Calculator
Generator Size Calculator
Size a standby or portable generator based on the loads you actually need to run.
Model reviewed August 2026
Recommended standby generator
18 kW
- Running watts
- 12,800 W
- Starting / surge watts
- 20,480 W
- Installed cost estimateIncludes transfer switch, pad and gas connection
- $8,640 – $15,300
Estimates only. Local pricing, site conditions and code requirements change the real number.
The formula
Running watts = sum of loads; Starting watts adds the largest motor's surge
Air conditioners have a large locked-rotor starting surge. A soft starter can cut the required generator size significantly.
Worked example
A 4-ton AC, electric water heater, refrigerator, well pump and lighting need about 18,000 running watts — a 22 kW standby unit.
Frequently asked questions
What size generator runs a whole house?
A typical 2,000-2,500 sq ft home with central AC needs a 22 to 26 kW standby generator for whole-home coverage, or 10 to 14 kW for essential circuits only.
Sources
Sources & verification
- NFPA 70 — National Electrical Code
- International Residential Code — electrical (Chapters 34-43)
- International Code Council (IRC/IBC)
Model vintage August 2026. The links above are the issuing agencies' own pages — we publish them so you can confirm the current rule yourself, not as evidence that each fact on this page was re-checked with them today. Cost figures are modeled estimates based on national baselines adjusted for local labor and material pricing, not quotes. Permit rules change; confirm with the building department before starting work.