What Size Generator Do I Need? A Complete Guide
Choosing the right generator size is one of the most important decisions for emergency power preparedness, backup electricity, or off-grid living. Size your generator too small and it will struggle under load, trip breakers, damage sensitive electronics, or burn out its engine. Size it too large and you will waste money upfront and spend more on fuel every time you use it.
Understanding Generator Wattage: Running vs. Surge
Running Watts (Rated Watts): The continuous power a device draws while operating normally. A refrigerator might run at 150-300 watts while its compressor is cycling.
Surge Watts (Starting Watts): The brief spike of power required to start motor-driven appliances. Electric motors require 2-4 times their running wattage for a fraction of a second when they first start up. Your generator must handle both requirements.
Always size your generator for surge watts, not just running watts. Failing to account for startup surges is the most common generator sizing mistake homeowners make.
How to Calculate Generator Size for Your Home
- List every appliance you need to run during an outage.
- Find running watts for each appliance from the nameplate label.
- Calculate surge watts for motor-driven appliances (2-4x running watts).
- Add total running watts for all appliances combined.
- Note the highest single-appliance surge watts.
- Apply a 20-25% safety margin by dividing total running watts by 0.8.
Worked Example: Medium Home
| Appliance | Running Watts | Surge Watts |
|---|---|---|
| Refrigerator | 150W | 600W |
| Sump Pump (1/2 HP) | 800W | 2,400W |
| Window AC (10K BTU) | 1,200W | 3,600W |
| Furnace Blower | 600W | 1,800W |
| LED Lights (10) | 100W | 100W |
| TV + Computer | 520W | 600W |
| Total | 3,370W | 9,100W peak |
With a 25% safety margin: 3,370 / 0.8 = 4,213W minimum. We recommend a 5,000W generator for this setup.
Whole House Generator Sizing
- Under 1,500 sq ft: 7,500-10,000 watts (7.5-10 kW)
- 1,500-2,500 sq ft: 10,000-16,000 watts (10-16 kW)
- 2,500-4,000 sq ft: 16,000-22,000 watts (16-22 kW)
- 4,000+ sq ft: 22,000-30,000+ watts (22-30+ kW)
Generator Fuel Types Compared
Gasoline: Most widely available, but degrades in 3-12 months without stabilizer. Best for regular use and occasional outages.
Propane: Stores indefinitely, starts reliably in cold weather. About 10-15% less output than gasoline but eliminates fuel-freshness concerns.
Natural Gas: Standby generators on natural gas never run out of fuel -- connected permanently to the utility line. The most cost-effective long-term option.
Diesel: Most fuel-efficient, longest engine life, preferred for commercial use. Diesel has a 1-2 year shelf life vs. 3-12 months for gasoline.
Dual-Fuel: Runs on gasoline or propane, giving flexibility during emergencies. Increasingly popular for home backup power.
Generator Safety Tips
- Never run a generator indoors, in a garage, or near windows. Carbon monoxide is odorless and deadly. Maintain 20-foot clearance from your home.
- Always use a transfer switch. Backfeeding utility lines is illegal, dangerous to lineworkers, and destroys your generator when power returns.
- Install a CO detector near sleeping areas.
- Never refuel a running generator. Let it cool first.
- Use the correct extension cords. 12-gauge minimum for motors and pumps.