Generator Size Calculator
Find the right generator size for your home or job site. Enter your running and surge watts to get the recommended generator capacity.
Calculate Generator Size
Result
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Recommended Size (Watts)
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Recommended Size (kW)
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Peak Demand (before margin)
Common Appliance Wattages
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Central AC (3 ton) | 3,500 W | 10,000 W |
| Window AC (10,000 BTU) | 1,200 W | 2,400 W |
| Refrigerator | 150–400 W | 600–1,200 W |
| Sump pump (1/2 HP) | 800 W | 2,000 W |
| Well pump (1 HP) | 1,000 W | 3,000 W |
| Electric water heater | 4,500 W | 4,500 W |
| Microwave | 1,000 W | 1,000 W |
| Lights (LED, 10 bulbs) | 120 W | 120 W |
| TV + cable box | 200 W | 200 W |
| Space heater | 1,500 W | 1,500 W |
Example Calculation
Powering essential circuits: refrigerator (400W), window AC (1,200W), lights (200W), and a sump pump (800W, 2,000W surge).
Total running watts = 400 + 1,200 + 200 + 800 = 2,600W
Highest surge = sump pump = 2,000W surge
Peak demand = max(running, surge) = 2,600W
With 20% margin = 2,600 × 1.20 = 3,120W recommended
Choose a 3,500W generator (next standard size up)
Highest surge = sump pump = 2,000W surge
Peak demand = max(running, surge) = 2,600W
With 20% margin = 2,600 × 1.20 = 3,120W recommended
Choose a 3,500W generator (next standard size up)
Frequently Asked Questions
What size generator do I need for my home?
Add up the running watts of all appliances you need simultaneously, identify the highest surge wattage, take the larger of the two, and add a 20-25% safety margin. Most homes need 5,000–12,000 watts for essential circuits. A whole-home generator is typically 15,000–22,000 watts.
What is the difference between running watts and starting watts?
Running watts is the steady power an appliance draws during operation. Starting (surge) watts is the brief spike of power needed when a motor starts — typically 2–3× the running watts. Air conditioners, refrigerators, and pumps all have significant surge requirements. Your generator must handle the surge without overloading.
Why add a safety margin to generator sizing?
Running a generator at 100% capacity continuously shortens its life and risks overloading. A 20–25% safety margin keeps the engine cool, improves fuel economy, and allows for unexpected additional loads. Generator manufacturers rate their units at peak — continuous operation should be at 80% or less.