Generator Sizing Calculator
Estimate generator kW needed for your loads.
About the Generator Sizing Calculator
Estimates the generator size (kW) needed from your total running load plus the extra starting surge of motor-driven appliances.
How to use this calculator
- List each load you want to power and note its running watts or amps.
- Add up the total running power, then account for any startup surge from motors or compressors.
- Convert the total to kilowatts by dividing watts by \(1000\).
- Compare the result with a generator rating that is at least as large as your total need.
The formula explained
The calculator estimates required generator size in kilowatts using total wattage, then converts that total to \(\text{kW}\) with \(\text{kW} = \frac{\text{watts}}{1000}\). If you have motor loads, it also helps you include a startup surge so the generator can handle the initial power spike.
- \(\\text{watts}\) = the total power needed by all connected loads, measured in watts
- \(\\text{kW}\) = generator size in kilowatts
- \(\\text{startup surge}\) = extra power needed when a motor or compressor first turns on
- \(\\text{running watts}\) = the power a device uses while operating normally
Step by step method
- Write down each device you want to run, such as lights, a fridge, or a pump, and note its running watts.
- Add the running watts to find the total continuous load.
- Add any startup surge for the largest motor or compressor load, if needed.
- Divide the final watt total by \(1000\) to get the generator size in \(\text{kW}\).
Worked example
Problem. You want to power a refrigerator at \(700\) watts, a set of lights at \(300\) watts, and a microwave at \(1200\) watts. The refrigerator has a startup surge of \(1200\) watts.
- Add the running watts: \(700 + 300 + 1200 = 2200\) watts.
- Include the refrigerator startup surge by adding the extra \(1200 - 700 = 500\) watts, so the peak becomes \(2200 + 500 = 2700\) watts.
- Convert to kilowatts: \(2700 \div 1000 = 2.7\) \(\text{kW}\).
Answer. You need a generator rated for at least \(2.7\) \(\text{kW}\), and choosing a little higher gives extra safety margin.
Tips and common mistakes
- Do not forget startup surge for motors, pumps, refrigerators, and air conditioners, because these can briefly need much more power than their running watts.
- A generator should usually be sized a bit above your calculated total, so it is not constantly working at its limit.
Frequently asked questions
Why account for starting watts?+
Motors (fridges, pumps, AC) briefly draw 2 to 3× their running watts at startup.
Should I add a safety margin?+
Yes, size ~20% above the peak so the generator isn't constantly maxed out.
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