Generator Wattage Calculator

About the Generator Wattage Calculator

The Generator Wattage Calculator helps you determine the right generator size by adding up the power requirements of all the appliances and devices you need to run. Every appliance has two wattage ratings: starting (surge) watts and running (continuous) watts. Your generator must handle both to operate reliably.

Undersizing a generator causes circuit breaker trips, unstable power, and potential equipment damage. Oversizing wastes fuel and money. This calculator lists common household and commercial appliances with their starting and running wattage, lets you build a custom load list, and recommends the appropriate generator size with a safety margin.

Whether you're preparing for power outages, equipping a job site, planning an outdoor event, or going off-grid, accurate wattage planning ensures your generator can handle the full electrical load without straining.

When This Page Helps

Generator sizing is easiest to get wrong when startup surges are ignored. This calculator keeps the running load, surge load, and safety margin in the same result so you can choose a generator that will actually start everything you care about.

How to Use the Inputs

1. Select appliances from the common presets or add custom items
2. Enter the quantity of each appliance you plan to run
3. The calculator totals both starting and running watts
4. View the recommended generator size with a 20% safety margin
5. Check the load breakdown table for individual contributions
6. Adjust quantities and see totals update in real-time
7. Compare recommended generator sizes from the reference table

Example Calculation

Tips & Best Practices

• Stagger appliance startups to avoid simultaneous surge loads
• Motor-driven appliances (fridge, pump, AC) have the highest starting surges
• LED lights use 75% less than incandescent — great for generator economy
• Never run a generator at more than 80% continuous load for longevity
• Consider an inverter generator for sensitive electronics (laptops, phones)
• Always run generators outdoors — carbon monoxide kills in enclosed spaces

Understanding Generator Ratings

Generators have two ratings: peak (starting) watts and rated (running) watts. Peak watts handle brief surges when motors start. Rated watts is the continuous output. A "5,000W generator" typically means 5,000 running watts and 6,250 peak watts. Always check both numbers when purchasing.

Common Household Power Requirements

Essential items during an outage: Refrigerator (200W run, 1200W start), Furnace Fan (700W run, 2350W start), Sump Pump (800W run, 2150W start), LED Lights (10W each), Phone Charger (25W), and WiFi Router (15W). These total about 2,000W running with a 3,550W peak — a 3,500W generator can handle essentials if you stagger startups.

Generator Types and Sizes

Portable generators (1,000-10,000W) are affordable and mobile but require manual startup. Inverter generators (1,000-4,000W) produce cleaner power for electronics but cost more. Standby generators (7,000-50,000W) start automatically during outages and power entire homes. Choose based on your load calculation, budget, and whether you need automatic backup.

Frequently Asked Questions

• What's the difference between starting and running watts?

  Starting (surge) watts is the brief spike of power needed to start a motor or compressor. Running (continuous) watts is the steady-state power draw. Starting watts are typically 2-3× higher.

• Do I add up all the starting watts?

  No — you add all running watts plus the highest single starting watt difference. Appliances don't all start simultaneously, so you account for the largest surge on top of the steady load.

• What size generator do most homes need?

  A 5,000-7,500W generator covers essentials (fridge, furnace, lights, well pump). A 10,000-12,000W generator runs most home circuits. A 20,000W+ whole-house generator powers everything.

• Can I run my AC on a portable generator?

  A 5,000 BTU window unit needs about 1,800 starting watts. A central AC needs 3,000-6,000 starting watts. You need a large portable or standby generator for central AC.

• What about power factor?

  Most generators are rated in watts (real power). Some appliances have a low power factor, drawing more apparent power (VA). For safety, the 20% margin accounts for power factor differences.

• Is the 20% safety margin enough?

  A 20% margin handles typical variations. If you're in extreme heat (AC runs harder) or extreme cold (heating loads increase), consider 25-30%.