Generator Size Calculator
What size generator runs your essentials?
⚠️ Generators produce carbon monoxide — outdoor use only, 20+ ft from openings. Never backfeed a house without a proper transfer switch.
How this is calculated
- Peak demand = running watts + largest motor start surge
- Motors briefly draw 2-3× their running watts when starting — only the LARGEST surge counts, they rarely start simultaneously
- The result snaps up to the next common generator size (2000, 3000, 4000, 5000, 6500, 7500, 10000...)
- Buying slightly larger than the math is fine; undersized generators trip breakers and burn out motors
Common questions
What size generator do I need to run a fridge?
A fridge runs at about 700 W but surges to ~1200-1500 W when the compressor kicks in. Add lights and phone chargers and a 2000 W generator handles it; a 3000 W inverter generator does it more quietly and safely for electronics.
How big a generator for essential home backup?
The classic essentials — fridge, freezer, furnace fan, some lights, TV/router — run around 3800 W with a ~2200 W surge, which lands on a 5000-6500 W generator. Add a well pump or window AC and you are at 7500-10000 W.
Why does only the largest surge count?
Surge is the extra power a motor needs for its first second or two. The fridge and the furnace fan almost never start in the same instant, so you plan for the worst single spike on top of the running total — not the sum of all spikes.
Inverter or conventional generator?
Inverter generators produce clean sine-wave power (safe for laptops and TVs), run quieter, and sip fuel at partial load — ideal for camping, tailgating, and essential-circuit backup up to ~4000 W. Conventional portables deliver more watts per dollar above that; whole-home standby units (14-20 kW) with automatic transfer switches are their own category.
What about connecting it to the house?
Never backfeed through a dryer outlet — it can electrocute line workers and is illegal. Use a transfer switch installed by an electrician (interlock kits are the budget option where allowed), or run heavy-duty extension cords to individual appliances. And always run the generator outdoors, at least 20 ft from any window: carbon monoxide kills.