Essentials (refrigerator, lights, router, phones) fit a 2,000–3,000 W unit; adding a sump pump, furnace blower and a well pump moves you toward 5,000–8,000 W; central air conditioning needs more again.
The method
- List every appliance you will run together and its running watts.
- Add them: that is the minimum running watts.
- Find the largest starting surge (starting − running) and add it to the total: that is the minimum starting watts.
- Add 10–20% of headroom so the generator is not at its limit.
- Check which fuel you will use — ratings on propane or natural gas are often lower than on gasoline.
Three worked examples
| Plan | Appliances | Running watts | Starting watts needed |
|---|---|---|---|
| Essentials | Refrigerator, router, lights, phones | 315 W | 765 W |
| Essentials + sump pump | …plus 1/3 hp sump pump | 1,115 W | 1,815 W |
| Essentials + pumps and furnace | …plus sump pump, well pump, furnace blower | 2,865 W | 3,965 W |
These totals use typical planning wattages; yours will differ. Add headroom, then compare against a generator's running and starting watts in our generator tables.
Inverter or open-frame?
Inverter generators produce cleaner power for electronics, are typically quieter and sip fuel at light loads, but cost more per watt. Open-frame generators are usually cheaper per watt and louder, with power quality that suits tools and appliances. Dual-fuel and tri-fuel models add propane and natural gas options, usually at reduced watts.
Connecting to the house
If you want a generator to power selected household circuits, you need a transfer switch or interlock kit and an inlet box, installed by a licensed electrician; see switch vs. interlock.
Questions readers ask
Is 3,500 watts enough to run a house?
Do I need an inverter generator?
Figures marked as Wattstanding calculations show their working; product figures come from the sources on each product page. How we work