Running Watts vs Starting (Surge) Watts
Running watts are what an appliance draws once it's up and going; starting watts are the brief spike (often 2-3x running watts) that motor-driven appliances like refrigerators, well pumps, and air conditioners need for that first second or two of startup. A generator's continuous wattage rating has to cover your total running load, and its surge rating has to cover the single biggest starting spike stacked on top of everything else already running.
Adding Up What Your Household Needs
Typical figures: a refrigerator draws 600-800 running watts (surging to roughly 1,200-2,200W), a sump pump draws 800-1,050W (surging to roughly 2,000-2,500W), a window AC unit runs 1,000-1,500W, a well pump runs 1,000-2,000W (with a higher surge), a microwave draws 600-1,200W, and a few LED lights plus a phone charger add up to under 200W combined. Add the running watts of everything you'll use at the same time, then add the surge watts of just the single biggest motor on that list (motors rarely all kick on at once) to land on your target generator size.
Portable Generators for Backup Power 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, enough for partial home backup or running power tools on a job site. Step up to 7,000-10,000 running watts and you can run a well pump, a window AC, and kitchen essentials all at once. Portables run on gasoline or propane, need to sit outdoors at least 20 feet from windows and doors because of carbon monoxide, and require manual startup along with 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 built to cover a home's entire electrical panel, central AC included, so outages barely register. Size these to your home's actual panel and peak demand (a licensed electrician can calculate this precisely), not square footage. Oversizing wastes money on capacity you'll rarely use; undersizing means the unit shuts down under peak load.
Inverter Generators for Sensitive Electronics
If you're powering laptops, medical equipment, or anything else sensitive to power fluctuations, use an inverter generator, which puts out cleaner, more stable sine-wave power than a conventional model. Inverter generators also run quieter and use less fuel at partial loads, though they typically cost more per rated watt and top out at a lower wattage than large conventional or standby units.
Frequently asked questions
What size generator do I need for a refrigerator and a few lights?
A portable generator in the 2,000-3,000 running watt range covers a refrigerator's running and starting watts plus lighting and small electronics, with 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 on their own) will typically overwhelm a 5,000W generator once other household loads are added in, so central air generally requires a larger portable or a whole-home standby unit.
Should I buy extra headroom above my calculated wattage?
Add 10-20% headroom above your calculated peak load so you're not running the generator flat-out continuously, which shortens engine life and leaves no cushion if you add another appliance down the road.
Do I really need a transfer switch?
Yes, for any generator powering household circuits during an outage. A transfer switch isolates your home from the utility grid, protects utility workers, prevents backfeed damage, and is required by electrical code for permanent generator hookups.