Generator Sizing Guide
A portable generator keeps the refrigerator cold, a window AC running, and a phone charged when the power is out for days. The trick is sizing it to what you actually need to run at the same time, not to your whole house.
Generators are rated in running watts and surge (starting) watts. Motors like refrigerators, well pumps, and AC compressors need two to three times their running wattage for a few seconds when they start. Add the running watts of everything you want on at once, then add the largest surge watt in the list. That total is the minimum generator size.
Common appliance wattage
| Appliance | Running watts | Surge watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Well pump (1/2 HP) | 1,000 | 3,000 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| Microwave (1000W) | 1,000 | — |
| Lights (LED, 10 bulbs) | 100 | — |
| Phone charger | 25 | — |
| Television | 200 | — |
Worked example
To run a refrigerator (700W), a window AC (1200W), a microwave (1000W), lights (100W), and a phone charger (25W) at the same time, you need 3,025 running watts. The largest surge is the AC at 3,600W. A 5,000-watt generator covers both with headroom. A 7,500W unit gives you room to add the well pump later.
Safety rules
- Never run a generator indoors, in a garage, or under a carport. Carbon monoxide kills more people after hurricanes than the storm does.
- Place it at least 20 feet from any door, window, or vent, with the exhaust pointing away from the house.
- Use a transfer switch or interlock for whole-panel connection. Backfeeding through a dryer outlet is illegal and can kill a lineman.
- Store fuel in approved containers, away from living spaces, and add stabilizer if it will sit for more than a month.