Recommended generator size
— kW
- Total running watts
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- Peak with largest motor starting
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- With 20% safety margin
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- Typical installed cost range
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- Monthly at 10.99% over 84 months (midpoint)
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Illustrative only. Not an offer of credit, a quote or financial advice. Lenders and contractors set real prices and rates.
How generator sizing works
Every appliance has a running wattage and, for anything with a motor or compressor, a higher starting wattage for a second or two. A generator has to carry the total running load plus the extra surge of the largest motor starting (motors rarely start at the same instant). Add a safety margin of about 20% so the generator isn't running flat out, then round up to the next standard size. The calculator does exactly that with typical wattages; your appliance labels or a dealer's clamp-meter reading give exact figures.
Essentials or whole house?
Covering essentials (fridge, sump pump, furnace blower, lights, a few outlets) usually fits a 10 to 14 kW unit. Adding central air conditioning is the big jump: a 3-ton AC alone can push the requirement to 18 to 24 kW. Load-management modules can let a smaller generator run the AC by shedding other loads while the compressor starts, which is why a dealer's calculation can come in smaller than this estimate. Electric ranges, dryers and water heaters are heavy loads most households leave off the generator.
What each size typically costs installed
| Size | Covers | Installed (typical) |
|---|---|---|
| 10 – 14 kW | Essentials | $7,000 – $12,000 |
| 18 – 24 kW | Most of an average home incl. one AC | $9,000 – $17,000 |
| 26 – 48 kW | Large home, multiple AC units | $13,000 – $35,000 |
Typical ranges for air-cooled and small liquid-cooled units including transfer switch, gas line, pad and permits. Long gas runs and difficult sites add cost.
Worked example
A home that wants the fridge, sump pump, furnace blower, a 3-ton AC, lights and the garage door covered totals about 7,200 running watts; with the AC's starting surge the peak is about 10,700 W, and with a 20% margin about 12,800 W, so a 14 kW unit is the recommendation (a dealer with load management might size it the same or one step smaller). Installed, that's typically $7,500 to $12,000, or about $167 a month at the midpoint over 84 months at an illustrative 10.99%. Our guide to whole house generator cost covers installation line items and running costs, and generator financing compares the ways to pay.
What the calculator leaves out
- Exact appliance wattages; check nameplates for anything large.
- Load-management options that allow a smaller unit.
- Propane tank cost if you don't have natural gas.
- Local permit, clearance and noise rules.
Frequently asked questions
What size generator do I need to run my whole house?
Most average homes with one central AC need 18 to 24 kW; large homes with multiple AC units need 26 kW or more. Essentials only usually fit 10 to 14 kW.
What size generator runs a 3-ton AC?
A 3-ton unit draws roughly 3,500 running watts and up to 7,000 starting. With essentials added, an 18 to 22 kW generator is typical, or a 14 kW with load management.
Is a 22 kW generator enough for a 2,500 sq ft house?
Usually yes for one central AC plus the main household loads, unless the home has electric heat, an electric range and dryer running at once.
How much does a 22 kW generator cost installed?
Typically $10,000 to $16,000 including the transfer switch, gas line, pad and permits, more for long gas runs or difficult sites.
Can I finance a standby generator?
Yes: unsecured loans, installer financing plans and home equity products. The lender pays the installer when the system is commissioned.
What size generator do I need for a 200 amp service?
Panel size doesn't set generator size; the loads you want powered do. Most homes on a 200 amp service that want essentials plus one central AC land at 18 to 24 kW; whole-house coverage with multiple AC units needs 26 kW or more. A load calculation or this calculator gives the real number.
Sources and further reading
Primary sources we rely on for this page. Program rules change, so check the source before you rely on a figure.