01
Formula
Watts = total running − largest running + largest starting.
Kilowatts = watts ÷ 1,000. The generator sizing calculator assumes one motor starts while the other loads are already on.
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Generator size calculator for running and starting watts
This generator size calculator adds every running watt, then swaps the largest motor's running watts for its starting watts. The generator sizing calculator is that same total. It does not apply a manufacturer derate for altitude. Add margin by raising the watts you type.
Generator watts
8,000W
Running total5,000 W
Starting bump3,000 W
Kilowatts8.00
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
Size for the moment the largest motor starts, with everything else already running.
Required watts equal total running watts minus the largest motor's running watts plus that motor's starting watts. Five thousand running watts, with a 1,500 watt motor that starts at 4,500 watts, needs 8,000 watts. That is 8.00 kilowatts. The bump is 3,000 watts, which is the starting watts minus the running watts of that one motor. If two large motors can start together, this single-motor swap is not enough. Add the second bump into the starting field or into the running total. Nameplate running watts and a published starting multiplier are better inputs than a guess. This page will not invent the multiplier.
02
Example
5,000 running watts, a 1,500 watt motor, 4,500 starting watts.
The generator size calculator shows 8,000 watts. The starting bump is 3,000 watts. If the motor's starting watts equal its running watts, the result stays at the running total.
03
Tips
Do not use starting watts for every load.
Lights and heaters do not have a start surge in this model. Only the largest motor is swapped. A well pump and an air compressor that might start together should be treated as one combined surge if that can happen.
04
Equipment
The generator is the machine.
Dengaro arranges financing for a generator. The watt total is not a loan amount. Fuel use for a generator you already own is on the fuel consumption calculator.
Important questions
Know what to expect.
Should I add 25 percent for margin?+
If you want margin, add it to the watts before you read the result, or pick the next size up from what the page shows. The formula itself does not hide a margin.
What if I do not know starting watts?+
Many motors are marked with a starting kVA or a code letter. Without that, do not pretend the running watts are enough. Leave a note and ask the motor supplier. This page will not guess.
Is this the same as the load calculator?+
The load calculator adds watts and a continuous adder for a feeder. This generator size calculator is about the start surge a generator has to cover.
Does altitude change the result?+
Not here. A generator's available watts fall as the air gets thinner. Derate the machine, not this arithmetic, when the site is high.
Financing built into the sale