Generator Size Calculator: Running and Starting Watts

How to do it

1. 1
   Step 1: Open Generator Size Calculator: Running and Starting Watts and add your input

   Type your numbers or text, add your file, or pick the options you need. Nothing is uploaded: everything stays in your browser.

2. 2
   Step 2: What Generator Size Calculator: Running and Starting Watts does

   Add up running watts and the largest starting surge to choose a generator size.

3. 3
   Step 3: Read the result and compare

   The result appears at once. Change one value to see how the answer moves, then keep the figure that fits your case.

4. 4
   Step 4: Copy or save it

   Copy the result or download it. For an important decision, confirm it with the official source or an adviser.

About this tool

List each appliance on its own line: name, running watts and the starting multiple (1 for lights and heaters, about 3 for motors). The calculator gives the total running watts, the extra starting load of the biggest motor, the peak and a recommended generator size.

How to size

Add the running watts of everything you will run at once. Motors such as refrigerators, pumps and air conditioners need two to three times their running watts for a moment when they start. Because they do not all start together, add only the largest extra surge to the total.

Margin

The recommended size is the larger of the running total plus a margin (20% by default) and the peak, rounded up to the next 500 watts.

Check the labels

Use the numbers on the appliance plates. For inverter generators the surge rating matters; for conventional generators the running rating matters. Run them within about 80% of the rating for long use.

Worked example

A refrigerator (700 W, ×3), furnace fan (600 W, ×2), lights (300 W), sump pump (800 W, ×3) and microwave (1,000 W) use 3,400 W running with a largest extra surge of 1,600 W, a peak of 5,000 W and a generator of about 5,000 W.

Tips for best results

Check every box before you rely on a result, because one wrong digit changes the whole answer. Use the exact figures from your payslip, bill, certificate or contract, and keep a note of the date you calculated. Rates, thresholds and rules change from time to time, so for tax, fees and legal matters treat the output as an estimate and confirm with the official source named on this page. You can run as many scenarios as you like; nothing is saved or sent, and refreshing the page clears the boxes.