Generator Wattage Calculator
Select your appliances to find the right generator wattage.
Your appliances
Running W = normal draw · Starting W = motor startup surge · Contruns 3+ hours (NEC continuous load) · Non-Contshort duty
1. Scenario A: (Continuous × 125%) + All Non-Continuous
2. Scenario B: All Running + Largest Startup Surge
3. Take the larger number
4. ÷ 0.8 (80% generator capacity)
5. Round up to nearest 500W
Formula: MAX(A, B) ÷ 0.8 → Round up 500W
Continuous loads × 125% (NEC safety margin)
+ All non-continuous loads at 100%
Example: Fridge + lights + TV + microwave all on at once
Total running watts + Largest motor's startup surge
Example: AC compressor kicks on while fridge, lights, TV are already running
Generator Selection Guide
Based on NEC calculation, you need a generator rated at 3,500W or higher. Check the generator's continuous rated watts (not peak/surge). Consider fuel type: gasoline, propane, diesel, or natural gas.
Fuel Cost Estimate
galCalculation breakdown
PickMyWatt.com
Generator Wattage Calculator
Selected Appliances
Calculated with PickMyWatt.com
NEC Article 220 Compliant
Data Sources: Energy Star, DOE, Manufacturer specs (Generac, Honda, Champion, Kohler)
What Size Generator Do I Need?
Our calculator uses the NEC (National Electrical Code) Article 220 methodology to determine the right generator size for your home. This is the same standard used by licensed electricians across the United States.
How Our Generator Calculator Works
Our calculation follows these steps based on NEC Article 220:
- Identify continuous loads — appliances that run 3+ hours (refrigerator, lights, HVAC, water heater, pumps, WiFi, medical equipment)
- Identify non-continuous loads — appliances that run less than 3 hours (microwave, coffee maker, hair dryer, tools, laundry)
- Sum continuous loads at full wattage
- Add only the largest non-continuous load — we assume you won't use the microwave, toaster, and hair dryer simultaneously
- Apply the 80% rule per NEC 210.20(A) — divide by 0.8 to get generator capacity
This approach is more accurate than simply adding all appliances together, because it accounts for the fact that not all appliances run at the same time.
Understanding Generator Load Types
The National Electrical Code (NEC) defines two types of electrical loads:
| Load Type | NEC Definition | Examples |
|---|---|---|
| Continuous Load | Maximum current expected to continue for 3 hours or more (NEC Article 100) | Refrigerator, lights, HVAC, water heater, sump pump, well pump, pool pump, WiFi, security system, medical equipment |
| Non-Continuous Load | Any load not expected to continue for 3 hours | Microwave, coffee maker, toaster, hair dryer, washing machine, dryer, dishwasher, power tools, garage door |
Why We Add a Safety Margin
Per NEC 210.20(A), continuous loads should not exceed 80% of a circuit's (or generator's) rated capacity. This means:
- A 5,000W generator can safely handle 4,000W of continuous load
- A 10,000W generator can safely handle 8,000W of continuous load
- The 20% headroom prevents overheating and extends equipment life
Generator Size Guide by Home Size
| Home Size | Typical Loads | Generator Size |
|---|---|---|
| Apartment | Fridge, lights, fan, phone | 2,000-3,500W |
| 1,500 sq ft | + window AC, TV | 5,000-7,500W |
| 2,500+ sq ft | + central AC, water heater | 10,000-15,000W |
| Whole House | All appliances | 15,000-25,000W |
Frequently Asked Questions
What size generator do I need for a 2,000 sq ft house?
For essentials (fridge, lights, window AC, TV): 5,000-7,500W. For whole-house with central AC: 10,000-15,000W.
How many watts does a refrigerator use?
Running: 150-400W. Starting: 600-1,200W. Energy Star models use less.
Why not just add up all my appliances?
Because you don't run all appliances at the same time. NEC Article 220 accounts for this by using demand factors. Our calculator uses the largest non-continuous load plus all continuous loads, which is the industry standard approach.
What size generator for a window air conditioner?
5,000 BTU: 500W. 10,000 BTU: 1,200W. 15,000 BTU: 1,800W.
How much does it cost to run a generator per day?
A 5,000W gas generator at 60% load for 8 hours: ~$15-20/day in fuel.
Can I run my whole house on a generator?
Yes, with a properly sized 15,000-25,000W standby generator and a transfer switch installed by a licensed electrician. See our load calculator to check your specific needs.
How Much Fuel Does a Generator Use?
Our fuel cost estimates use industry-standard consumption rates based on manufacturer specifications:
| Fuel Type | Consumption Rate | Source |
|---|---|---|
| Gasoline | 0.075 gal/kW/hr | Generac, Honda |
| Diesel | 0.05 gal/kW/hr | Cummins, Caterpillar |
| Propane (LPG) | 0.1 gal/kW/hr | Generac, Champion |
| Natural Gas | 12 cu ft/kW/hr | US EIA |
Note: Actual consumption varies by generator model, load factor, altitude, and temperature. The Average Load is calculated as Base Load × Diversity Factor. Base Load is the ratio of your selected continuous appliance watts to generator watts. The Diversity Factor (default 75%) reflects that not all appliances run simultaneously at full power — per Caterpillar / ISO 8528, residential standby generators typically operate at 50–75% average load factor. You can adjust the Diversity Factor slider to match your usage pattern.
References & Sources
- NFPA 70 - National Electrical Code (NEC)
- NEC Article 100 - Definitions (Continuous vs Non-Continuous Load)
- NEC Article 220 - Branch-Circuit, Feeder, and Service Calculations
- NEC 210.20(A) - Continuous Load Sizing (80% Rule)
- DOE Appliance Energy Calculator