What Size Generator Do I Need? Load List & Surge Math
Quick answer: add the running watts of everything you'll run at once, add the starting surge of the biggest motor (AC, well pump or sump), then add 20%. Essentials land around 5,000–7,500 W; whole-house with central AC runs 12,000–20,000 W.
Essentials vs whole house
Decide what an outage should keep running. Essentials — fridge, lights, furnace blower, sump pump, phone chargers, a few outlets — typically need 5,000–7,500 running watts. Whole house — add central AC, electric water heater, oven — starts at 12,000–20,000 watts, which means a standby unit and a bigger fuel bill.
Most homes are happiest with essentials plus a manual transfer switch; that covers food, heat, water and sump — the four things an outage actually threatens.
Build the load list
Walk the house and write down running watts from appliance labels:
| Appliance | Running W | Starting surge |
|---|---|---|
| Refrigerator | 600–800 | 1,800–2,400 |
| Sump pump (1/2 hp) | 800–1,100 | 1,900–2,300 |
| Well pump (1 hp) | 1,000–1,500 | 3,000+ |
| Furnace blower | 800–1,200 | 2,000–3,000 |
| Central AC (3-ton) | 3,000–3,500 | 7,000+ |
| Electric water heater | 4,000–4,500 | — |
The surge that decides size
Motors draw 2–4× their running watts for a few seconds when they start. The generator's surge rating must cover the largest single motor start — usually the AC compressor or well pump — because that start happens while everything else is already running. A 5,500-W generator rated 8,000 surge handles essentials with a 1,100-W well pump; add a 3,500-W AC compressor and it trips.
Portable, standby & the transfer switch
- Portable: 3,000–12,000 W, run it 20+ ft from the house, fuel it manually. Pair with a manual transfer switch or interlock.
- Standby: 12,000–22,000 W, wired to a transfer switch, starts automatically. Costs 3–5× a portable but runs your whole panel.
- Never backfeed through a wall outlet — without a transfer switch you can electrocute a lineman. It's illegal in most jurisdictions.
Sizing mistakes
- Ignoring surge — the breaker trips the first time the AC starts.
- Running the generator at 100% — add 20% so it breathes.
- Buying the biggest you can afford — low-load running wastes fuel and harms the engine.
- Forgetting the transfer switch — the most dangerous mistake in the category.
- Not testing in summer — AC load is the true test; run it once before you need it.
Size it once, safely
Enter your loads in the generator size calculator, add the transfer switch to the budget, and test the setup before storm season. The generator that starts your AC and your sump pump without tripping is the right one.