% headroom rounded up to the nearest standard size.
Why are starting watts higher than running watts? +
Anything with an electric motor or compressor draws several times its normal current for a fraction of a second at switch-on. The table reflects that: a sump pump lists 800 W running against 2,000 W starting, while a purely resistive space heater lists 1,500 W for both.
Does it add up the starting watts of every appliance? +
No. It sums all running watts, then adds only the largest single surge in your selection, swapping out that appliance's running figure. Adding every surge together would badly oversize the generator, since motors almost never start at the same moment.
Can a portable generator run central air conditioning? +
The Central AC (3 ton) row is 3,500 W running and 6,000 W starting, which pushes the recommendation past 8,500 W on its own. It carries a footnote saying a load that size typically calls for a whole-house standby generator rather than a portable one.
How much headroom is built into the recommended size? +
A flat 20% is added to the starting-watt total before rounding up to the next standard rating. The results table lists both figures separately, so you can see the raw requirement and the padded, rounded recommendation side by side.
Starting watts against running watts
Anything with an induction motor — fridges, freezers, pumps, compressors, air conditioners — draws a large surge at startup, commonly three to seven times its running figure for a second or two. A generator sized to running watts alone will stall when the compressor kicks in.
The sizing rule that follows is to sum the running watts of everything that will be on, then add the single largest starting surge, since it is unlikely that two motors start at exactly the same instant. Staggering startup deliberately is how a smaller generator can run a larger load.
Power quality, fuel and the things that do not tolerate a rough waveform
Sensitive electronics need a clean sine wave. Inverter generators produce one; cheaper conventional sets produce a rougher waveform with higher total harmonic distortion that can damage switch-mode supplies and confuse anything with a microprocessor.
Fuel consumption scales with load rather than with rating, so an oversized generator running lightly is inefficient and, on diesel, prone to wet stacking — unburnt fuel fouling the exhaust from prolonged light running.
Everything about generator use is downstream of one safety point: they must never be run indoors or in an attached space, and they must never be back-fed into house wiring without a proper transfer switch, which is a lethal hazard to line workers.
