What Is a Generator Size Calculator and How Does It Work?
A generator size calculator adds up the running watts of every appliance you want to power, then adds the largest starting surge, to find the minimum generator output in watts or kVA. Sizing correctly prevents overloaded circuits, tripped breakers, and damaged appliances during a power outage.
The sizing rule used here is: Generator size = max(Running watts × (1 + margin), Running watts + largest starting surge). Running watts is the sum of quantity × rated watts for each appliance. Starting surge is the starting watts minus running watts of the single biggest motor load, because motors rarely all start at the same instant.
According to the U.S. Department of Energy, portable generators should never be run indoors or within 20 feet of a home, and the Consumer Product Safety Commission (CPSC) attributes roughly 70 deaths a year in the United States to carbon monoxide from portable generators. Correct sizing helps, but placement and a properly installed transfer switch matter just as much.
How to Use This Generator Size Calculator
Enter the quantity of each appliance you want to run at the same time. The generator size updates instantly:
- Appliance quantities: Type 1 (or more) next to each appliance. Leave blank or 0 for appliances you will not run at the same time.
- Custom load: Enter running and starting watts from an appliance nameplate for anything not on the list, such as a power tool or an RV air conditioner.
- Safety margin: The headroom above running watts. 20% is a common default, which keeps the generator near 80% load.
Wattage figures are typical North American values. In the UK, EU, and Australia, appliances usually run on 230 V, so the watts are similar but amps are roughly half those at 120 V. Always check the nameplate on your own equipment, since motor starting watts vary widely by model.
How Many Watts Do Common Appliances Need?
Resistive loads such as heaters need the same power to start as to run, while motor loads such as refrigerators and air conditioners need two to three times more at startup. Typical values are below.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Furnace fan (1/2 HP) | 875 | 2,300 |
| Window A/C (10,000 BTU) | 1,200 | 3,600 |
| Central A/C (3 ton) | 3,500 | 7,000 |
| Electric water heater | 4,000 | 4,000 |
| Electric dryer | 5,400 | 6,750 |
A typical 3,500 to 5,500-watt portable generator covers essentials, while the U.S. Energy Information Administration notes the average US home draws about 1.2 kW on average but can peak far higher when air conditioning and electric heating run together.
Frequently Asked Questions
What size generator do I need to run my house?
For essentials only — a refrigerator, sump pump, furnace fan, lights, and electronics — a 3,500 to 4,000-watt generator is usually enough. Whole-home backup including central air conditioning typically needs 7,500 to 12,000 watts. Add up running watts, then add the single largest starting surge.
What can a 5,000-watt generator run?
A 5,000-watt generator can run about 5,000 watts continuously and a short surge above that, typically 6,250 watts. That covers a refrigerator, sump pump, furnace, lights, and a microwave together, but not central air conditioning or an electric dryer at the same time.
What is the difference between running watts and starting watts?
Running watts (rated watts) are the continuous power an appliance draws once it is operating. Starting watts (surge watts) are the brief extra power motors and compressors pull for a second or two at startup, often 2 to 3 times the running figure.
Should I add a safety margin when sizing a generator?
Yes. Add a safety margin of 10 to 25 percent so the generator runs at no more than about 80 percent of its rated output. This improves fuel efficiency, reduces wear, and leaves room for loads you forgot or appliances that cycle on together.