Generator sizing calculator
Motors draw several times their running current at start-up, and that surge is what really sizes a generator.
%
Generator size
8.25kVA
- Required output
- 6.60 kW
- Peak load including starting surge
- 5500
- Running load in kVA
- 4.38 kVA
- Altitude derating factor Engines lose about 3% of output per 300 m of altitude.
- 1.0000
- Rough fuel use at full load
- 1.98 L/h
Show the working
- Total running load = 3500
- Largest motor load = 1000
- Starting surge multiple = 3
- Power factor = 0.8
- Site altitude = 0
- Headroom = 20
- Peak load including starting surge = 3500 - 1000 + 1000 * 3 = 5500
- derate = 1 - 0 / 100 * 0.03 / 3.048 * 3.048 / 100 * 100 = 1
- Altitude derating factor = max(1 - (0 / 300) * 0.03, 0.5) = 1
- required_watts = 5500 * (1 + 20/100) / 1 = 6600
- Required output = 6600 / 1000 = 6.6
- Generator size = 6.6 / 0.8 = 8.25
- Running load in kVA = 3500 / 1000 / 0.8 = 4.375
- Rough fuel use at full load = 6.6 * 0.3 = 1.98
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- peak load = running load + largest motor x (start multiple - 1)
- kVA = kW / power factor
Only the largest motor is assumed to start at once. If two can start together, add the second surge as well.
Running a generator lightly loaded for long periods glazes the bore on a diesel. Aim for 50 to 80% load.
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