What Size Generator Do I Need?
To keep the essentials running in an outage — fridge, some lights, Wi-Fi, phone charging, a few outlets — most homes need a 3,000–5,000 watt generator. To run nearly everything including central AC or electric heat, you're looking at 10,000–20,000+ watts (usually a standby unit). The right size comes from adding up what you'll actually run.
Running watts vs starting watts
Two numbers matter for every appliance:
- Running (rated) watts — steady draw once it's going.
- Starting (surge) watts — the brief spike when a motor or compressor kicks on, often 2–3× running watts.
Your generator must cover the total running watts of everything on at once, plus the single largest starting surge on top. Miss the surge and the generator stalls when the fridge or well pump cycles on.
Common appliance wattages
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator/freezer | 150–200 | 900–1,200 |
| Sump pump (1/2 HP) | 800–1,050 | 1,300–2,150 |
| Well pump (1 HP) | 1,000–1,200 | 2,000–3,000 |
| Furnace blower (gas heat) | 600–900 | 1,500–2,350 |
| Window AC (10,000 BTU) | 900–1,200 | 1,800–2,600 |
| Central AC (3 ton) | 3,000–3,500 | 7,000–9,000 |
| Lights (per room, LED) | 20–60 | — |
| Wi-Fi + electronics | 50–150 | — |
| Microwave | 1,000–1,500 | — |
| Electric water heater | 3,800–4,500 | — |
Worked example: essentials backup
Say you want fridge, furnace blower, sump pump, lights, and Wi-Fi during an outage:
- Running total: 175 (fridge) + 750 (blower) + 900 (sump) + 200 (lights) + 100 (Wi-Fi) = 2,125 W
- Largest surge: sump pump adds ~1,150 W over its running figure
- Peak need: 2,125 + 1,150 ≈ 3,275 W
A 4,000–5,000 W generator handles this comfortably with headroom. Add central AC and you'd jump to 8,000–10,000 W+.
Typical sizes by goal
| Goal | Generator size | Type |
|---|---|---|
| Fridge + phones + a few lights (camping/minimal) | 2,000–3,000 W | Portable inverter |
| Essentials (fridge, furnace, sump, lights, Wi-Fi) | 4,000–7,500 W | Portable |
| Essentials + window AC or well pump | 7,500–10,000 W | Large portable |
| Most of the house incl. central AC | 10,000–16,000 W | Standby |
| Whole house, all systems | 18,000–26,000 W | Standby |
A few sizing tips
- Leave 20% headroom. Running a generator at max continuously shortens its life and leaves no margin for surges.
- Stagger big loads. Not everything needs to start at once — you can run a smaller generator by not firing the AC and well pump simultaneously.
- Inverter generators are quieter, more fuel-efficient at partial load, and produce clean power for electronics.
- Never backfeed. Connect through a transfer switch or interlock installed by an electrician — never plug a generator into a wall outlet.
Size it exactly
The generator size calculator lets you add your appliances and returns the right size with starting surge included. For anything you'll wire in, the watts to amps and wire size tools cover the electrical side.
FAQ
Will a 5,000 watt generator run a house?
It runs the essentials — fridge, furnace blower, sump pump, lights, and electronics — but not central AC or an electric water heater at the same time. For those you need 10,000 W+.
What size generator runs a refrigerator?
A fridge needs about 150–200 running watts but 900–1,200 to start, so even a 2,000 W generator handles one comfortably with room for lights and a phone charger.
Do I add starting watts for every appliance?
No — only the single largest starting surge, added to the total running watts, because appliances rarely start at the exact same instant.
Related tools
- Generator Size — add your appliances, get the size.
- UPS / Battery Runtime — battery backup as an alternative.
- Appliance Wattage Chart — running and surge watts for 60+ devices.