Quick answer: a generator for a whole house typically needs 5,000 watts to run essentials (fridge, furnace, lights, internet) and 14,000 to 20,000 watts to run everything including central AC. Most homeowners land on a 10,000 to 14,000 watt standby unit, which costs $3,000 to $7,000 installed.
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"Generator for a whole house" means different things to different people. For one family it is keeping the freezer cold and the sump pump running through a summer storm. For another it is running two air conditioners, an electric water heater, and a well pump for three days straight. The size, fuel, and cost of the right generator all follow from that first question: what has to stay on when the grid goes down?
This guide walks through the types available, how to size one without guessing, what the real costs look like, and which fuel makes sense for your house.
What a whole house generator actually needs to power
Start by listing what you want running during an outage, then add up the watts. Running watts keep a device going; starting watts are the brief surge when a motor kicks on, and they run two to three times higher for fridges, well pumps, and AC compressors.
Typical running loads in a US home:
| Appliance | Running watts | Starting surge |
|---|---|---|
| Refrigerator | 150 - 400 | 600 - 1,200 |
| Furnace fan (gas heat) | 300 - 600 | 900 - 1,800 |
| Sump pump | 600 - 800 | 1,300 - 2,000 |
| Lights and electronics | 300 - 800 | Minimal |
| Central AC (3-ton) | 3,000 - 3,500 | 7,000 - 9,000 |
| Electric water heater | 4,000 - 4,500 | Minimal |
| Well pump (1/2 hp) | 900 - 1,100 | 2,200 - 3,000 |
Three tiers cover most situations. Essentials only (fridge, furnace, some lights, phone charging) fits inside 3,000 to 5,000 watts. Essentials plus one heavy load, usually the AC or the well pump, needs 7,000 to 10,000 watts. Whole-home comfort with central air and electric appliances takes 14,000 to 20,000 watts.
If you would rather not do the math by hand, our generator size calculator walks through your specific appliance list and returns a wattage target.
Types of generators for a whole house
Three designs compete for the job, and they differ more in convenience than in raw power.
A standby generator (also called a whole home standby generator) is permanently installed outside, wired to your panel through an automatic transfer switch, and plumbed into natural gas or a large propane tank. It starts itself within seconds of an outage and shuts down when utility power returns. This is the only type that truly runs a house unattended, and it is what most people picture when they search for a full house generator.
A portable generator runs on gasoline or propane you store yourself. Move it outside, start it, and either run extension cords or wire it through a manual transfer switch. It will power the essentials in any mid-size home at a fraction of the standby cost, but you do the work every time, and refueling means going outside in the storm.
An inverter generator produces cleaner power for electronics and sips fuel by throttling to the load. Portables with inverter technology work well for partial-home backup, though large inverter models cost more per watt than conventional ones. Our standby vs portable comparison puts the three side by side on cost, runtime, and what each can realistically run.
What size generator for a whole house
Sizing comes down to one honest question: does central air conditioning count? It is usually the single largest surge load in the home.
- Essentials-only homes: 5,000 to 7,500 watts handles fridge, furnace, lights, sump pump, and devices.
- Homes with central AC: add roughly 4,000 to 6,000 watts of headroom for the compressor start, pushing the target to 10,000 to 14,000 watts.
- All-electric homes: electric heat, water heating, and cooking push requirements past 20,000 watts, and many all-electric homes are better served by managing loads than by one giant unit.
House square footage matters less than the appliance list. A 3,000 square foot gas-heated home with window units can run on less than a 1,500 square foot all-electric one. If your list adds up near the boundary between tiers, round up; generators age, and starting surges do not shrink.
How much a whole house generator costs
Budget for the equipment and the installation together, because installation often rivals the unit price.
| Type | Equipment | Installed total |
|---|---|---|
| Portable, essentials | $600 - $1,500 | $700 - $2,000 (with transfer switch) |
| Inverter portable | $1,200 - $4,000 | $1,500 - $4,500 |
| Standby, 10kW - 14kW | $2,500 - $5,500 | $5,000 - $9,000 |
| Standby, 20kW+ | $4,500 - $8,000 | $8,000 - $15,000+ |
The standby installed number includes the transfer switch, a concrete pad, gas line work, and permits, which vary a lot by region and by how far the gas meter sits from the install spot. Portable units cost almost nothing to install beyond a transfer switch or interlock kit if you want them wired into house circuits.
Ongoing costs differ too. A standby on natural gas costs pennies per hour but needs annual service ($200 to $500). A portable burns $5 to $15 of gasoline per day of heavy use, and storing that much fuel safely is its own project.
Fuel options for whole-home backup
Natural gas is the default where the utility line reaches, because runtime is effectively unlimited and nothing needs refueling. Propane stores indefinitely and works where gas lines do not, but a whole-house draw needs a large tank; sizing one is covered in our propane tank sizing guide. Gasoline is cheap and available in small engines' quantities, yet it degrades in months and is genuinely hard to store in outage-sized amounts. Diesel shows up mainly in larger portable and commercial units, with good fuel storage life. Solar generators pair clean operation with limited output, fine for essentials but rarely a whole-house answer.
For most homes the fuel question answers itself: gas line on site means standby on natural gas, and no gas line means propane or a portable.
How to pick your setup
Match the generator to your outage tolerance. If outages near you are short and seasonal, a portable that covers the fridge, furnace, and some lights is the pragmatic buy; our power outages by state page shows how often your state actually loses power. If you have a well pump, medical equipment, work-from-home needs, or long multi-day outages, a 14kW standby with automatic start pays for itself in the first real storm. If you want to browse current models and prices before deciding, the whole house generator listings on Amazon give a live view of the market.
One more decision worth making early: who installs. Standby units almost always go through a licensed dealer, both for the gas work and for warranty terms. Portable-to-panel wiring also needs an electrician; backfeeding through a dryer outlet is dangerous and illegal in most places.
FAQ
How many watts do I need to run my whole house?
Add the running watts of everything you want on at once, then add the largest motor's starting surge. Essentials land near 5,000 watts, homes with central AC near 10,000 to 14,000, and all-electric homes above 20,000.
Will a 10,000 watt generator run a whole house?
It runs the essentials plus either central AC or an electric water heater, one at a time, in most gas-heated homes. It will not carry an all-electric house.
Is it cheaper to run a generator on propane or gasoline?
Per hour of equivalent load, gasoline usually costs less and propane costs more but stores far longer. Whole-home standby economics favor natural gas or a large propane tank over either.
Do I need a permit to install a whole house generator?
Most jurisdictions require electrical and sometimes gas permits for standby installation. A licensed dealer typically pulls them as part of the job; ask before you sign.
Conclusion
Choosing a generator for a whole house is a sequencing problem: list the loads, pick the tier that matches your outage reality, then choose standby for automation or portable for budget. Size it with our generator size calculator, compare the types on our generator comparison page, and you will end up with backup power that fits the house instead of the showroom.
