Generator load calculation and sizing
Most generator sets that behave badly were sized badly. A set that dips every time a chiller starts, trips on under-frequency when the lifts recall, glazes its bores at a fraction of its rating, or overheats in August but not February — none of those are usually faults. They are sizing decisions catching up with their owner.
Two numbers, not one
Sizing asks two separate questions
They usually have different answers, and the set is sized on the larger of the two. This is a guide, not a design service — a final sizing for a real installation is done against the actual load schedule, the actual starting sequence, and the manufacturer's data for the actual machine.
Question one
Running load — a kW question
- How much power the set has to supply continuously
- Sets the engine size and the fuel consumption
- Decides whether the set spends its life comfortably loaded or lightly loaded
Question two — usually the decider
Starting load — a kVA question
- How much the set has to absorb in an instant
- Motors, transformers and some drives draw a large multiple of running current at the moment they start
- The alternator has to supply that inrush without the voltage collapsing
- A building with a modest running load and one big direct-on-line motor is sized by the motor
kVA is apparent power — what the alternator and cables carry. kW is real power — what the engine produces. Power factor is the ratio, and the standard basis for a three-phase genset rating is 0.8 lagging. So an alternator can be fully loaded in kVA while the engine is only partly loaded in kW — an installation full of lightly loaded motors works the set hard, not the engine, and still glazes its bores. And improving power factor does not release engine capacity the way people expect: capacitive correction that is fine on a mains supply can cause instability and even self-excitation on a generator, and correction banks generally have to be disconnected when a site transfers to generator supply. If your site has automatic correction, say so at the sizing stage.
Step 1
Build a real load schedule
Not a guess, and not last year's schedule. Then note which of these actually run at the same time. Diversity is real, and applying it honestly is how you avoid buying a set twice the size you need; applying it optimistically is how you end up with one that trips.
| Load type | What to record | Why it matters |
|---|---|---|
| Motors — pumps, chillers, lifts, compressors, fans | Rated kW, starting method, whether they can be staged | The starting method changes the inrush by a factor of several |
| Resistive — heaters, filament lighting, ovens | kW | Unity power factor, no inrush, easy load |
| Lighting and small power | kW, and the type of ballast or driver | Modern electronic drivers behave differently from old ballasts |
| UPS systems | kVA, and the UPS input characteristic | The awkward one — see below |
| Variable speed drives | kW, and whether the drive has an input filter | Non-linear load, harmonic current |
| IT and data loads | kW, and diversity | Rarely as diverse as claimed |
| Life safety — fire pumps, smoke extract, lift recall | kW, starting method, and their obligation | These start at the worst possible moment: during the outage |
| Welders and site plant | Duty cycle | Highly intermittent, high peak |
Step 2 — where sizing is decided
The starting load
A three-phase induction motor started direct on line draws a starting current in the region of six to eight times its full load current, at a poor power factor, for the second or two it takes to run up. The alternator supplies that as kVA while the engine absorbs the mechanical load.
Direct on line
Highest inrush, simplest, cheapest — and the sizing driver on most small and medium installations.
Star-delta
Substantially reduced starting current, but with a transient at the changeover, and only usable on loads that can start unloaded.
Soft starter
Inrush limited by design, at the cost of a longer run-up.
Variable speed drive
The gentlest start from the generator's point of view — but it brings harmonic current with it, which is a different problem on the same set.
- Start the biggest motor first, or last, deliberately. The worst case is the largest motor starting while the set is already fully loaded. Sequencing the starts with time delays in the control system is often cheaper than a larger generator.
- The voltage dip is the acceptance criterion, not the current. What matters is how far voltage falls, how long recovery takes, and whether the rest of the site tolerates it. Contactors drop out, drives trip and UPS units transfer to battery at dip levels the generator itself survives easily.
- Load acceptance in steps is a machine characteristic. ISO 8528-5 defines performance classes, commonly referred to as G1 to G4, describing how much frequency and voltage deviation a set permits on load application and how quickly it must recover. A site full of drives, IT and UPS equipment needs a tighter class than a workshop with a couple of pumps — and if your load is sensitive, that class belongs in the specification.
Step 3
The rating on the nameplate is a duty, not just a number
The same physical set carries several ratings depending on how it is allowed to be used. They are defined in ISO 8528 and they are not interchangeable.
| Rating | What it means | Typical use |
|---|---|---|
| ESP — emergency standby | Maximum power available during a mains failure, at varying load, for a limited number of hours per year. No overload capability. | Building standby sets |
| PRP — prime | Unlimited hours at varying load, with a defined limit on the average load over time | Off-grid sites, sites with no mains |
| COP — continuous | Unlimited hours at constant load | Base-load and grid-parallel operation |
| LTP — limited-time running | Full rated power for a limited number of hours per year | Peak-lopping and specific contracted uses |
Buying a set on its standby rating and running it as prime power — a site with no mains, a temporary works compound, a set covering an extended outage — overloads it against its own rating and shortens its life. And average load matters as well as peak: prime ratings carry a limit on the mean load over a running period, so a set that never exceeds its nameplate can still be over-used.
Step 4 — and it matters more here than anywhere
Derating
Every engine and alternator is rated at reference conditions. Away from them the machine produces less: hot air is less dense, so the engine gets less oxygen per stroke and can burn less fuel — and hot air also carries heat away from the alternator windings less effectively, so the alternator's continuous output falls too.
A set specified at reference conditions is being asked to deliver that output in ambient temperatures far above them, frequently inside a plant room that is hotter still. It holds its load in winter and struggles in summer. It approaches its high-coolant-temperature trip on a day it always used to survive — often diagnosed as a cooling fault when it is a sizing and ventilation fault, see generator overheating. And voltage regulation on load degrades in the hottest months, see generator voltage problems.
Ask for the derating curves at the point of purchase, and specify the temperature you actually have — including the plant room temperature, not the shade temperature outside it.
- Plant room ventilation
A generator rejects a great deal of heat into its enclosure. If the air path cannot supply cool air and remove hot air at the rate the set needs, it is recirculating its own heat and every derating calculation you did is wrong. Designed at installation — see installation and commissioning.
- Dust
Airborne dust and sand load radiator cores and air filters faster here than any published service interval anticipates, and a loaded core has the same effect as a further derate.
- Use the maker's own curves
Published for the specific engine and the specific alternator — not a rule of thumb from a forum.
Step 5
The loads that break the rules
UPS systems
A rectifier load. Older or unfiltered designs draw heavily distorted current, heating the alternator disproportionately and distorting the voltage waveform — which is why UPS suppliers frequently specify a generator oversized relative to the UPS kVA. Get the figure from the UPS manufacturer in writing before sizing the set.
Drives and non-linear loads
Harmonic currents heat the alternator in ways a simple kW meter does not show, and can interact with the AVR's sensing. Where drives dominate the load, the alternator specification — not just its size — becomes part of the question.
Regenerative loads
Lifts, cranes, hoists and some drives feed energy back on deceleration. A generator cannot absorb it the way a mains supply can, and the result is an over-voltage or overspeed condition. Identify it at design stage.
Fire pumps and life safety
They start under the worst conditions, are frequently excluded from load-shedding by regulation, and their obligation is legal rather than commercial. Size for them being on, not available.
A worked example
A small industrial unit, diversity honestly applied
| Load | Running kW | Notes |
|---|---|---|
| Process pump, 22 kW motor, DOL | 22 | The largest single motor |
| Compressor, 15 kW, star-delta | 15 | |
| Ventilation fans, 3 × 4 kW | 12 | Run continuously |
| Lighting and small power | 18 | |
| Office and IT | 10 | |
| Total running load | 77 kW |
Running requirement. 77 kW. At 0.8 power factor that is roughly 96 kVA — so a set whose nameplate sits just above the running total covers the running case with very little to spare.
Starting requirement. The 22 kW direct-on-line motor is the worst case. At full load it draws in the region of 40 A; started at six to eight times that, it briefly demands several hundred kVA of apparent power at a poor power factor, on top of whatever is already running.
So the running total is not the answer. Either the set is sized up so the dip on that motor start stays within what the site tolerates, or the 22 kW motor gets a soft starter, or the sequence is arranged so it starts before the rest of the load is applied. All three are legitimate; the cheapest depends on the site. What is not legitimate is buying to the running total and finding out in the power cut. Then derate for the actual plant room temperature.
The arithmetic is straightforward. The judgement is in the load schedule, the starting sequence and the honesty of the diversity — which is why it is worth doing properly once rather than approximately three times.
If you already own the set
Prove it rather than assume it
Most people reading this already have a generator, and the real question is not "what size do I need" but "is what I have adequate". That is a measurement, and it has a specific answer.
A load bank test puts the set through real load steps up to its full rating and records voltage, frequency, dip and recovery on each application, coolant and exhaust temperatures and oil pressure — in the ambient you actually have. It answers three questions nothing else answers: can the set still make its rating (an engine with worn injectors, a tired turbocharger or a loaded core cannot, and will not tell you until the outage); how does it behave on load steps (which is what a motor start is); and is it the right size for the load it now has (buildings add load over years without anyone revisiting the standby capacity). It also runs a lightly loaded set hot enough to burn off the deposits that light running leaves.
Where we come into this. We are a repair workshop rather than a supplier. We measure what an existing set can actually do and give you the answer with figures behind it. Where a set is undersized for a load it has grown into, we say so. Where it is oversized and suffering for it, we say that too — the fix there is operational rather than capital. And whether a set is worth keeping at all is a measurement, not an opinion: let us look at it before you price a replacement.
Undersized
What it looks like
- Under-frequency trips — see under-frequency shutdown
- Deep voltage dips
- Breakers tripping on inrush
- High exhaust temperatures
- An engine working at the top of its range every hour it runs
Oversized — the quieter, commoner fault
What it costs
- Cylinder temperatures never reach proper combustion
- Bores glaze; carbon builds in the exhaust
- Oily deposits weep from manifold and stack joints
- The set gradually loses its ability to take load — wet stacking
- The set bought with generous headroom becomes the set that cannot carry what it was bought for
It needs a deliberate loading policy, not just a service schedule. Generator maintenance covers the regime; a load bank covers the proving. And if the answer does turn out to be a change of set, used generators covers what to check on one.
Common questions
Sizing, answered
How do I calculate what size generator I need?
Total the running load in kW, convert to kVA at the rating power factor, then separately work out the worst-case starting demand — normally the largest motor starting direct on line into an already loaded set. Size on whichever is larger, then derate for your actual plant room temperature. The starting case decides it more often.
Why does my generator dip so badly when a large motor starts?
A direct-on-line start briefly demands several times the motor's running current at a poor power factor, and the alternator has to supply it. If the dip drops contactors or trips drives, the options are a larger set, a softer starting method, or sequencing the starts.
What is the difference between standby, prime and continuous ratings?
Different duties for the same machine. Standby is limited-hours emergency use at varying load with no overload capability; prime is unlimited hours at varying load with a limit on average load; continuous is unlimited hours at constant load. Running a standby-rated set as a prime set is a common and expensive mistake.
Do I need to derate a generator for UAE temperatures?
Yes, and by more than most specifications assume — the number that matters is the temperature of the air the set actually breathes, which in a plant room is higher than the shade temperature outside. Ask the manufacturer for the derating data for that engine and alternator, and specify the plant room condition rather than the weather forecast.
Can I run a generator at 100 % of its rating?
At its standby rating, only for the limited hours that rating permits, with no overload capability above it. Loading any set to its ceiling leaves nothing for the next motor start and nothing for a hot afternoon.
Is it bad to run a generator on a very light load?
Yes, and it is the more common problem of the two. It produces incomplete combustion, glazed bores, carbon in the exhaust and a set that progressively loses its ability to take load. A set lightly loaded by design needs periodic running at real load — one of the things a load bank test is for.
Related
Where to go next
Get the set measured
Send the engine and alternator data plates, the set's rating, and what it feeds. If the question is whether your existing set can carry what you now have, the answer comes from a load test rather than from a calculation on a page.