Generator Size & Wattage FAQ
Straight answers about running watts, surge and picking the right size.
Common appliance wattage quick-reference
| Appliance | Running watts | Starting (surge) watts |
|---|---|---|
| Refrigerator / freezer | 150 W | 800 W |
| Well pump (½ HP) | 1,000 W | 2,200 W |
| Sump pump (½ HP) | 800 W | 1,800 W |
| Furnace blower | 800 W | 1,800 W |
| Central AC (3 ton) | 3,500 W | 7,000 W |
| Window AC (10k BTU) | 1,200 W | 2,200 W |
| Microwave oven | 1,000 W | 0 W |
| Space heater | 1,500 W | 0 W |
| Washing machine | 500 W | 1,200 W |
| TV / computer | 200 W | 0 W |
| Lights & electronics | 300 W | 0 W |
Figures are typical industry averages for planning only. The nameplate on your unit sets the real number — when in doubt, size up.
Frequently Asked Questions
What is the difference between running and starting watts?
Running watts are the steady power an appliance draws while on. Starting (or surge) watts are the brief extra jolt a motor needs for a second or two when it kicks on. A generator must cover the starting watts, which are usually two to three times the running watts for anything with a motor.
How do I size a generator for my whole house?
List every appliance you want on during an outage, add their running watts, then add the single largest starting wattage on top (assume one motor starts at a time). Multiply that peak by about 1.2 for headroom. Use the home-backup calculator on this site to do it automatically.
Should I get a portable or a standby generator?
A portable generator (2–7.5 kW) covers a fridge, a few lights and maybe a furnace during short outages, and you fuel it by hand. A standby unit (10–20 kW) wires into the panel, starts automatically and can run central AC and well pump together, but costs far more to install.
Can a 7500 watt generator run a house?
A 7,500 W portable handles a fridge, sump pump, furnace blower, lights and a microwave, but usually not a central air conditioner (which can need 7,000 starting watts on its own). For whole-house cooling you typically need a 10–15 kW standby unit.
Why does my generator keep tripping under load?
Two common causes: the starting surge of a motor exceeded the generator's rated surge, or the total running load passed the unit's continuous rating. Size for the largest surge plus everything else running, with ~20% headroom.
Is this calculator professional advice?
No. It is a planning tool using typical wattage. For a permanent install, have a licensed electrician size the transfer switch and confirm loads against your real nameplates.