About the Generator Wattage Calculator
This calculator sizes a generator using the standard "running plus surge" method that appliance and generator manufacturers use for portable and standby units. Instead of just adding up nameplate wattages, it accounts for the extra current that motor-driven appliances draw for a second or two when they start.
The formula
Required starting wattage = Total running watts (of everything you plan to run at once) + (Highest starting watts among those appliances − that same appliance's running watts).
The logic: when your highest-surge appliance switches on, every other appliance you listed is already running at its steady-state (running) wattage. Only that one appliance briefly spikes above its own running watts — up to its starting-watts rating — so you only need to add the difference between its starting and running watts on top of the running total, not its full starting wattage a second time.
The calculator then applies a safety margin, commonly 20-25%, on top of that required starting wattage to arrive at a recommended generator size. The margin covers voltage sag under load, appliances you may have missed, and de-rating for altitude, heat, or fuel type.
How to gather your inputs
- Total running watts: add up the running (rated) wattage of every appliance you want powered at the same time. This is usually printed on a nameplate or in the manual, in watts (or amps × volts if only amps are listed).
- Highest starting watts: among those appliances, find the one with the largest starting/surge rating — typically a refrigerator, air conditioner, well pump, sump pump, or power tool with an electric motor. Non-motor loads like lights and electronics have no meaningful surge (starting watts ≈ running watts).
- That appliance's running watts: the same appliance's steady-state wattage, already counted inside your running total.
- Safety margin: a percentage buffer above the calculated peak; 20-25% is a common starting point, higher if you expect to add loads later.
Practical context
This method sizes for one motor starting at a time. If you might start two large motors simultaneously (for example, an AC unit and a well pump at the same instant), treat their combined starting-minus-running delta as the surge figure instead of just the single largest one. Always check a specific generator's continuous and surge wattage ratings against your result, and consult an electrician for permanent standby or whole-house installations.