What size generator do you need? The watts-and-surge method
Updated · Reviewed against the current BackupAtlas dataset
Short answer: add up the running watts of everything you'll run at once, add the single largest starting surge (motors rarely all start together), then add 20% headroom. That number — not a round "whole house" figure — is the generator size to shop for. The sizing calculator does the arithmetic for your actual appliance list.
Two numbers per appliance, and why both matter
Every generator (and most motor-driven appliances) has a running watts rating and a starting watts rating. Running watts is the load a generator sustains continuously; starting watts is the brief surge available for a few seconds while a motor spins up. Compressors, pumps, and blower motors draw two to six times their running power for that instant — resistive loads like lights and heaters have essentially no surge. See starting vs running watts for the full explanation. Sizing on running watts alone is the single most common mistake: a generator that comfortably covers everyone's steady-state load can still stall the instant a refrigerator compressor kicks on.
The three-step method
- Sum running watts. Add the running watts of every appliance you want on at the same time. This is the load the generator must sustain for hours, not seconds.
- Add one surge, not all of them. Find the single largest difference between an appliance's starting and running watts, and add just that one. Motor starts are momentary and effectively never synchronized in practice — if you know two large motors genuinely start at the same instant (say, a well pump and central AC on the same automatic transfer switch), add both.
- Add 20% headroom. Running a generator at its rated limit runs it hot, loud, and thirsty, and dual-fuel or tri-fuel units make fewer watts on propane or natural gas than their gasoline rating (see the derating math). Multiply the peak figure by 1.2 for a size with margin.
Typical wattages for planning
These are commonly cited planning figures, not measurements of your appliances — a nameplate or owner's manual value always wins when you have one. The full table, with a running calculator, is on the sizing tool.
| Appliance | Running W (typical) | Starting W (typical) |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Furnace blower (gas furnace) | 800 | 2,350 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Well pump (1/2 HP, 240 V) | 1,000 | 2,100 |
| Central AC (3-ton, no soft starter) | 3,500 | 8,750 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Lights (whole-house LED) | 300 | 300 |
| TV + internet router | 200 | 200 |
Worked example
Say the plan is a refrigerator, the furnace blower, whole-house LED lighting, and a TV with a router — a common essentials-plus-heat outage list.
- Running watts: 700 + 800 + 300 + 200 = 2,000 W
- Largest single surge: the furnace blower, 2,350 − 800 = 1,550 W
- Peak demand: 2,000 + 1,550 = 3,550 W
- With 20% headroom: 3,550 × 1.2 ≈ 4,300 W recommended
That lands squarely in the small-portable or entry inverter class — see best generators for a refrigerator and essentials. Add a well pump or window AC and the same method pushes the number toward the 30-amp partial-home class instead.
When to size up further
- 240 V appliances (well pumps, electric dryers, central AC, EV charging) need a generator with 120/240 V output regardless of wattage — a 120 V-only unit cannot run them at any size.
- Dual-fuel and tri-fuel units make less power on propane or natural gas than their headline gasoline rating — size against the fuel you'll actually run on, detailed in the dual-fuel and tri-fuel guide.
- Central air conditioning has an unusually large starting surge relative to its running load — see can a generator run central AC? before assuming a mid-size unit covers it.
- Whatever the number comes out to, connecting it to house wiring safely requires a transfer switch or interlock installed by a licensed electrician — never a backfeed cord.