Generator Size Calculator

Find the right size generator for your power needs

What Size Generator Do I Need?

Use this free calculator to find the right generator size for your home, RV, or job site. Select the appliances you want to power, and we'll calculate your total running watts, peak surge watts, and recommend generator sizes with a 20% safety buffer.

Select your appliances

How to Calculate Generator Size

Choosing the right generator size is essential for reliable backup power. A generator that's too small won't run all your appliances, while one that's too large wastes fuel and money. Here's how to calculate exactly what you need.

Understanding Running Watts vs. Starting Watts

Running watts (also called rated watts) is the continuous power an appliance needs to operate. Starting watts (also called surge watts) is the extra burst of power needed when a motor first starts up. Appliances with motors—like refrigerators, air conditioners, sump pumps, and power tools—typically need 2-3 times their running watts for the first few seconds when starting.

The Generator Sizing Formula

  1. Add up the running watts of all appliances you'll use at the same time
  2. Find the appliance with the highest starting watts
  3. Add that starting wattage to your running total
  4. Add 20% buffer for safety and efficiency

Common Generator Sizes

  • 2,000-3,000W: Camping, tailgating, small power tools
  • 3,000-5,000W: RV, essential home circuits (fridge, lights, phone chargers)
  • 5,000-7,500W: Most home backup needs (fridge, sump pump, window AC, lights)
  • 7,500-10,000W: Larger homes, multiple air conditioners, well pumps
  • 10,000W+: Whole-house backup, central air conditioning

Tips for Accurate Calculations

  • Check appliance labels for actual wattage—estimates vary widely
  • Not all appliances run simultaneously—calculate based on realistic usage
  • Consider which appliances are essential vs. nice-to-have during outages
  • Factor in climate—heating and cooling are often the biggest loads