Generator Size Calculator
Tick everything you want running at the same time. The calculator adds running watts, the biggest startup surge and a 20% safety margin.
Method: Running watts plus the largest motor surge, plus a 20% margin. Appliance wattages from published charts such as Briggs & Stratton's.
Recommended generator
7,500 W
You need a large portable generator, or a small standby unit.
| Running watts | 3,780 W |
| Largest startup surgeSump pump (½ HP) | +1,800 W |
| Peak demand | 5,580 W |
| With 20% safety margin | 6,700 W |
Wattages are typical values. Check the label on your own appliances, and have an electrician install a transfer switch before connecting a generator to your home wiring.
How generator size is calculated
Add up the running watts of everything you want on at the same time. Then add the extra startup surge of the single biggest motor (fridge, AC, pump), since motors rarely all start at once. Finally, add a 20% safety margin and round up to the next common generator size.
Appliance wattages come from published generator wattage charts, such as Briggs & Stratton's, and industry rules of thumb. Your own appliances may differ, so check the nameplate label for exact figures.
Generator size chart
| What you want to power | Typical generator size |
|---|---|
| Fridge, lights, phones | 2,000–3,500 W |
| House essentials during an outage | 5,000–7,500 W |
| 30 amp RV with one AC | 3,500–4,500 W |
| 50 amp RV with two ACs | 7,500–12,000 W |
| 1,500 sq ft house with 2-ton central AC | 12,000–14,000 W |
| Whole house, everything on | 17,500–22,000 W standby |
More generator size guides
- What size generator do I need to run a refrigerator
- What size generator do I need to run my house during an outage
- What size generator to run a 1500 sq ft house
- What size generator do I need for a whole house
- What size generator do I need for a 30 amp RV
- What size generator do I need for a 50 amp RV
- What size generator do I need for a sump pump
- What size generator do I need for a well pump
Frequently asked questions
What size generator do I need to run a house?
To run essentials during an outage (fridge, freezer, furnace fan, sump pump, lights), most homes need 5,000–7,500 watts. To run a whole house with central air, plan on a 17,500–22,000 watt home standby generator.
What size generator do I need for a refrigerator?
A typical refrigerator needs about 700 running watts and about 2,200 watts to start. A 2,000–3,500 watt generator runs it, plus lights and phone chargers.
What is the difference between running watts and starting watts?
Running watts are what an appliance uses continuously. Starting (surge) watts are the brief extra power motors need to start up, often 2–3 times the running watts. A generator must handle your running total plus the largest single surge.
What size generator for a 30 amp RV?
A 30 amp RV can draw up to 3,600 watts. With one rooftop air conditioner, a 3,500–4,500 watt generator is typical.
Why add a safety margin?
Running a generator at 100% load shortens its life and risks tripping its breaker. A 20% margin keeps it running comfortably and leaves room for an extra appliance.