Running watts vs starting (surge) watts
Running watts covers the steady draw of an appliance, while starting watts is the momentary jump, frequently 2-3x the running figure, that motor-driven equipment such as refrigerators, well pumps, and air conditioners pulls during the first second or two of startup. Your generator's continuous rating has to carry the full running load, and its surge rating has to absorb your biggest single starting spike stacked on top of everything already operating.
Adding up a real household load
Some representative numbers: a refrigerator at 600-800 running watts (surging to roughly 1,200-2,200W), a sump pump at 800-1,050W (surging to about 2,000-2,500W), a window AC unit at 1,000-1,500W, a well pump at 1,000-2,000W (with a higher surge), a microwave at 600-1,200W, and a handful of LED lights plus a phone charger totaling under 200W. Sum the running watts of everything you'll run at the same time, then tack on the surge watts of just the single biggest motor in the group (motors seldom fire at precisely the same moment) to arrive at your target generator size.
Portable generators for backup and job sites
A portable unit rated 3,000-5,000 running watts will run a refrigerator, sump pump, some lights, and a few small electronics, enough for partial home backup or powering tools on a job site. Move up to 7,000-10,000 running watts and you can run a well pump, a window AC, and kitchen basics all at once. Portables burn gasoline or propane, must sit outdoors at least 20 feet from any window or door because of carbon monoxide, and rely on manual startup with extension cords or a transfer switch.
Whole-home standby generators
Whole-home units generally fall in the 14kW to 22kW+ range, mount permanently with an automatic transfer switch, and are sized to power a home's entire electrical panel, central AC included, so an outage barely registers. Sizing them should be based on your home's real panel and peak demand, something a licensed electrician can figure out exactly, rather than square footage alone; go too big and you pay for installed capacity that mostly sits idle, go too small and the unit trips offline when demand peaks.
Inverter generators and sensitive electronics
When your load includes laptops, medical devices, or other electronics that react badly to power fluctuations, reach for an inverter generator, since it delivers cleaner, steadier sine-wave power than a conventional model. Inverter units also run quieter and sip less fuel under partial loads, but they carry a higher price per rated watt and don't reach the wattage ceilings of big conventional or standby generators.
Frequently asked questions
What size generator do I need to run a refrigerator and a few lights?
A portable generator rated 2,000-3,000 running watts easily handles a refrigerator's running and starting demands along with lights and small electronics, leaving a bit of headroom.
Will a 5,000-watt generator run central air conditioning?
Just a small window or portable AC; the starting watts of a central AC compressor, often 3,000-5,000W by themselves, will generally swamp a 5,000W generator as soon as you factor in the rest of the household loads, so central air usually demands a larger portable or a whole-home standby unit.
How close to sizing exactly should I buy, or should I buy extra headroom?
Build in 10-20% of headroom over your calculated peak load so the generator isn't pinned at its maximum around the clock, which wears the engine faster and leaves nothing in reserve should you plug in another appliance down the road.
Do I need a transfer switch?
Yes, for any generator you'll wire into household circuits during an outage; a transfer switch cleanly separates your home from the utility grid, keeping utility workers safe and preventing backfeed damage, and electrical code mandates one for any permanent generator hookup.