Generator Size Calculator

Calculate generator wattage needed based on appliances. Accounts for starting watts on motor loads.

By Konstantin Iakovlev · Updated April 2026 · Source: DOE

Select Appliances to Power

Use the Generator Size Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

How It Works

Sizing a generator for 2026 means matching its output to the appliances you actually intend to run, and that figure hinges on two kinds of wattage. Get it wrong in either direction and you either overpay for capacity you'll never use or end up with a unit that stalls the moment a compressor kicks on. The tool weighs both running and starting demands, with particular care for motor-driven equipment.

Calculation starts by totaling the running watts of every appliance you select. Because motors rarely start simultaneously, the tool then finds the single highest starting wattage in your list and adds it on top of that running total. The result is the minimum surge wattage your generator has to deliver.

Counting only running watts is where most people go wrong. Loads like refrigerators and air conditioners draw far more power to start than to run, so that surge figure matters. Build in a 10-20% buffer for loads you add later or didn't anticipate, and account for the small stuff too, phone chargers and LED fixtures add up once you string enough of them together.

Example: Home Backup Scenario

  1. 1 Input your appliances: Refrigerator (700 running, 2200 starting watts), Microwave (1500 running, 1500 starting watts), 50-inch Smart TV (120 running, 120 starting watts), 10 LED Light Bulbs (10 watts each, 100 running total).
  2. 2 Calculate: Total running watts = 700 + 1500 + 120 + 100 = 2420 watts. Highest starting wattage (from refrigerator) = 2200 watts. Therefore, required surge wattage = 2420 (running) + 2200 (highest starting) = 4620 watts. Continuous running wattage = 2420 watts.
  3. 3 Result: You need a generator capable of at least 2420 continuous running watts and 4620 surge (starting) watts. A 5000-watt generator would provide a comfortable buffer.
  4. 4 Context: This calculation ensures your generator can handle the initial power spike when a major appliance, like your refrigerator, kicks on, preventing overload and ensuring continuous power for your other devices. Always consult your appliance manuals for exact wattage figures.

Source: DOE · Last updated: April 2026

Frequently Asked Questions

What size generator do I need for my house?
Most homes need a 7,500 to 10,000 watt generator for essentials like the fridge, sump pump, lights, and a few outlets. A whole-house standby generator for central air and all circuits typically requires 20,000 to 25,000 watts.
What is the difference between running watts and starting watts?
Running watts is the continuous power an appliance uses during normal operation. Starting watts (or surge watts) is the extra power needed for the first few seconds when a motor starts, typically 2-3 times the running watts.
Can a generator run a central air conditioner?
Yes, but a 3-ton central AC unit requires about 3,500 running watts and 4,500-6,000 starting watts. You need at least a 7,500-watt generator to run AC along with other basic loads.

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