Generator Size Calculator
Select the appliances you need to power during an outage. This calculator accounts for both running watts and starting watts (surge) to recommend the right generator size for your home.
Check the appliances you want to power. Adjust wattage if your appliance differs.
| Appliance | Running W | Starting W |
|---|
What is Generator Size?
Generator size refers to the electrical power output capacity of a generator, measured in watts (W) or kilowatts (kW). Choosing the correct generator size is critical: an undersized generator will overload and shut down (or in older models, sustain damage), while an oversized generator wastes fuel and purchase cost. Residential generators range from 2,000W portable units suitable for camping and small power outages, to 20,000W+ standby generators that can power an entire home including air conditioning and electric appliances continuously.
A key distinction in generator sizing is between running watts (also called rated watts) and starting watts (also called surge watts). Running watts is the continuous power the generator can sustain; starting watts is the brief peak power available for a fraction of a second to start motor-driven appliances. Electric motors — in refrigerators, air conditioners, well pumps, and power tools — require 2–3× their running wattage at startup due to the surge current needed to overcome motor inertia. A refrigerator that runs on 150W may require a 400–600W startup surge, and the generator must accommodate this peak without tripping.
Generator sizing follows a simple process: list all appliances you need to run simultaneously, sum their running watts, identify the single appliance with the highest starting-watt surge, and add that surge to the sum of all other running watts. Add 20–25% safety headroom to avoid running the generator at its rated maximum capacity continuously, which shortens engine life. This calculator helps you build an appliance load list, automatically accounts for starting surges, and recommends the minimum generator size (running and starting watts) for your scenario.
Starting vs Running Watts Explained
Motor-driven appliances (refrigerators, AC, pumps) require a brief surge of power at startup — 2 to 3 times their running wattage. Your generator must handle this surge, even though it only lasts a fraction of a second.
How the Generator Size Calculator Works
Formula, assumptions, and calculation steps for this solar & energy tool.
Formula Used
Required Wattage = sum of Running Watts of Essential Appliances + Highest Starting Watts
Methodology
Sums the continuous running wattage of appliances to be powered simultaneously, then adds the single highest surge or starting wattage.
Calculation Steps
- Enter wattage, usage time, tariff, battery, or panel assumptions.
- Convert power and time into energy units such as kWh.
- Apply cost, savings, or sizing formulas.
- Show monthly, annual, or per-charge estimates.
Assumptions and Limits
- Weather, shading, tariffs, and equipment losses affect real results.
- Utility net-metering rules vary by location.
- Use installer or utility data for final sizing.
Frequently Asked Questions
Add up the running watts of all appliances you want to power simultaneously. Then find the appliance with the highest starting watt surge (usually a motor: refrigerator, AC, pump). Your generator must handle total running watts plus that highest starting surge. Add 25% safety margin. Result: recommended generator size in watts.
For most homeowners, the refrigerator, sump pump, and heating/cooling system are critical. A well pump is essential if you are on a private well. Medical equipment (oxygen concentrators, CPAP) should be included if applicable. For whole-home backup, include the HVAC system — this typically requires a 10,000W+ standby generator.
Portable generators (1,000–12,000W) are less expensive ($500–3,000), run on gasoline, and must be manually started and refueled. They suit occasional use and partial home coverage. Standby generators (7,000–22,000W+) run on natural gas or propane, start automatically within seconds of an outage, and can power an entire home. They cost $3,000–10,000+ installed.
A 5,000W gasoline generator typically consumes 0.5–0.75 gallons per hour at 50% load. Running 8 hours uses 4–6 gallons. A 10,000W standby generator running on natural gas uses approximately 150–200 cubic feet per hour. Always ensure you have adequate fuel reserves before an anticipated storm or outage.
Real-World Applications
Common Mistakes
Common Appliance Wattage Reference
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 150–400 W | 800–1,200 W |
| Central AC (2.5 ton) | 3,500 W | 7,500–10,000 W |
| Window AC (10,000 BTU) | 1,200 W | 2,200–3,600 W |
| Well Pump (1/2 HP) | 750 W | 1,500–2,000 W |
| Sump Pump (1/3 HP) | 800 W | 1,300–2,500 W |
| Microwave | 1,000 W | 1,000 W (no surge) |
| LED Lighting (10 bulbs) | 90–150 W | 90–150 W |
References
- U.S. Department of Energy. Portable Generator Safety. Energy.gov, 2023.
- Briggs & Stratton. Generator Sizing Guide. Briggs & Stratton, 2024.
- Generac Power Systems. What Size Generator Do I Need? Generac, 2024.
- National Electrical Manufacturers Association. NEMA MG 1 — Motors and Generators. NEMA, 2021.
- National Fire Protection Association. NFPA 70 National Electrical Code Article 702 — Optional Standby Systems. NFPA, 2023.
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