Generator Size
Estimate the generator wattage needed for essential loads.
About Generator Size
A generator must cover the largest appliance's startup surge, which can be double its running load. Adding a margin gives a safe continuous rating. Generators rated below the startup surge will stall or trip, so sizing past the peak demand prevents failure exactly when you need power.
Formula
Generator Size
Size = max(running, largest startup x surge) x (1 + margin)
- surge
- Motors briefly draw multiple times their run watts
Worked example
With the default values below, Generator Size returns Worst-Case Demand of 4,400. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Essential running wattsminimum 0 W
- 2,500 W
- Largest startup wattsminimum 0 W
- 2,200 W
- Surge factorminimum 1 x
- 2 x
- Safety marginaccepts 0–50 %
- 20 %
Results
- Worst-Case Demand
- 4,400 W
- Recommended Generator
- 5,280 W
- Recommended Size (kW)
- 5.3 kW
- Headroom
- 880 W
Step by step
- 1Values used
- 2Generator Size
- 3Worst-Case Demand= 4,400 W
- 4Recommended Generator= 5,280 W
- 5Recommended Size (kW)= 5.3 kW
- 6Headroom= 880 W
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Generator Size calculated?
Size = max(running, largest startup x surge) x (1 + margin) A generator must cover the largest appliance's startup surge, which can be double its running load. Adding a margin gives a safe continuous rating.
Why does Generator Size matter?
Generators rated below the startup surge will stall or trip, so sizing past the peak demand prevents failure exactly when you need power.
What values do I need to enter?
This calculator takes 4 inputs: Essential running watts, Largest startup watts, Surge factor, Safety margin. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.