Generator sizing · estimate
How big a generator do you need?
Tick what you want running during an outage. The calculator adds the running watts, the biggest starting surge and some headroom, then suggests a standard size. It is an estimate: the site visit confirms it at your panel.
Load calculator
The arithmetic
How the estimate is worked out
- Running load. Add the running watts of everything you ticked.
- Starting surge. Motors (pumps, compressors) draw extra for a second or two when they start. We add the biggest one, because they rarely all start together.
- Load management. If you turn it on, heavy loads take turns, so only the biggest of them counts.
- Fuel. On natural gas, most air-cooled units make about 10% less power, so we size up for it.
- Headroom. Add 10%, then round up to the next standard size.
Bigger is not automatically better. A generator that runs at a small fraction of its rating most of the time costs more to buy and to fuel. We quote the smallest size that covers what you told us.
Typical watts we plan with
| Appliance | Running | Starting |
|---|---|---|
| Furnace or boiler (gas, oil or propane) | 800 W | 1,600 W |
| Central air, about 2 tons | 2,400 W | 4,000 W |
| Central air, about 3 tons | 3,500 W | 5,800 W |
| Central air, about 4 to 5 tons | 5,000 W | 8,200 W |
| Heat pump, ducted, about 3 tons | 4,000 W | 6,500 W |
| Ductless mini-splits, 2 to 3 heads | 2,500 W | 3,000 W |
| Well pump | 1,500 W | 3,000 W |
| Sump pump | 1,050 W | 2,150 W |
| Septic or ejector pump | 1,000 W | 2,000 W |
| Electric water heater | 4,500 W | 4,500 W |
| Refrigerator | 300 W | 1,200 W |
| Second fridge or chest freezer | 250 W | 900 W |
| Microwave | 1,200 W | 1,200 W |
| Dishwasher | 1,300 W | 1,300 W |
| Washing machine | 1,200 W | 2,300 W |
| Electric range and oven | 5,000 W | 5,000 W |
| Electric dryer | 5,500 W | 5,500 W |
| EV charger (Level 2) | 7,700 W | 7,700 W |
| Lights, TV, outlets and chargers | 1,500 W | 1,500 W |
| Internet, computers and home office | 400 W | 400 W |
| Garage door opener | 600 W | 1,200 W |
| Medical equipment (oxygen, CPAP) | 400 W | 400 W |
Central air and heat pump figures assume a soft-start kit, which we usually fit ($350 to $500).
Not sure what is in the basement? Send a photo of your electrical panel and the label on the furnace or well pump. Ruth will tell you which boxes to tick, or just book the free site visit.
Site visits are free
An estimate is a start. The site visit makes it a quote.
Send the details in five short steps and we will book a free site visit, where we read your panel and confirm the size.