What size generator do I need?
Short answer: generator size = (running watts + the largest motor's extra starting watts) ÷ 0.9. A fridge, sump pump, furnace fan, six bulbs, a router and a microwave run at 4,755 W. The sump pump's start adds 4,500 W more (3 × its 1,500 running watts), for a 9,255 W peak. Divide by 0.9 and you need about 10,300 W. Add your own devices below.
1. What you want to run at once
Watts are typical values. Your appliance label or manual beats them. Tick "Motor" for anything with a motor or compressor; its extra starting watts default to 3 times running watts, added on top of running watts (Home Depot's method). Type your own figure if the label lists additional starting watts.
2. Generator size
- Running watts
- 4,755 W
- Peak watts
- 9,255 W
- Recommended rating
- 10,284 W
Everything on at once
Running + 4,500 W extra starting from Sump pump
Peak ÷ 0.9, so you use no more than 90% of capacity (Home Depot)
- Run a generator outdoors only, at least 20 feet from the house. Never in a garage, basement, shed or on a porch, even with doors open (CPSC).
- Keep it away from doors, windows and vents, and have working carbon monoxide alarms inside (CPSC).
- Never plug a generator into a wall outlet. Backfeeding can electrocute utility workers. Have a licensed electrician install a transfer switch or interlock (ESFI).
How this is calculated
- Running watts = sum of running watts × quantity.
- Peak = running watts + the extra starting watts of one unit. Only the single largest extra is added, since motors rarely start at the same moment.
- Motor extra starting watts default to 3 × running watts, added on top of running watts, as in Home Depot's example (350 W fridge: 350 + 1,050 = 1,400 W). Lights and coffeemakers need no extra.
- Recommended rating = peak watts ÷ 0.9. Round up to the next generator size you can buy.
How the sizing works
- Running watts are what a device draws once it is going. Add them for everything on at the same time.
- Extra starting watts are the brief surge a motor needs to start, added on top of its running watts. Home Depot estimates this at about 3 times running watts: a 350 W fridge needs 350 + 1,050 = 1,400 W. Lights and coffeemakers need no extra.
- Only the biggest surge counts. Motors rarely start at the same moment, so the calculator adds the extra for one motor only.
- Leave margin. Home Depot advises using no more than 90% of a generator's capacity, so the total is divided by 0.9.
Worked example: household essentials
| Item | Running watts | Extra starting watts (3× running for motors) |
|---|---|---|
| Refrigerator/freezer | 800 | 2,400 |
| Sump pump | 1,500 | 4,500 |
| Furnace fan (gas furnace) | 875 | 2,625 |
| 6 incandescent bulbs, 60 W each | 360 | 0 |
| Wi-Fi router and modem | 20 | 0 |
| Microwave | 1,200 | 0 |
| Total running | 4,755 |
| Step | Math | Watts |
|---|---|---|
| Running total | sum of the table | 4,755 |
| Largest extra starting watts (sump pump) | 1,500 × 3 | 4,500 |
| Peak load | 4,755 + 4,500 | 9,255 |
| Recommended rating | 9,255 ÷ 0.9 | 10,284 |
Running watts come from Home Depot, except the furnace fan and bulbs (Honda) and the router (our assumption). The same method and numbers appear in our generator sizing guide.
Typical running watts
From Home Depot's sizing guide. The calculator presets use the top of each range. Check your own labels.
| Appliance | Running watts | Notes |
|---|---|---|
| Refrigerator/freezer | 600–800 | Motor |
| Sump pump | 1,500 | Motor |
| Window air conditioner | 600–1,500 | Motor |
| Water pump | 1,000–2,000 | Motor; well pumps often 240 V |
| Central air conditioning | 2,000–4,000 | Motor; 240 V |
| Water heater (electric) | 3,000–4,500 | 240 V |
| Space heater | 1,250 | |
| Microwave | 1,200 | |
| Television | 100–350 | |
| Coffeemaker | 400–800 | No extra starting watts |
Large 240 V loads like central AC and well pumps need a generator with a 240 V outlet and a proper hookup. Read the nameplate and ask a licensed electrician.
Safety: carbon monoxide and backfeeding
- CPSC says to run a generator outdoors only, at least 20 feet from the house, away from doors, windows and vents.
- Never run one inside a home, garage, basement, crawlspace, shed or on a porch (CPSC).
- Never plug a generator into a wall outlet. ESFI warns that backfeeding can electrocute utility workers. Use a transfer switch or interlock installed by a licensed electrician.
Common questions
What size generator do I need to run a refrigerator?
An 800 W fridge with 3× extra starting watts peaks at 800 + 2,400 = 3,200 W. Divided by 0.9, that is about 3,560 W. Your fridge's label may show a much lower figure.
Is a 5,000-watt generator enough for a house?
For a short list, yes. The essentials example above needs about 10,300 W, so you would trim or stagger loads.
Why does starting wattage matter?
Motors draw a surge for a moment when they start. If the generator cannot cover that surge, the motor may fail to start or the generator may overload.
Should I get a power station instead?
For a few small loads, a battery power station is quiet and makes no fumes. Compare them inpower station vs generator, or size one with thepower station size calculator.