Sizes a standby generator from a staged load schedule: running kVA per step, the
order-dependent cumulative peak demand as motors start, a genset kVA recommendation with a growth
margin, and an optional candidate-genset headroom + transient volt-dip check.
Tender-stage only: paralleling and fuel/tank sizing are out of scope; verify final design with the genset supplier.
For design guidance only. Always verify final design with the genset supplier.
About this generator sizing calculator
This free generator / standby set sizing calculator sizes a diesel standby genset from a staged load schedule. It works out the running kVA per load step, the order-dependent cumulative peak demand as motors start, a recommended genset kVA with a growth margin, and an optional headroom and transient volt-dip check against a candidate machine. The ISO 8528 rating classes (ESP, PRP, LTP, COP) are shown for reference so you can match the duty. It is aimed at UK M&E and electrical designers and contractors sizing standby power at tender stage. Everything runs in your browser. Nothing is uploaded.
How the generator size is calculated
Each row’s running load is kVA = kW ÷ pf. A motor row’s starting kVA is the running kVA × a starting-method multiplier (DOL ≈7×, star-delta ≈2.33×, soft starter ≈3×, VSD/VFD ≈1.5×). Rows are grouped by their step number, and a step’s peak demand = cumulative running kVA through that step + the single largest motor starting surge within it (a staggered-start assumption: only one machine is taken to be mid-start at once). The recommended genset is the worst step’s peak demand × (1 + growth margin). The optional dip check follows NEMA MG1 32.18.5.2: dip% = X′d% × (worst-step increment kVA ÷ candidate genset kVA), compared against your chosen limit.
Frequently asked questions
What growth margin should I apply?
A margin of 20–25% on the worst-step peak demand is the common convention, leaving headroom for future load and for the reduced capability of an aged or de-rated alternator. The default is 25%; adjust it to suit the project and the ISO 8528 rating class you are sizing to.
What does the step number do?
The step number sequences the load-acceptance schedule. Loads on the same step energise together; higher-numbered steps come on later on top of the running load already present. The calculator adds only the largest single motor surge on each step, so grouping big motors onto separate steps reduces the transient peak the genset has to swallow.
Does it size fuel, tanks or paralleling?
No. It sizes a single set on kVA demand and transient dip only, fuel consumption, tank sizing and multi-set paralleling are out of scope. Treat the output as a tender-stage starting point and confirm the final selection, rating class and volt-dip with the genset supplier.