What This Calculator Does
This Generator Sizing Calculator helps you choose a generator that can power all your appliances at once. Motor-driven devices (refrigerators, air conditioners, pumps, power tools) briefly draw a large startup surge — often 2-3 times their running watts — when their motor kicks in. A correctly sized generator must supply the steady running load of everything running together, plus that one big surge.
How to Use It
For each appliance, enter its running watts (the continuous draw) and its surge watts (the momentary startup spike). If a device has no surge, set the surge equal to its running watts. The calculator adds all running watts, then adds the single largest surge above its running level, since only one motor typically starts at a time.
The Formula Explained
The minimum wattage is the total running watts plus the largest startup surge contribution: $$\text{Required} = \sum(\text{running}) + \max(\text{surge} - \text{running})$$ We then suggest a recommended size with 25% headroom so the generator is not constantly maxed out, which extends its life and handles voltage dips.
Worked Example
Suppose you run a fridge (800 W running, 2200 W surge), a window AC (1200 W, 1200 W surge), and a well pump (600 W, 1800 W surge). Total running = \(800 + 1200 + 600 = 2600\) W. Surge contributions are 1400, 0, and 1200 W; the largest is 1400 W. Required = \(2600 + 1400 =\) 4000 W. Recommended with headroom = \(4000 \times 1.25 = 5000\) W.
FAQ
Why only the largest surge, not all of them? Motors rarely start at the exact same instant, so industry practice sizes for the running total plus the single worst-case startup spike.
Running watts vs. surge watts? Running (rated) watts is the continuous draw; surge (starting/peak) watts is the brief inrush when a motor or compressor starts.
Why add 25% headroom? Operating near 100% capacity strains a generator. Headroom improves reliability, efficiency, and lets you add small loads later.
