Running watts vs. starting watts, explained
This is the single concept that determines whether you buy the right generator. Get it wrong and a generator that looks big enough on paper will stall the moment your refrigerator's compressor kicks on.
Running watts: the steady draw
Running watts (also called rated or continuous watts) is the power an appliance draws while operating normally. A 1,500 W space heater draws 1,500 watts the entire time it's on. A modern refrigerator draws roughly 150–400 watts while the compressor runs. Simple.
Starting watts: the surge nobody budgets for
Starting watts (surge or peak watts) is the brief burst of extra power — lasting a few seconds — that electric motors and compressors need to get moving from a dead stop. The multiplier is typically 2 to 3 times the running watts:
- Refrigerator: ~250 W running → ~1,200 W starting
- Well pump (1/2 HP): ~1,000 W running → ~2,100 W starting
- Window AC (10,000 BTU): ~1,500 W running → ~3,500 W starting
- Furnace blower (1/2 HP): ~875 W running → ~2,350 W starting
Resistive loads — heaters, toasters, incandescent lights, kettles — have no surge. What they draw is what they draw. Electronics like TVs and laptops also have no meaningful surge.
The formula pros use
You don't need to budget surge for every motor at once — motors rarely all start in the same second. The standard method used in manufacturer sizing worksheets:
Generator peak watts needed = (sum of all running watts) + (largest single motor's additional starting watts)
Where "additional starting watts" = starting watts minus running watts for that one motor. Then add headroom: plan to load a generator to about 80% of its running rating continuously, not 100%.
Our calculator does exactly this math, with 25% headroom on running watts and a 10% margin on the peak.
The marketing trap: peak vs. running ratings
Portable generators are advertised by their peak watts — the number they can sustain for seconds. The running watts rating, usually about 80% of peak, is the number that actually powers your house. A "7,500 W" portable typically delivers ~6,000 running watts. Always size by the running rating and treat peak as your surge budget.
Quick reference
- Motors and compressors: budget 2–3x running watts for starting.
- Heating elements and lights: starting = running.
- Size for the largest single motor start, on top of everything already running.
- Buy by running watts; keep 20–25% headroom.
- When in doubt, check the appliance nameplate — our calculator lets you override every value.