Running watts, starting watts, and the arithmetic in between

Almost every sizing guide gives the same instruction. Add up the running watts of everything you want to power, find the appliance with the highest starting wattage, and add that on top.

That arithmetic counts one appliance twice.

Where the double-count creeps in

A refrigerator with 700 running watts and 2,200 starting watts is not drawing 2,900 watts at startup. It draws 2,200. The 700 is already inside the 2,200, because starting wattage is the total the appliance pulls at that instant, not an extra charge on top of its normal draw.

So if the fridge is in your running total at 700 and you then add 2,200 for its surge, you have paid for 700 watts of fridge twice.

The correction, in one line

Add only the surge above the appliance’s own running draw.

For that fridge, 2,200 minus 700 is 1,500. If your running total across everything is 3,000 watts, the peak the generator has to survive is 4,500, not 5,200. At the sizes generators are actually sold in, 700 watts is frequently the difference between two models and a few hundred dollars.

Why one surge and not all of them

Motors start at different moments and surges last a fraction of a second. The chance of every compressor in the house starting in the same instant is small enough that sizing for it buys a machine that spends its whole life barely loaded.

What matters is the largest single surge, because it sets the highest point the generator ever has to reach. The generator size calculator does exactly this, adding the running total, the biggest surge above running, and a margin so the unit is not pinned at its ceiling, then rounding up to a size that exists on a shelf.

The assumption has one real failure mode, and it is worth knowing. A well pump and an air conditioner that can both kick in unprompted are a genuine risk of a simultaneous start, and that pairing is the usual reason an apparently correctly sized generator stalls.

Reading the two numbers on the box

Generators are sold with two ratings, and they describe different jobs.

The running or rated figure is what the machine will produce continuously, all day, which is the number to compare against your running total. The starting, surge or peak figure is what it can deliver for a second or two, and it is the one your largest surge has to fit inside.

A unit advertised as 3,500 watts with 4,375 peak is a 3,500 watt generator. Buying it against a 4,000 watt running load because the big number on the carton says 4,375 is the most common way people end up with a generator that shuts down every evening.

Converting when the label gives amps

Plenty of nameplates give amps rather than watts. Multiply amps by volts to get watts, so a 6 amp motor on a 120 volt circuit is 720 running watts. The watts calculator handles the conversion in both directions, including three-phase and power factor if you are sizing for something beyond a house.

Where a nameplate gives a locked rotor amp figure, that is the startup draw, and it is often three to six times the running current. Multiply it out before assuming the surge is modest.

The number this changes

Run the two methods against the same house and the gap is usually between 500 and 1,500 watts, almost always in the direction of a larger generator than you need.

That gap is not academic. It is frequently a whole size class, which is several hundred dollars to buy, more fuel burned per hour for the life of the machine, and a heavier unit to drag out of a shed in the rain.