Generator Size Calculator
Add up running and starting watts for your essential household appliances to determine the right generator size. This calculator uses the sizing methodology published by Generac and Champion Power Equipment.
Generator Sizing Tool
Click common household items below to add them to your load estimate. Values are standard planning estimates from manufacturer charts.
Essentials
Climate
Water
Kitchen
Home Office
Other
Selected Appliances (3)
Refrigerator/Freezer
400W run • 1200W start — Planning estimate. Check nameplate on rear or inside.
LED Lights (5 bulbs)
75W run • 75W start — 8–15W per bulb.
Wi-Fi Router/Modem
20W run • 20W start — 10–20W continuous.
Results
Total Running Load
495 Watts
Largest Additional Surge
+800 Watts
Motors surge on start; only the largest surge is added to continuous load.
Estimated Peak Requirement
1,295 Watts
Practical Planning Range
1,300 – 1,600 W
Includes ~20% margin for future loads and output variation.
How Our Sizing Methodology Works
This calculator follows the sizing methodology published by leading generator manufacturers Generac and Champion Power Equipment:
- 1.Sum Running Watts: Total the continuous wattage required by all appliances you plan to run simultaneously.
- 2.Identify Largest Surge: Appliances with motors (refrigerators, AC units, sump pumps) require extra power to start. The calculator determines the single highest starting surge among your selected items, because you will not start all motorized loads at the exact same instant.
- 3.Add Headroom Margin: We add a practical 20% safety buffer to prevent running the generator near 100% capacity continuously, which reduces engine life and fuel efficiency.
Running Watts vs. Starting Watts
Running (Rated) Watts
Continuous electrical power an appliance consumes during normal operation. Resistive loads like incandescent lights, toasters, and heaters use only running watts with zero starting surge.
Starting (Surge) Watts
The initial 2-to-3-second burst of electric current required to overcome mechanical inertia when an electric motor starts up (compressors, blowers, pumps). Surge can be 2× to 3× continuous wattage.
Generator Wattage Reference Guide
Typical ratings compiled from Champion and Generac reference charts:
| Appliance | Running Watts | Starting Watts | Surge Factor | Source |
|---|---|---|---|---|
| Refrigerator/Freezer | 400 W | 1200 W | 3.0x | Champion Power Equipment |
| LED Lights (5 bulbs) | 75 W | 75 W | None | Champion Power Equipment |
| Wi-Fi Router/Modem | 20 W | 20 W | None | Champion Power Equipment |
| Phone/Laptop Charger | 100 W | 100 W | None | Champion Power Equipment |
| Window AC (10,000 BTU) | 1200 W | 2400 W | 2.0x | Champion Power Equipment |
| Window AC (5,000 BTU) | 600 W | 1200 W | 2.0x | Champion Power Equipment |
| Furnace Blower (1/2 HP) | 800 W | 1600 W | 2.0x | Champion Power Equipment |
| Space Heater | 1500 W | 1500 W | None | Champion Power Equipment |
| Sump Pump (1/2 HP) | 1050 W | 2150 W | 2.0x | Champion Power Equipment |
| Well Pump (1/2 HP) | 1000 W | 2000 W | 2.0x | Champion Power Equipment |
| Microwave | 1200 W | 1200 W | None | Champion Power Equipment |
| Coffee Maker | 1000 W | 1000 W | None | Generac |
| Desktop Computer | 300 W | 300 W | None | Multi-source estimate |
| Laptop | 65 W | 65 W | None | Multi-source estimate |
| Monitor | 40 W | 40 W | None | Multi-source estimate |
| Television (32–55") | 200 W | 200 W | None | Champion Power Equipment |
| Garage Door Opener | 350 W | 875 W | 2.5x | Multi-source estimate |
Critical Safety & Electrical Guidelines
Carbon Monoxide Hazard — Never Run Indoors
Portable generators emit deadly carbon monoxide (CO) gas. Never operate a generator inside a home, garage, basement, crawl space, or near open windows and doors. Position generators at least 20 feet away from exterior openings with exhaust pointed away from the structure.
Transfer Switch Required for Panel Connection
Never plug a generator directly into a standard wall outlet (“backfeeding”). Backfeeding can electrocute utility line workers restoring power. Connecting a generator to your household panel requires a code-compliant transfer switch or interlock kit installed by a licensed electrician.
Frequently Asked Questions
Running (rated) watts are the continuous power an appliance draws during normal operation. Starting (surge) watts are the temporary burst of power needed to start motor-driven equipment like refrigerators, air conditioners, and pumps. Generac states starting wattage is typically two to three times the running figure. Your generator must handle both the total running load and the largest starting surge.
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