Energy
Generator Size Calculator
Estimate generator capacity from total running load, power factor and the starting demand of the largest load.
Include all loads expected to operate at the same time, including the largest starting load.
Running power of the largest motor or other load that creates a significant starting surge.
Ratio of starting apparent demand to normal running apparent demand. Use manufacturer starting data where available.
%
Optional allowance for load variation and future capacity.
Recommended Generator Capacity
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Preliminary generator capacity based on coincident running and starting demand.
Running apparent load
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Largest load running demand
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Largest load starting demand
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Peak starting demand
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Capacity margin
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Recommended capacity
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Equivalent kW at entered PF
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Method:
Running apparent load:
Srun =
Prun ÷ PF
Largest load running apparent demand:
SL =
PL ÷ PF
Estimated largest-load starting demand:
Sstart =
SL × starting multiplier
Because the largest load is already included in the total running load, the peak starting demand is:
Speak =
Srun +
SL ×
(starting multiplier − 1)
Recommended generator capacity:
Sgen =
Speak ×
(1 + margin)
This is a preliminary sizing method. Actual generator sizing for motors and nonlinear loads can also depend on motor starting current, starting power factor, voltage dip limits, alternator reactance, starting method, load sequencing, harmonics and generator manufacturer performance data.
A motor’s locked-rotor current multiple should not automatically be treated as an identical kVA multiplier unless the applicable starting data supports that assumption.