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Whole House Generator Size Calculator
We loaded a typical home with central air. Remove what you won't run, add what you have, and see the generator size your house really needs.
- 240 V appliances flagged
- Motor surge + 20% margin
- Runs in your browser
| Appliance | Qty | Run W | Start W | Hrs/day | Remove |
|---|
More options: altitude
Matched to your list
Models that can run your list
The smallest models whose manufacturer specs cover your running watts and your biggest motor start. Smaller usually means cheaper, lighter and quieter.
Power station
Silent, safe indoors, no fuel. Best for fridges, CPAP and electronics.
Inverter generator
Quiet, clean power for electronics. Runs for days with fuel. Outdoors only.
Portable generator
The most watts per dollar, often dual fuel and 240 V. Loud. Outdoors only.
We may earn a commission if you buy through these links, at no extra cost to you. It never changes which models appear: they're picked only by the math above. How we pick.
Three levels of "whole house"
The biggest decision isn't the brand. It's what you'll run at the same time. Here's what each level needs with typical appliances:
| Level | Running watts | Starting watts | 240 V? |
|---|---|---|---|
| Essentials only Fridge, freezer, sump pump, lights, Wi-Fi, chargers | 1,800 W | 2,600 W | No |
| Most of the house Adds furnace blower, well pump, microwave, TV, washer | 6,200 W | 6,600 W | Yes |
| Whole house + central AC Adds a 3-ton central AC, more lights and TVs, gas dryer | 10,400 W | 15,000 W | Yes |
What drives the size up
- Central air and heat pumps. The single biggest load: around 3,000 to 5,000 running watts and 8,000 to 10,000 to start. Skipping AC during an outage can cut the generator you need in half.
- Electric water heaters, dryers and ranges. 4,000 to 5,500 watts each, all 240 V. Run them one at a time, or not at all.
- Well pumps. Usually 240 V with a big start-up surge. If you're on a well, you need a 240 V generator and a transfer switch.
- Motors starting together. The planner adds the biggest start-up surge once. If two big motors could start at the same moment (an AC and a well pump), add margin.
Portable generator or home standby?
A large portable generator (about 9,000 to 12,000 running watts, 120/240 V, often dual fuel) connected through a transfer switch or interlock can run most homes for a fraction of the cost of a standby unit. That's what the planner recommends above.
A home standby generator starts by itself within seconds and runs on natural gas or propane for days, but it's sold and installed through dealers and electricians, not online, so we don't list them. The largest portables top out around 9,500 running and 12,500 starting watts. If your result is above that (central air is the usual reason), you have three options: skip the AC during outages, ask an HVAC technician about a soft start kit for the AC compressor, or get a standby quote from a licensed installer.
- Use a transfer switch or a panel interlock with an inlet box, installed by a licensed electrician. Never "backfeed" through a dryer or wall outlet.
- Keep the generator at least 20 feet from the house, exhaust pointing away from doors and windows, with CO alarms inside.
Frequently asked questions
What size generator do I need to run my whole house?
It depends on what "whole house" means for you. Essentials (fridge, freezer, sump pump, lights, Wi-Fi) usually need about 2,000 running watts and 2,500 to 3,000 starting watts. Adding a well pump, furnace blower and kitchen items takes it to about 6,000 to 8,000 watts. Running central air at the same time typically needs 10,000 watts or more, which is portable generator territory at the very top, or a home standby generator.
Can a portable generator run a whole house?
A large 9,000 to 12,000-watt portable generator with a 240 V outlet can run most homes, including a central AC on many systems, if you don't run everything at once. It must be connected through a transfer switch or interlock kit installed by a licensed electrician, never through a wall outlet.
Portable or standby generator for the whole house?
A home standby generator starts automatically and runs on natural gas or propane, but costs several thousand dollars installed and is bought through a dealer or electrician. A large portable generator plus a transfer switch costs far less, but you have to roll it out, start it and refuel it during the outage.
Does square footage tell me the generator size?
Not reliably. Two homes of the same size can need very different generators: one with gas heat, gas water heater and no AC may need 5,000 watts, while one with central air, a well pump and an electric water heater can need 15,000 watts or more. List the appliances instead.
What is a transfer switch and do I need one?
A transfer switch (or a panel interlock with a power inlet box) connects a generator to your home's circuits safely and isolates your house from the utility lines. It's the only safe way to power hardwired and 240 V appliances like a well pump, furnace or central AC, and it must be installed by a licensed electrician.
Estimates from typical appliance values; check your own labels. Product specs from manufacturers, last checked October 21, 2026. More appliances in the generator wattage chart.