How to size a generator
Generators list two ratings. The running (rated) watts must cover everything on at the same time. The starting (surge) watts must cover that load plus the brief extra draw of the largest motor when it starts. Starting motors one at a time keeps the surge requirement down.
Example: essential home circuits
Refrigerator (700 W), freezer (500 W), sump pump (1,000 W), furnace fan (800 W) and lights and electronics (1,000 W) total 4,000 W. With 20% margin you need 4,800 W running, so a 5,000 W generator. The sump pump needs about 2,000 W extra to start, so the starting rating should be at least 6,000 W.
Typical wattages
| Appliance | Running W | Extra starting W |
|---|---|---|
| Refrigerator | 600–800 | 1,200–2,200 |
| Sump pump (1/2 hp) | 1,000 | 2,000 |
| Furnace fan (1/2 hp) | 800 | 1,300 |
| Well pump (1 hp) | 1,000 | 2,000 |
| Window A/C (10,000 BTU) | 1,200 | 1,800 |
| Central A/C (3 ton) | 3,500–4,000 | 4,000–6,000 |
Frequently asked questions
What size generator do I need to run a house?
Essential circuits usually need 5,000–7,500 W. Running central air and an electric range as well typically takes a 15–22 kW standby unit.
Can I plug a generator into a wall outlet?
No. Back-feeding is dangerous for you and utility workers. Use a transfer switch or a listed interlock kit.
Why do motors need starting watts?
A motor draws several times its running current for a second or two while it gets up to speed.