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Key points
- Size from running watts plus the single largest motor’s starting surge.
- Portable: most power per dollar, but loud and manual. Inverter: quieter, cleaner power, less output. Standby: automatic, permanently installed.
- CPSC: run generators outdoors only, at least 20 ft from doors, windows and vents.
- Never back-feed a house through an outlet; use a transfer switch installed by an electrician.
Start with size, not type
List what must run during an outage, then add up running watts. Motors — refrigerators, sump pumps, well pumps, furnace fans and air conditioners — briefly need two to three times their running power to start. Because they rarely start at the same moment, you add only the largest extra starting load to the running total.
A typical essentials list — refrigerator, sump pump, some LED lights, TV and router, and phone chargers — comes to about 2,200 running watts and 3,700 starting watts. With a 20% safety margin, that points to a generator around 4.5 kW.
Generator Size CalculatorAdd up what you need to power in an outage and get running watts, starting watts and a sensible generator size.
Open calculator →The three types
| Conventional portable | Inverter | Standby | |
|---|---|---|---|
| Typical use | Essentials in an outage, job sites | Camping, RVs, sensitive electronics, quieter backup | Whole-home or large-circuit backup |
| Power | Most watts per dollar | Usually smaller; some can be paired | Largest; sized to the house |
| Noise | Loud | Much quieter, especially at light load | Enclosed, moderate |
| Power quality | Fine for most appliances | Clean, stable output | Clean |
| Fuel | Gasoline (some dual-fuel) | Gasoline, propane on some | Natural gas or propane |
| Starting | Manual; you wheel it out and connect it | Manual | Automatic via transfer switch |
| Installation | Transfer switch or inlet recommended | Cords or transfer switch | Professional install and permits |
How to choose
- Short, occasional outages and a tight budget: a conventional portable sized for essentials, connected through a transfer switch or power inlet.
- Noise matters, or you want it for camping too: an inverter generator. They run quieter and adjust engine speed to the load, saving fuel when you only need a few hundred watts.
- Long outages, medical equipment, or no one home to start it: a standby generator that starts automatically.
Safety rules that are not optional
Carbon monoxide from generators kills people every year. The U.S. Consumer Product Safety Commission’s guidance is direct:
- Run generators outdoors only — never in a home, garage, basement, crawlspace or shed, even with doors open.
- Keep them at least 20 feet from doors, windows and vents.
- Install carbon monoxide alarms with battery backup and test them monthly.
- Never plug a generator into a wall outlet to power the house. Back-feeding can electrocute utility workers and neighbors. Use a transfer switch installed by a licensed electrician.
Fuel planning
Store fuel safely and rotate gasoline, which degrades over months. Run the generator for a few minutes every month or two so it starts when you need it. If you rely on a portable for long outages, plan how many hours of runtime each tank gives at your expected load — the manual lists it at 25% and 50% load.
Electricity Cost CalculatorFind what an appliance really costs to run — per day, month and year.
Open calculator →Ways to connect a generator to your home
- Extension cords: run heavy-duty outdoor cords from the generator to individual appliances. Simple, but you cannot power hard-wired equipment like a furnace or well pump.
- Manual transfer switch or power inlet: an electrician installs a switch that safely connects selected circuits to the generator and isolates them from the grid. This is the safest way to run a portable on house circuits.
- Automatic transfer switch: standard with standby generators. It senses an outage, starts the generator and switches over without you.
Never improvise with a “double male” cord into an outlet. It back-feeds the utility lines and can kill line workers.
Fuel options
| Fuel | Pros | Cons |
|---|---|---|
| Gasoline | Available everywhere; most power per engine size | Degrades in storage; hard to buy during widespread outages |
| Propane | Stores for years; cleaner running | Usually somewhat less output than on gasoline; tanks to manage |
| Natural gas | Never needs refilling (standby units) | Requires a gas line and professional installation |
| Dual-fuel | Flexibility in an emergency | Costs a little more |
Typical appliance loads
These are the planning values used in our generator calculator. Check the nameplates of your own appliances when you can.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Sump pump (½ hp) | 1,050 | 2,150 |
| Furnace blower (½ hp) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Microwave | 1,000 | 1,000 |
| Space heater | 1,500 | 1,500 |
Generator Size CalculatorAdd up what you need to power in an outage and get running watts, starting watts and a sensible generator size.
Open calculator →Keep it ready
- Run it under some load for a few minutes every month or two.
- Use fuel stabilizer in stored gasoline, or run the carburetor dry before storage as the manual directs.
- Change the oil on schedule — new engines often need an early first change.
- Keep a spare spark plug, oil and the right extension cords with the generator.
- Protect it from rain with a cover or enclosure made for running generators, not a garage or shed.
Frequently asked questions
Can a portable generator run central air conditioning?
Central air has a large starting surge. It usually needs a big generator, a soft-start device on the compressor, or a standby unit. Check the AC nameplate and ask an electrician.
What size generator runs a whole house?
It depends entirely on what runs at once. Add up your loads with the calculator; homes with electric heat, water heaters and central air need far more than homes on gas.
Are inverter generators worth it?
If you value quiet operation, fuel efficiency at light loads and clean power for electronics, yes. For maximum watts per dollar, a conventional portable wins.
How far from the house should a generator be?
CPSC guidance is at least 20 feet from doors, windows and vents, outdoors only, with CO alarms inside the home.