Generator Watt Calculator
Calculate total running and starting (surge) wattage to determine the right generator size for your home, jobsite, or emergency backup.
Calculate Generator Size
Enter total continuous running load and starting surge load to determine your required generator size.
How to Size a Generator
Generator sizing requires calculating both running watts (continuous) and starting watts (surge). Running watts is the constant power each appliance needs. Starting watts is the brief power spike when motors start — typically 2-3× running watts for compressors and pumps.
Common Appliance Running & Starting Watts
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150-400 W | 1,200-2,000 W |
| Sump Pump (⅓ HP) | 800 W | 1,300 W |
| Window AC (10,000 BTU) | 1,200 W | 3,600 W |
| Central AC (3 Ton) | 3,000 W | 6,000 W |
| Furnace Fan | 700 W | 2,100 W |
| Well Pump (½ HP) | 1,000 W | 2,100 W |
| Microwave | 1,000 W | 1,000 W |
| LED Lights (10 bulbs) | 100 W | 100 W |
Generator Size Guide
| Generator Size | Best For | Can Power |
|---|---|---|
| 3,000-4,000 W | Emergency basics | Fridge, lights, phone chargers |
| 5,000-7,500 W | Essential circuits | + Well pump, sump pump, window AC |
| 10,000-12,000 W | Most home circuits | + Electric range, central air (small) |
| 15,000-20,000 W | Whole house backup | All circuits including central AC & electric water heater |
Frequently Asked Questions About Generator Sizing
What size generator do I need for my house?
Most homes need a 5,000-7,500 watt generator for essential circuits (fridge, well pump, lights, furnace fan). For whole-house backup including central AC, you need 15,000-20,000 watts. List all appliances you want to run, add running watts, and add the largest starting surge.
What is the difference between running and starting watts?
Running watts is the continuous power an appliance needs during normal operation. Starting watts (surge watts) is the extra power burst needed for 1-3 seconds when electric motors first start. A refrigerator may run at 200W but needs 1,200W to start its compressor.
Can I run my AC on a generator?
Yes, but you need a large enough generator. A window AC (5,000 BTU) needs ~1,800W starting. A central AC (3-ton) needs ~6,000W starting. Ensure your generator's starting wattage exceeds the AC's starting requirement.
What happens if I overload a generator?
Overloading trips the circuit breaker, causes voltage drops, or damages the generator and connected equipment. Always size your generator with a 20-25% safety margin above calculated peak load. Never exceed the rated wattage.
Do I need a transfer switch?
Yes, a transfer switch is required by the NEC to connect a generator to your home's electrical panel. It prevents backfeeding electricity into the utility grid (which is dangerous to line workers) and allows you to power hard-wired circuits like furnaces and well pumps.