Generator Sizing Calculator
Figure out what size generator you need: portable, inverter or home standby. Check off the appliances you want to keep running during an outage; the calculator adds their running watts plus the single largest startup surge and recommends a generator size. Common whole-house backup plans are worked out below.
Pick what you want to keep running
Recommended generator
6.9 kW
≈ 6,900 W starting capacity
- Running watts (all on)
- 4,150 W
- Peak with largest surge
- 5,700 W
Sized as running watts plus the single largest motor surge, with ~20% headroom. Wattages are typical: check your appliances' labels. A portable generator needs a manual transfer switch or interlock; never back-feed an outlet. Have an electrician handle the connection.
What size generator do I need for my house?
It depends on what you want running, not on the size of the house. Each row below is a common backup plan run through the same math as the calculator: total running watts, plus the single largest motor start surge, plus 20% headroom. Treat it as a starting point, then tick your own loads above.
| Backup plan | Running | Peakwith surge | Recommended+20% headroom | Generator class |
|---|---|---|---|---|
| Keep the food coldRefrigerator / freezer only | 700 W | 2,200 W | 2,700 W | Small inverter or large power station |
| Bare essentialsFridge, lights & outlets, TV / Wi-Fi | 1,600 W | 3,100 W | 3,800 W | Mid-size portable (3,000–6,000 W) |
| Winter, gas heatBare essentials + furnace blower | 2,400 W | 3,950 W | 4,800 W | Mid-size portable (3,000–6,000 W) |
| Basement with a sump pumpWinter set + sump pump | 3,450 W | 5,000 W | 6,000 W | Mid-size portable (3,000–6,000 W) |
| On a private wellWinter set + 1/2 HP well pump | 3,400 W | 4,950 W | 6,000 W | Mid-size portable (3,000–6,000 W) |
| Comfortable outageWinter set + sump, window AC, microwave | 5,650 W | 7,200 W | 8,700 W | Large portable (6,000 W+) |
| Most of the house (no central AC)Every load in the calculator | 11,370 W | 12,920 W | 15,600 W | Home standby |
Central air conditioning is not in these rows. Its start surge depends on the compressor and on whether a soft starter is fitted, and on a whole-house plan it usually decides the generator size on its own: see whole-house vs partial backup. Anything on 240 V (well pump, electric water heater) needs a 120/240 V generator connected through a transfer switch or interlock kit, not extension cords.
Portable, standby or power station?
- Under about 3,000 W: a small inverter generator or one of the larger battery power stations. Enough for a fridge, lights and the internet. A power station runs indoors and silently, but only for as long as its battery lasts: how big a power station for home backup.
- 3,000 to 10,000 W: a portable generator, ideally a 120/240 V unit with a twist-lock outlet so it can feed a transfer switch. This is where most essentials plans land.
- Above about 10,000 W, or if you want power back without anyone going outside: a home standby generator on natural gas or propane with an automatic transfer switch. See portable vs standby.
Every fuel-burning generator runs outdoors, well away from doors and windows, with a battery-backed CO alarm inside the house.
Running and starting watts of common loads
Motor-driven loads (compressors, pumps, blowers) draw a brief surge when they start. Resistive loads like lights and heating elements start at the same wattage they run at. These are the typical values the calculator uses; the nameplate on your own appliance is the better number.
| Load | Running watts | Starting watts |
|---|---|---|
| Water heater (electric) | 4,000 W | 4,000 W |
| Furnace blower (gas heat) | 800 W | 2,350 W |
| Refrigerator / freezer | 700 W | 2,200 W |
| Sump pump | 1,050 W | 2,150 W |
| Well pump (1/2 HP) | 1,000 W | 2,100 W |
| Window AC (10k BTU) | 1,200 W | 1,800 W |
| Garage door opener | 720 W | 1,400 W |
| Microwave | 1,000 W | 1,000 W |
| Lights & outlets | 600 W | 600 W |
| TV, Wi-Fi, devices | 300 W | 300 W |
For appliances not listed, the appliance wattage chart lists running watts for many more, and the generator runtime calculator estimates how long a tank lasts at your load.
Inverter generators by size
Match the running watts above to the generator's running rating, not its peak, and check its starting watts against your largest motor. A mid-size unit covers a furnace blower, fridge and sump pump; a small one covers a fridge and electronics. Run any generator outdoors, well away from doors and windows.
We earn a commission if you buy through links on this page, at no extra cost to you. It does not change what we recommend. See our affiliate disclosure. As an Amazon Associate I earn from qualifying purchases.
Mid-size inverter (3,000–6,000 W running)
WEN DF680iX
Best for: Furnace blower, fridge and sump pump in an outage, or an RV with one air conditioner
Small inverter (under 3,000 W running)
Honda EU2200i (EU2200ITAN)
Best for: A refrigerator, internet and electronics through an outage, or quiet power at a campsite
Frequently asked questions
Ask AI about this
Open an AI assistant with a question grounded in this page.
Sizing tells you the watts you need. Battery power stations cover the essentials tier silently and indoors, where a fuel generator cannot. See the portable power station picks.
Related guides
Related calculators
Home Energy Score Calculator
Get a 0–100 home energy score versus similar homes, plus a rough monthly bill, from a few quick questions about your home.
Open calculatorFurnace Size Calculator
Find what size furnace you need in BTU from square footage, climate zone, and insulation: output BTU your home needs vs the input rating you buy, rounded to a real furnace size.
Open calculatorMini Split Sizing Calculator
Find what size mini split you need for a room in BTU and tons (from square footage, climate, sun, and use) rounded to a real unit size, with an oversizing warning.
Open calculatorHeat Pump Cost Calculator
Estimate heat pump installation cost by size and type (ducted, ductless, or geothermal) with an equipment, labor, and adder breakdown and a cost-by-type comparison.
Open calculatorCite or link to this pageAPA · MLA · Chicago · HTML
Free to cite, quote and link. Copy a ready-made citation: the link back is all we ask.
- APA
WattSimple. (2026, June 26). Generator Sizing Calculator. https://wattsimple.com/calculators/generator-sizing
- MLA
"Generator Sizing Calculator." WattSimple, 26 June 2026, https://wattsimple.com/calculators/generator-sizing.
- Chicago
WattSimple. "Generator Sizing Calculator." Last modified June 26, 2026. https://wattsimple.com/calculators/generator-sizing.
- HTML link
<a href="https://wattsimple.com/calculators/generator-sizing">Generator Sizing Calculator</a> (WattSimple)
Sources & standards
This tool's methodology and data follow US standards and authoritative public datasets: