How to Use the Generator Capacity Calculator
When sizing a generator for camping or backup power, running watts alone aren't enough. Motor-driven appliances like refrigerators, air conditioners, and well pumps briefly draw far more power the instant they switch on — that spike is called starting or surge watts. This calculator takes your total running watts and expected surge watts, then recommends a generator size based on whichever requirement (running with a 20% margin, or surge) is higher.
If you don't know your surge wattage, you can leave it blank — the calculator estimates it as 1.5x your running load. If you have several motor-driven appliances, check each device's label for a more accurate figure, since actual surge can run higher.
Sizing a generator to exactly match your running load risks a shutdown when appliances start simultaneously. As a rough guide: 1-3kW suits basic camping needs, 5-7kW covers a small home backup setup, and 10kW or more is common when running several large appliances at once.
Frequently Asked Questions
Running watts are what a device draws continuously once it's on. Starting (surge) watts are the brief spike motor-driven appliances like refrigerators or A/C units pull the instant they switch on. A generator needs to handle whichever value is higher.
Sizing a generator to exactly match your running load risks shutdown when multiple appliances start at once. Adding at least a 20% margin above the running load keeps the generator from overloading.