Running watts vs starting (surge) watts
Running watts is what an appliance draws continuously; starting watts is the brief spike (often 2-3x running watts) that motor-driven appliances like refrigerators, well pumps, and air conditioners need for the first second or two of startup. A generator's continuous wattage rating must cover your total running load, and its surge rating must cover your single largest starting spike layered on top of everything else already running.
Adding up a real household load
Typical figures: refrigerator 600-800 running watts (surge to ~1,200-2,200W), sump pump 800-1,050W (surge to ~2,000-2,500W), window AC unit 1,000-1,500W, well pump 1,000-2,000W (surge higher), microwave 600-1,200W, a few LED lights and a phone charger under 200W combined. Add the running watts of everything you'll use simultaneously, then add the surge watts of only the single largest motor on that list (since motors rarely all start at the exact same instant) to get your target generator size.
Portable generators for backup and job sites
Portable generators in the 3,000-5,000 running watt range cover a refrigerator, sump pump, some lighting, and a few small electronics, good for a partial home backup or job site power tools. Stepping up to 7,000-10,000 running watts covers a well pump plus a window AC plus kitchen essentials simultaneously. Portables run on gasoline or propane, need to be sited outdoors at least 20 feet from windows and doors due to carbon monoxide, and require manual startup and extension cords or a transfer switch.
Whole-home standby generators
Whole-home units, typically sized 14kW to 22kW+ and installed permanently with an automatic transfer switch, are sized to cover a home's full electrical panel, including central AC, so outages are seamless. Sizing here should be done against your home's actual panel and peak demand (a licensed electrician can calculate this precisely), not just square footage; oversizing wastes money on installed capacity you'll rarely use, undersizing means the unit shuts down under peak load.
Inverter generators and sensitive electronics
If you're powering laptops, medical equipment, or other electronics sensitive to power fluctuations, choose an inverter generator, which produces cleaner, more stable sine-wave power than a conventional generator. Inverter models are also quieter and more fuel-efficient at partial loads, though they typically cost more per rated watt and top out lower in wattage than large conventional or standby units.
Frequently asked questions
What size generator do I need to run a refrigerator and a few lights?
A portable generator in the 2,000-3,000 running watt range comfortably covers a refrigerator's running and starting watts plus lighting and small electronics, with some headroom to spare.
Will a 5,000-watt generator run central air conditioning?
Only a small window or portable AC unit; a central AC compressor's starting watts (often 3,000-5,000W alone) will typically overwhelm a 5,000W generator once other household loads are added, so central air generally requires a larger portable or whole-home standby unit.
How close to sizing exactly should I buy, or should I buy extra headroom?
Add 10-20% headroom above your calculated peak load to avoid running the generator at its absolute ceiling continuously, which shortens engine life and leaves no margin if you add an appliance later.
Do I need a transfer switch?
Yes for any generator you'll use to power household circuits during an outage; a transfer switch safely isolates your home from the utility grid, which protects utility workers and prevents backfeeding damage, and is required by electrical code for permanent generator hookups.