What Is the Generator Wattage Calculator?
This calculator helps you choose a generator that is big enough to power all your appliances at once. Motor-driven devices like refrigerators, air conditioners, and pumps briefly draw a large "starting" (surge) wattage when they switch on, on top of their normal "running" wattage. A correctly sized generator must supply every device's running watts continuously, plus enough extra headroom to absorb the single largest surge that occurs at startup.
How to Use It
Add up the running watts of everything you intend to power and enter that as Total Running Watts. Then find the device with the biggest starting surge, take only its extra surge wattage (starting watts above its running watts), and enter that as Largest Starting (Surge) Watts. The calculator returns the minimum generator capacity required, plus a recommended size with a 20% safety margin.
The Formula Explained
Required wattage = sum of running watts + the maximum starting surge. We only count the single largest surge because appliances rarely start at exactly the same instant, so the worst-case load is everything running plus the biggest one kicking on. The recommended figure multiplies the requirement by 1.2 so the generator is not constantly maxed out, which extends its life and efficiency.
$$\text{Recommended} = \left(\text{Running Watts} + \text{Surge Watts}\right) \times 1.2$$
Worked Example
Suppose your running load totals 3,000 W and your refrigerator needs an extra 1,500 W to start. Required = \(3{,}000 + 1{,}500 = 4{,}500\ \text{W}\). With 20% headroom, the recommended generator is \(4{,}500 \times 1.2 = 5{,}400\ \text{W}\). A 5,500 W generator would comfortably cover this.
FAQ
Do I add up all the surge watts? No — use only the single largest surge, since devices rarely start simultaneously.
Why add 20% headroom? Running a generator near 100% load constantly causes overheating and shortens its lifespan. Headroom provides a safety buffer.
What is the difference between running and starting watts? Running watts power a device continuously; starting watts are the short surge needed to spin up a motor or compressor.
