Generator Wattage Calculator
Add up the running and starting watts of your appliances to find the generator size you need for backup power.
%
Peak watts plus your safety margin
- 1
Total running watts
Σ (running × qty) = 0Sum of all appliances' continuous wattage times quantity. - 2
Largest starting surge
1,200Highest single (starting − running) watts across all appliances. - 3
Peak load (W)
0 + 1,200 = 1,200 - 4
Recommended generator size
1,200 × 1.2 = 1,440Includes 20% safety margin.
How does this calculator work?
Total your appliances' running watts, add the biggest single starting surge to get peak watts, then multiply by a safety margin (about 1.2). The result is the minimum continuous generator size you should buy so motors can start without overloading the unit.
Formula
How this is calculated
Enter each appliance on its own line as "name: running watts, starting watts". The first number is the continuous (running) wattage; the optional second number is the starting/surge wattage that motors draw for a fraction of a second when they switch on. Append "x3" to a name to set a quantity, and the calculator multiplies that appliance's running watts accordingly.
The running load is the sum of every appliance's running watts times its quantity — this is the steady power the generator must supply continuously. Only one large motor typically starts at a time, so the peak demand is the total running load plus the single largest starting surge, calculated as max(starting − running) across your list. The recommended size adds a safety margin (20% by default) so the generator is never run flat out, which protects efficiency and lifespan.
Values are in watts (1 kW = 1000 W). The model assumes appliances without a listed starting wattage are purely resistive (no surge), and that motor loads do not all start simultaneously. For very large or simultaneous motor starts, size up; results are a planning estimate, not a substitute for an electrician's load calculation.
Frequently asked questions
Running watts are the continuous power an appliance draws once on. Starting (surge) watts are the brief, higher draw motors need at startup — often 2 to 3 times the running watts for fridges, pumps and air conditioners.
Running a generator at 100% of its rating reduces efficiency, increases heat and shortens its life. A 20% margin keeps it in a comfortable load band and leaves room for an extra device.
Usually not. This calculator assumes only the single largest motor surges at once, which is the standard sizing assumption for staggered startups.
Also known as
TG we-Calculate Editorial Team. (2026). Generator Wattage Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/generator-wattage-calculator
TG we-Calculate Editorial Team. "Generator Wattage Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/generator-wattage-calculator.
TG we-Calculate Editorial Team, "Generator Wattage Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/generator-wattage-calculator
@misc{wecalculate_generator_wattage_calculator, title = {Generator Wattage Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/generator-wattage-calculator}}, year = {2026}, note = {TG we-Calculate} }
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