Not sure what size generator you need? Add the appliances and equipment you want to power, and this generator sizing calculator adds up the running watts, allows for the extra starting current of the largest motor, adds a safety margin and recommends a standard generator size for a home or a commercial building.
How to size a generator
- List the loads you need during an outage. Use the nameplate where possible; typical values are pre-filled in the calculator.
- Add the running watts of everything that may run at the same time.
- Allow for motor starting. Motors (refrigerators, pumps, air conditioners) draw 2–3 times their running watts for a few seconds when they start. Only the largest motor usually starts at one time, so its extra starting watts are added on top of the running total.
- Add a margin of about 20–25% so the generator does not run near full load continuously.
- Choose the next standard size at or above the result.
Worked examples
Example 1: essential home circuits
A refrigerator (700 W running / 2,200 W starting), sump pump (1,050 / 2,150 W), furnace fan (800 / 2,350 W), LED lights (500 W), TV and electronics (500 W) and a microwave (1,000 W) total 4.55 kW running. The largest extra starting load is the furnace fan (1,550 W), giving a 6.1 kW peak. With a 25% margin the requirement is 6.1 kW, so a 7.5 kW generator covers it.
Example 2: adding a 3-ton central air conditioner
Adding a 3-ton AC (3,800 W running / about 11,400 W starting) raises the running load to 8.35 kW, and the AC start pushes the peak to 15.95 kW. The calculator recommends a 16 kW generator. A soft starter on the AC can cut its starting watts by roughly half and allow a smaller generator.
Example 3: commercial building
For a 120 kW three-phase load with a 25% margin, the requirement is 150 kW, so a 150 kW (187.5 kVA at 0.8 PF) generator is suitable. Use our kVA calculator to convert to amps and size the transfer switch.
Typical running and starting watts
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Sump pump (1/2 hp) | 1,050 | 2,150 |
| Well pump (1 hp) | 2,000 | 4,000 |
| Furnace fan (1/2 hp) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,800 | 11,400 |
| Electric water heater | 4,500 | 4,500 |
| Washing machine | 1,150 | 2,250 |
| Electric dryer | 5,400 | 6,750 |
| Microwave | 1,000 | 1,000 |
These are typical figures; models vary widely. Check the nameplate or the manufacturer’s data for accurate sizing.
Installation notes
- A permanently installed standby generator needs a transfer switch so it can never back-feed the utility grid (NEC Article 702 for optional standby systems).
- Load-management modules can shed large loads such as AC units or water heaters, letting a smaller generator serve the whole house.
- Generators are rated in kW at a power factor (usually 0.8 for three-phase units); the kVA rating is kW ÷ PF.
- Read our generator testing step-by-step guide for commissioning checks.
Frequently asked questions
What size generator do I need to run a house?
Most homes need 7.5–10 kW to run essentials such as the refrigerator, sump pump, furnace fan, lights and electronics. Running a central air conditioner or electric water heater as well typically requires 16–22 kW or more.
What is the difference between running watts and starting watts?
Running watts are the continuous power an appliance uses. Starting (surge) watts are the extra power a motor needs for a few seconds when it starts, often two to three times its running watts.
Will a 22 kW generator run a whole house?
A 22 kW generator can run most average homes including one central air conditioner, provided large electric loads such as dryers, ranges and water heaters are not all running at the same time. Add up your loads with the calculator to check.
How much bigger should a generator be than the load?
Allow about 20–25% above the calculated load so the generator is not running at full capacity continuously, and make sure it can handle the starting surge of the largest motor.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.
