How to Size a Whole-Home Generator (Sizing Calculator + Guide)
By LumoWatt Editorial Team · Published 2026-04-01 · Updated 2026-10-15
Looking for a licensed installer?
Compare standby generator installers near you. Every one has a state electrical license we checked.
Find installers near you →Generator sizing & cost planner
This is an educational estimate, not an electrical load calculation, engineering plan, quote, or installation recommendation. Final sizing and installation must be completed by qualified professionals. Use this to get in the right ballpark before you request quotes.
Optional, for local pricing
Why sizing matters
An undersized generator trips or shuts down when the load is too big, and Generac warns that overloading can damage the generator and connected devices. An oversized one costs more up front. Sizing depends on which circuits you want backed up, not just square footage.
The three sizing tiers
These are rough starting points. Your load calculation decides.
- Essential circuits only (10 to 14 kW): refrigerator, well pump, a few lights and outlets, one AC zone.
- Most of the home (16 to 20 kW): the above plus most lighting circuits and a larger AC system.
- Whole home (22 kW and up): everything, including multiple AC zones and an electric range or dryer. Generac's air-cooled models go up to 28 kW; bigger loads need a liquid-cooled unit.
What a load calculation actually adds up
A licensed installer will build a load calculation from your electrical panel's circuit list. The reason the numbers matter: Generac notes that induction motors need about three times their running watts to start, for a few seconds. A generator that can run a big AC all day may still trip if two AC zones start at the same time. Load managers, sold by both Generac and Kohler, hold back large loads so they don't all start at once. Understanding the difference between running watts and starting surge is what separates a correctly sized generator from one that trips under real-world conditions.
| Appliance | Running watts | Starting surge | Notes for backup planning |
|---|---|---|---|
| Central AC, 24,000 Btu (2 ton) | 3,800 | About 3 times running | Usually the largest load; a load manager can delay its start |
| Central AC, 40,000 Btu (about 3.3 ton) | 6,000 | About 3 times running | Start it before other motor loads |
| Well pump, submersible 1 HP | 2,000 | About 3 times running | Often the top priority on a well |
| Well pump, submersible 1/2 HP | 1,500 | About 3 times running | The surge must fit the generator's starting capacity |
| Sump pump | 800 to 1,050 | About 3 times running | Critical in flood-prone basements |
| Refrigerator | 700 | About 3 times running | Surges at every compressor start |
| Freezer | 700 | About 3 times running | Same as a refrigerator |
| Electric water heater, 40 gallon | 4,500 | None (heating element) | Elements take turns, so 4,500 is the peak; often managed or left off |
| Electric range | 1,500 per element | None (heating element) | Use 1 or 2 elements during an outage |
| Electric clothes dryer | 5,750 | Drum motor only | Optional; run it when other big loads are off |
| EV charger, Level 2 (32 A at 240 V) | 7,680 | None | 32 amps times 240 volts; often left off or managed |
| TV, microwave, lights, chargers | TV 200 to 500, microwave 700 to 1,000 | Small | Add up what you will run at the same time |
Running watts are approximate figures from Generac's wattage reference guide; the water heater figure is Rheem's standard 40 gallon tank. The real numbers are on each appliance's data label, and if it lists only volts and amps, multiply them to get watts. Standby spec sheets list motor starting ratings too (Kohler's 20RCA, for example, lists 41 kVA peak motor starting at 240 V), so your installer can check your AC against the generator you are quoted.
How installers calculate it
A licensed installer will do a load calculation based on your electrical panel's circuit list, not a square-footage rule of thumb. Treat any quote that skips this step as a red flag.
Use the sizing calculator for a starting estimate based on the appliances you want backed up, then get 2 to 3 quotes from license-checked local installers for a real load calculation.
See our generators by home size guide for a home-size-based starting point, and once you have a rough size in mind, our whole house generator cost guide covers what installation actually costs. Ready to move forward? Browse license-checked installers in your area.
Sources
Figures and rules in this guide were checked against these sources on October 11, 2026. Programs, prices, and terms change, so confirm the current details before you rely on them.
- Generac: GP15000E and GP17500E owner's manual, wattage reference guide and overload warning (PDF)
- Generac: 10, 14 and 18 kW air-cooled standby generator spec sheet (PDF)
- Generac: 22 to 28 kW air-cooled standby generator spec sheet, including load managers (PDF)
- Kohler (Rehlko): 20RCA home generator spec sheet, motor starting and load shed kit (PDF)
- Rheem: Professional Classic electric water heater specifications (PDF)
Frequently asked questions
- Can I just size based on my square footage?
- No, square footage is a poor proxy for electrical load. Two identically-sized homes can need very different generator capacity depending on appliances, HVAC zones, and how many circuits you want backed up.
- What happens if I undersize my generator?
- It trips its breaker or shuts down when the load is more than it can carry, and your backed-up circuits lose power until the load comes down. Generac also warns that overloading can damage the generator and connected devices.