---
title: "DG set sizing: why motor starting usually governs"
description: "Two calculations decide a generator rating and most quotations do only one. A worked example where the starter choice halves the frame size."
date: "2026-04-23"
author: "Divakar B"
source: "https://energycalchq.com/blog/dg-set-sizing-motor-starting"
license: "© 2026 Divakar B. Quote with attribution to https://energycalchq.com/blog/dg-set-sizing-motor-starting"
---

Ask for a DG quotation and you will usually be asked one question: what is the
load? Add up the kW, divide by power factor, add a margin, pick the next
standard rating. That covers the set running steadily, and it says nothing about
the moment somebody presses the start button on the largest motor — which is
when generators actually misbehave.

There are two calculations. The rating you need is the larger of them, and on
plants with one big motor it is usually the second.

## Calculation one: the running load

Straightforward, and the one everybody does.

```
Running kW  = connected load × demand factor
Running kVA = running kW / power factor
Required    = running kVA × (1 + future margin)
```

Take a plant with 120 kW connected, a demand factor of 0.7 and a power factor of
0.8:

```
84 kW running → 105 kVA → 121 kVA with 15 % margin
```

Two things to be careful about. The demand factor should come from measurement
if the plant exists — see [connected load, maximum demand and
diversity](/blog/connected-load-maximum-demand-diversity). And the power factor
matters more on a generator than on a utility supply, because an alternator is
rated in kVA at 0.8 power factor and hits its current limit long before the
engine runs out of kW.

## Calculation two: the step load

When a large motor starts, it draws a very large current at a very poor power
factor for a second or two. The alternator has to supply that transient
*without* the voltage collapsing far enough to drop out the contactors — which
is around 25 % dip in practice.

A brushless alternator will hold roughly **2.5 times its continuous rating** for
that period within an acceptable dip. So:

```
Step requirement = (other load already running, kVA)
                 + (starting kVA of the largest motor / 2.5)
```

Starting kVA depends entirely on how the motor is started:

| Starting method | kVA per kW of motor |
|---|---|
| Direct on line | 8.5 |
| Star–delta | 2.9 |
| Soft starter | 3.9 |
| VFD | 1.5 |

Continue the example with a 45 kW motor as the largest single load:

```
Direct on line:
  Starting kVA   = 45 × 8.5 = 382 kVA
  Other load     = 105 − (45 / 0.8) = 49 kVA
  Step required  = 49 + 382 / 2.5 = 202 kVA     ← governs

Star–delta:
  Starting kVA   = 45 × 2.9 = 130 kVA
  Step required  = 49 + 130 / 2.5 = 101 kVA     ← running load governs at 121
```

![Bar comparison showing 202 kVA required for a DOL start against 101 kVA for star-delta on the same plant](/blog/dg-sizing-what-governs.svg "The running load did not change. The starting method moved the answer from a 250 kVA set to a 125 kVA set.")

**A 250 kVA set against a 125 kVA set, decided entirely by the starter.** A
star–delta starter or a soft starter on that one motor costs a fraction of the
difference between the two frame sizes — and the smaller set then runs at a
healthier load factor for the rest of its life. The [DG sizing
calculator](/tools/dg-sizing) runs both calculations and tells you which one
governed.

The same logic applies in reverse when a plant is already built: if a genset
browns out on motor starting, changing the starter is nearly always cheaper than
changing the generator.

## Derate for the site

The engine is an air pump. Thinner, hotter air means less power:

- Roughly **3 % per 300 m** of altitude above 1,000 m.
- Roughly **2 % per 5 °C** of ambient above 40 °C.

A set installed at 1,800 m in a hot summer can be 15 % down on its nameplate. The
derating applies at your site, so the *ordered* rating must exceed the calculated
requirement, not equal it.

## Read the rating definition, not just the number

ISO 8528 defines three ratings, and a quotation usually shows the largest one:

| Rating | Means | Typical relationship |
|---|---|---|
| Standby (ESP) | Emergency use, varying load, no overload capability | The headline figure |
| Prime (PRP) | Unlimited hours, varying load, 10 % overload for 1 hour in 12 | ~90 % of standby |
| Continuous (COP) | Unlimited hours at constant load | ~80 % of standby |

If the set is genuinely a backup that runs during outages, size against standby.
If it is your regular supply — common on sites with poor grid availability —
size against **prime**, or you will be running an emergency-rated machine
continuously and its maintenance interval will tell you so.

There is also a load acceptance class (G1 to G4) covering how much step load the
set will take and how fast it recovers. G2 suits general industrial and
commercial use; G3 and G4 are for sensitive loads. If you have a large step load
relative to the set, this class is worth specifying rather than assuming.

## Load factor, fuel and the cost of oversizing

An oversized diesel is not a safe choice, it is a different problem. Below about
30 % load a diesel runs cool, burns fuel incompletely and **wet-stacks** —
unburnt fuel and carbon accumulate in the exhaust, injectors glaze, and
maintenance cost climbs. Aim for 60–80 % load in normal running.

Fuel consumption is roughly:

| Load | Litres per kWh |
|---|---|
| 100 % | 0.25 |
| 75 % | 0.27 |
| 50 % | 0.30 |
| 25 % | 0.38 and rising |

Over a year of daily running, the gap between a well-loaded set and a lightly
loaded one is usually larger than the price difference between the two sets.

If the load genuinely varies that much, two smaller sets in synchronism beat one
large one: run one at good load factor most of the time and bring the second in
at peak. It costs more to buy and it is cheaper to own, and it gives you
redundancy that a single set cannot.

## Capacitors and generators do not mix casually

A capacitor bank sized for the utility supply can destabilise a generator. On a
lightly loaded set, a large fixed bank drives the power factor leading, the AVR
hunts, and voltage becomes unstable.

Switch the bank out when running on DG, or use an
[APFC controller](/blog/apfc-panel-step-sizing) that is told which source is
live and adjusts its target accordingly. This is a two-wire interlock at the
changeover, and it gets forgotten on most retrofits.

## Non-linear load needs more alternator than you think

Motor starting is the case everyone eventually hears about. The one that
catches modern installations is UPS and drive load, and it fails differently:
nothing stalls, the voltage waveform just deteriorates until sensitive
equipment starts misbehaving for reasons nobody can trace.

A generator is a much weaker source than the grid. Its subtransient reactance
is typically 12–18 %, against something nearer 5 % for a distribution
transformer, so the same harmonic current produces three times the voltage
distortion. Load that behaves perfectly on mains can push total voltage
distortion into double figures on a genset.

The usual guidance is to keep non-linear load below about a third of the set
rating, and to oversize where it is higher — for a UPS with a six-pulse front
end, sizing the alternator at 1.5 to 2 times the UPS rating is common advice
from set manufacturers. The cheaper fix is usually at the load: a twelve-pulse
or active front end, or input reactors on the drives, cuts the harmonic current
at source and lets you buy the smaller set.

If the site has significant drive load, ask the genset supplier for the
alternator's harmonic derating explicitly rather than accepting the standard
rating. It is a question they expect.

## Before you release the order

- Both calculations done, and you know **which one governed**.
- Demand measured rather than estimated, where the plant exists.
- Power factor corrected before sizing, not after.
- Starting method for the largest motor decided — it may be the cheapest
  decision on the project.
- Derating applied for altitude and ambient at the actual site.
- Rating definition confirmed: standby, prime or continuous.
- Expected load factor between 60 % and 80 %.
- Capacitor bank interlocked to the changeover.

Run your own figures through the [DG sizing
calculator](/tools/dg-sizing) — it does both calculations, applies the
deratings, snaps to standard Indian frame sizes and shows the fuel burn at the
resulting load point.