Running vs Starting Watts
Running (rated) watts is what a generator can supply continuously. Starting (surge or peak) watts is a much higher number it can supply for only a fraction of a second, to cover the brief spike a motor draws when it first turns on. Marketing leads with the bigger surge number; sizing a generator correctly means matching BOTH numbers to what you actually plan to run.
The one thing to know: Every portable generator has two wattage numbers, and the marketing headline is almost always the bigger one: 'starting' or 'surge' watts, the brief spike a motor draws for a fraction of a second when it first switches on. The number that actually limits what you can run all day is the smaller 'running' or 'rated' watt figure. A generator can have 3,500 surge watts and only 3,000 running watts — enough to START a refrigerator, but not enough to run it continuously alongside anything else.
Why the surge happens
The surge only applies to motor-driven appliances — anything with a compressor or pump: refrigerators, freezers, well pumps, sump pumps, air conditioners, furnace blowers, and shop tools. Purely resistive loads (lights, most microwaves, phone chargers, space heaters) draw close to their running watts immediately with little to no surge. Motor surge also isn't fixed: it varies by the motor's age (older motors and worn bearings surge harder), design (PSC vs capacitor-start vs LRA-rated compressors), and ambient temperature (cold compressor oil is thicker and harder to start, raising the surge).
A concrete example
A standard refrigerator draws roughly 400-800 running watts once its compressor is spinning steadily, but needs a starting surge of roughly 1,200-3,200 watts for the fraction of a second the compressor motor takes to spin up (see the Startup Watt Index for the full appliance-by-appliance table). A generator rated at "2,200 starting watts / 1,800 running watts" can start that fridge alone, but would struggle to also start a sump pump at the same moment — even though its running-watt total might otherwise seem to have headroom, because the SURGE has to cover whichever single motor is starting, not the sum of everything.
How to size correctly
- Add up running watts for everything you intend to have on simultaneously.
- Identify your single largest starting-watt draw among the motor-driven appliances in that list — not the sum of all their starting watts.
- Make sure the generator's surge rating clears that largest single starting-watt figure on top of the running watts of everything else already switched on.
- Sequence your startup — switch on the highest-surge appliance first (or one at a time), rather than all at once, to reduce the instantaneous demand on the generator.
This page explains general generator-sizing principles and is not a substitute for your specific appliance's nameplate rating or your generator's owner's manual.
Frequently Asked Questions
Running (rated) watts is the continuous power a generator can supply hour after hour. Starting (surge/peak) watts is a much higher figure the generator can only supply for a fraction of a second, used to cover the brief power spike a motor draws when it first switches on. A generator's box headline is almost always the starting-watt number.
An electric motor at rest has very little resistance in its windings, so when power is first applied, current briefly spikes far above the running current until the motor spins up to speed and its back-EMF limits the draw. This lasts a fraction of a second to a few seconds depending on the load the motor is starting against.
Anything with a compressor, pump, or motor: refrigerators and freezers, well pumps, sump pumps, window and central air conditioners, furnace blowers, and shop tools like table saws and air compressors. Purely resistive loads — incandescent and most LED lighting, phone chargers, and many microwaves — draw close to their running watts immediately with little to no surge.
Yes. The same refrigerator model can surge harder on a cold day (compressor oil is thicker) or as it ages (motor bearings wear, capacitors weaken), so published ranges are planning estimates, not a guaranteed number for your specific unit.
The motor may fail to start, trip the generator's overload protection, or in poorly protected setups stress the motor windings. It won't typically destroy the generator itself if overload protection works correctly, but repeated failed start attempts are hard on the appliance's motor.