Generator Wattage Calculator
Size emergency standby and portable generators by calculating total running watts and motor surge starting watts with safety margins.
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Running Watts vs. Starting (Surge) Watts
Generators have two distinct power ratings: Running Watts (the continuous electrical output a generator can deliver) and Starting / Surge Watts (the short burst of maximum power delivered for 2 to 3 seconds to start electric motors).
The Locked Rotor Amps (LRA) Rule
Electric induction motors (found in refrigerators, air conditioners, well pumps, and compressors) require 2x to 4x their running wattage when the motor starts from a standstill.
How Generator Capacity is Calculated
To prevent generator stall, the peak starting wattage is calculated by summing the running wattage of all connected devices, plus the single largest motor starting surge differential ($Starting - Running$).
Frequently Asked Questions
Can a generator run at 100% capacity continuously?
No. Running a generator at 100% continuous load causes premature engine wear, overheating, and frequent breaker trips. It is strongly recommended to operate at 70% to 80% of rated capacity.
Do inverter generators handle starting surges better?
Inverter generators produce cleaner power (< 3% THD) safe for sensitive electronics. However, conventional open-frame generators often have slightly higher short-term motor-starting surge headroom due to heavy rotor momentum.
How many watts do I need to power a house during an outage?
Basic essentials (fridge, lights, router, TV, sump pump) require 3,500W to 5,000W. If running a well pump or central AC, a 7,500W to 12,000W generator with a 30A or 50A transfer switch is typically required.