What size generator for a 3,000 sq ft house?20.5 kW
Add the loads you want to run together, including the motor that needs the most power to start.
Your starting point
20.5 kW
Planning range
16.5–23 kW
A screening estimate under the assumptions below. The range is a shopping guide, not a guarantee.
Planning estimate. A licensed contractor must confirm the final size, installation and local code requirements. Electrical work needs a licensed electrician.
Adjust your loads
The calculation, step by step.
Concurrent running watts + largest incremental startup watts; selected headroom; round up to 0.5 kW.
| Input or step | Calculated amount | Startup |
|---|---|---|
| Refrigerator / freezer x 1 | 700 W running | 2,900 W total start each |
| Second refrigerator / freezer x 1 | 700 W running | 2,900 W total start each |
| Lighting (10 x 60 W) x 1 | 600 W running | 600 W total start each |
| Gas furnace fan (1/2 hp) x 1 | 800 W running | 3,150 W total start each |
| Central AC (24,000 BTU/h) x 2 | 7,600 W running | 8,750 W total start each |
| Microwave (1,000 W) x 1 | 1,000 W running | 1,000 W total start each |
Peak before reserve: 16,350 W = 11,400 W running + 4,950 W incremental startup.
What moves this number?
Motor startup
Use the largest total-start requirement minus its own running watts.
Concurrent loads
Add only appliances you plan to use at the same time.
Reserve capacity
25% headroom is a planning preference, not a code requirement.
Your location and assumptions.
Location does not change this model. The selected appliance loads, room conditions or peak usage determine the result.
- Running load: 11,400 W
- Largest incremental surge: 4,950 W
- Selected headroom: 25%
- Shopping category: standby capacity to discuss
- Square footage chooses an example AC count; it does not establish household electrical demand.
What might it cost to run?
Fuel cost = manufacturer fuel use at your actual load × your local fuel price. Rated kW alone cannot predict fuel consumption or outage runtime.
Before you choose equipment.
Startup is assumed one motor at a time. Simultaneous starts can require more. Fuel, altitude, temperature and outlet ratings can reduce usable output. Never operate a generator indoors or backfeed a wall outlet.
This example covers only the displayed loads. Electric heat, cooking, water heating, EV charging and unlisted circuits are excluded unless you add them.
Bring this calculation to your installer. They will check the measured load, manufacturer performance, distribution or connections, installation conditions and local requirements.
A few more sizing questions.
What size generator do I need to run these loads?
The default screen gives 20.5 kW, with a planning range of 16.5–23 kW. Change the inputs above to reflect your actual home. The selected loads total 11,400 running watts, plus 4,950 incremental startup watts.
Does square footage or service amperage determine generator size?
No. Area only selects an illustrative load list here. Your panel rating is a connection limit; simultaneous demand and motor startup determine the required output.
Do I add all starting watts together?
This model assumes one motor starts at a time. It adds the largest extra startup demand to all concurrent running loads. If motors may start together, an electrician must account for their combined surge.
Can a portable generator power my house?
It may serve selected loads when its running and surge ratings, fuel rating, receptacles and each supply leg are sufficient. A licensed electrician must provide suitable transfer equipment. Never connect through a wall outlet.
What changes the 25% reserve?
Reserve is a shopping preference in this calculator. Actual derating, voltage regulation, fuel and manufacturer start capability can require a different margin.
Is this a final equipment size?
No. A licensed contractor must measure your building or actual loads, check manufacturer performance and local code, and confirm the final selection.