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This generator size calculator answers the question “what size generator do I need?” Tick the appliances you want to run during a power outage, add your lights, and it totals the running watts, adds the largest motor start-up surge and recommends a generator size with safe headroom.
How to Use the Generator Size Calculator
- Tick every appliance you want to run at the same time.
- Enter the total wattage of the light bulbs you will use.
- Add any other loads in running watts, using the label on the appliance.
- Read the recommended generator size and check whether you need a 240 V outlet.
Running Watts vs Starting Watts
Running (rated) watts are the power an appliance draws while it runs. Starting (surge) watts are the brief extra power a motor needs to start, often two to three times its running watts. Refrigerators, sump pumps, furnace fans and air conditioners all have a start-up surge.
Generators list both a running and a starting rating. Your generator must cover all running loads plus the largest single surge, because motors rarely start at exactly the same moment.
How the Calculation Works
Peak watts = Total running watts + Largest (starting − running) surge
Recommended running capacity = Total running watts ÷ 0.8, rounded up to the next 500 W
Running a generator at no more than about 80% of its rated output keeps it from overloading, runs cooler and leaves room for an extra load. The calculator recommends the larger of the headroom figure and the peak starting watts.
Appliance Wattage Chart
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator or freezer (ENERGY STAR) | 132–192 | 1,200 |
| Sump pump, ⅓ hp | 800 | 1,300 |
| Sump pump, ½ hp | 1,050 | 2,150 |
| Furnace fan, ½ hp (gas or oil) | 875 | 2,350 |
| Window AC, 10,000 BTU | 1,500 | 2,200 |
| Microwave, 1,000 W | 1,500 | 1,500 |
| Electric water heater | 4,500 | 4,500 |
| Washing machine | 1,200 | 1,200 |
| Electric clothes dryer | 5,400 | 6,750 |
| Electric range, 8-in element | 2,100 | 2,100 |
| Dishwasher | 216 | 540 |
| Coffee maker, 4 cup | 600 | 600 |
| Garage door opener | 720 | 1,420 |
| Flat-screen TV, 46 in | 190 | 190 |
| Desktop computer | 600–800 | 600–800 |
| Laptop | 200–250 | 200–250 |
Figures are from Honda’s generator wattage estimation guide. Your own appliances may differ, so check the data plate where you can.
Example
Powering a refrigerator, ½ hp sump pump, furnace fan, microwave, coffee maker, TV and 300 W of lights needs about 4,700 running watts. The furnace fan adds the largest start-up surge (1,475 W), so the peak is about 6,200 W. With 20% headroom, a generator of about 6,500 W is the right size.
Generator Sizes at a Glance
| Generator size | What it typically runs |
|---|---|
| 2,000–3,500 W (inverter) | Fridge, lights, phone charging, TV, small appliances |
| 5,000–7,500 W (portable) | Essentials plus a sump pump, furnace fan and microwave |
| 8,000–12,000 W (large portable) | Most of a house except central AC, electric range and dryer at the same time |
| 14,000–26,000 W (standby) | Whole-home backup, including central air conditioning |
Safety Tips
- Never run a generator indoors, in a garage or near open windows: carbon monoxide can kill.
- Connect to house circuits only through a transfer switch or interlock installed by an electrician.
- Use heavy-duty outdoor extension cords rated for the load.
- Start the largest motor first, then add smaller loads.
Sources
Frequently Asked Questions
What size generator do I need to run a house?
Most homes can run essentials such as a fridge, sump pump, furnace fan, lights and a microwave on a 5,000–7,500 W generator. Running central air or an electric range and dryer needs a 14,000 W or larger standby unit.
How many watts does a refrigerator need on a generator?
A modern ENERGY STAR refrigerator uses about 130–200 running watts but needs about 1,200 starting watts for its compressor.
Will a 3,500 watt generator run a refrigerator?
Yes. A 3,500 W generator can run a refrigerator plus lights, a TV and phone chargers, but not a sump pump and microwave at the same time.
What is the difference between running and starting watts?
Running watts are the continuous power an appliance uses. Starting watts are the short burst a motor needs to start, often two to three times higher.
Can I plug a generator into a wall outlet?
No. Backfeeding through an outlet is dangerous and illegal in most places. Use a transfer switch or interlock installed by a licensed electrician.
Read the guide: What Size Generator Do I Need to Run My House?
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