Sizing & Specification

Generator sizing is easy to get wrong. We make sure it's right.

A generator is only as good as its specification. Undersize it and critical loads trip; oversize it and you've wasted capital and shortened the engine's life. Our engineers size and specify it properly — first time.
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Most sizing mistakes don't show up on day one. They show up the first time the load steps, the plant room heats up, or the power's actually needed. By then it's a retrofit, not a revision — and far more expensive than getting the brief right at the start.
The Method

How a standby generator is sized properly

There's no single number that does this. Sizing is a sequence of engineering decisions — here's the one our engineers work through, and what each step is really about.
01
Build a real load schedule
List every load with its actual running kW and whether it's continuous, intermittent or standby — then apply realistic diversity. The most common error starts here: summing nameplate ratings instead of measured running loads, which leads straight to an oversized, inefficient set.
Why it matters: a diesel set running lightly loaded for long periods will wet-stack — unburnt fuel and carbon fouling the engine.
02
Find the load that governs the size
It's rarely the steady-state total. It's the worst-case transient — usually the largest motor starting direct-on-line, which draws an inrush of typically 6–7 times its full-load current for a few seconds. The set has to hold voltage and frequency within tolerance while that happens, or downstream loads drop out.
The lever: star-delta, soft starters and VSDs cut the starting surge — and often the size of set you need with it.
03
Characterise the load
UPS, variable speed drives and IT loads are non-linear — they draw harmonic currents that heat the alternator and distort the voltage waveform. The alternator is often oversized to keep distortion within limits. Lightly loaded UPS can also present a leading power factor that destabilises the set.
04
Convert to kVA at the right power factor
Generators are rated in kVA at 0.8 power factor. kVA = kW ÷ pf, so a 400 kW load at 0.8 pf is a 500 kVA demand. Get the power factor assumption wrong and the set is wrong before you've started on anything else.
05
Select the correct duty rating
ISO 8528-1 defines emergency standby (ESP), prime (PRP) and continuous (COP) ratings. They're not interchangeable: running a prime application on a standby-rated set will shorten engine life dramatically. The duty has to match how the set will actually be used.
06
Derate for the site
A rating quoted at sea level and 25°C is not what you get in a hot, enclosed plant room at altitude. Altitude, ambient temperature and exhaust back-pressure all reduce available output. The set has to be sized for the conditions it'll actually work in.
07
Design for resilience and autonomy
Sizing the set is only part of the picture. Fuel storage and autonomy, changeover times, paralleling and synchronising, N+1 redundancy and — in healthcare — HTM 06-01 segregation of essential services all shape the final specification. This is where a kVA figure becomes a working system.
Want this done for your project — properly, first time?
Get your specification reviewed
Beyond the Set

The specification is more than a kVA figure

A correctly sized set in a poorly designed plant room will derate, overheat or trip. These are the disciplines that turn a generator into a reliable system — and the questions a good specification answers early.
Plant room & ventilation
Air in, air out, heat away
  • Combustion air and radiator cooling air sized together — cooling air volumes are large and routinely underestimated.
  • Attenuated intake and discharge louvres to control boundary noise (BS 4142 assessment).
  • Heat rejection managed so intake temperatures stay within limits and the set doesn't self-derate.
  • Fire-rated construction, fire dampers, and access for maintenance and engine removal.
Ref: ISO 8528 · BS 4142 · BS 7671
Exhaust & flue systems
Get the gases out, quietly
  • Residential- or industrial-grade silencers selected for the attenuation the site needs.
  • Back-pressure kept within the engine maker's limit — excess means lost power, high exhaust temperatures and DPF regeneration faults on Stage V engines.
  • Thermal expansion bellows, support, lagging and condensate drainage.
  • Termination height and position for dispersion, clear of air intakes, and compliant with permitting.
Ref: MCPD · Specified Generator regs · Stage V
Life safety & resilience
HTM 06-01 & critical loads
  • Healthcare premises designed to HTM 06-01 — secondary supplies, autonomy and segregation of essential services.
  • N+1 configurations where a single set can't be a single point of failure.
  • Defined fuel autonomy to ride through extended outages, with bulk storage and bunding.
  • Fire-resistant cabling to life-safety and fire-fighting circuits (BS 8519), with no-break supplies via UPS where seconds matter.
Ref: HTM 06-01 · BS 8519 · BS 7671

Not sure where to start?

Tell us the application and a rough load. We'll work out what you actually need — no obligation.
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Avoid the pitfalls

Where generator specifications go wrong

These are the mistakes we're called in to fix — most of them avoidable with the right brief at the start.
Sizing on connected load, not running load
Summing nameplate ratings ignores diversity and gives a set that's too big — costly to buy, and prone to wet-stacking when lightly loaded.
Ignoring motor starting
The largest motor's inrush often governs the size. Miss it and the set can't hold voltage on start-up — tripping the very loads it's there to protect.
Forgetting to derate
A sea-level, cool-day rating isn't what a hot, enclosed plant room delivers. Without derating, the set can't make its numbers when it's needed.
Undersized ventilation
Cooling and combustion air are underestimated, intake temperatures climb, and a correctly sized set quietly derates itself in service.
Exhaust back-pressure over the limit
An undersized or over-long flue robs power and drives high exhaust temperatures and DPF regeneration faults on modern engines.
No fuel autonomy plan
The set runs — until it doesn't. Storage, autonomy and refuelling have to be designed in, not discovered during an outage.
Wrong duty rating
Specifying a standby rating for a prime application looks fine on paper and wears the engine out years early in the field.
Oversizing "to be safe"
The instinct to round up wastes capital, worsens load factor and creates the wet-stacking problems it was meant to avoid.
Get a second pair of expert eyes before you commit.
Have your specification checked
By sector

Sizing considerations by application

The headline kVA is only half the story — the load profile and resilience requirement change everything.
HEALTHCARE
Hospitals & care
HTM 06-01 resilience, life-safety segregation, rapid changeover and fuel autonomy for extended outages. Often N+1.
DATA CENTRE
IT & comms
Non-linear UPS load, tight voltage and frequency tolerance, block-load step changes and high availability targets.
MANUFACTURING
Process & plant
Large motors starting DOL dominate the sizing. Process continuity and safe-shutdown loads drive the worst-case transient.
MANUFACTURING
Process & plant
Large motors starting DOL dominate the sizing. Process continuity and safe-shutdown loads drive the worst-case transient.
UTILITIES
Water & waste
Pump motor starting, remote sites and continuity-of-supply obligations. Often prime-rated, not standby.
COMMERCIAL
Offices & retail
Life safety, lifts and IT on standby supply; mixed linear and non-linear loads with modest step changes.
Working with us

What getting it right looks like

Whether you're at feasibility, specifying now or replacing a failed set, the route in is the same — and it starts with a conversation.
Step 01
Tell us the basics
Application, a rough load and your site. Don't worry if it's incomplete — that's what we're here for.
Step 02
We do the engineering
We build or review the load schedule and step-loading and pin down the duty, derating and resilience.
Step 03
We specify the system
Set, plant room, ventilation, flue, fuel and changeover — a specification you can build and tender with confidence.
Step 04
Supply, install, support
From a single set to full design, install, commissioning, load bank testing and ongoing service.
Start the conversation
Engineer Review

Get your sizing and specification reviewed — free

Send us your project and a Solent Power engineer will sense-check it and come back with a clear view on the set, plant room and resilience. The more you tell us, the more useful our response.
  • A rough load and application is enough to get started — we'll work out the rest with you.
  • South Coast based, UK-wide service. Generators, UPS, field service, load bank testing and fuel.
  • Engineers, not a call centre. No hard sell — straight answers from people who specify these systems every week.
Consent
Common Questions

Generator sizing FAQs

Online calculators give a rough figure by dividing a load by a power factor — but they miss the things that actually govern the size, like motor starting, non-linear loads and site derating. That's why they so often get it wrong. We'd rather size it with you properly: send us your load and application and we'll give you a figure you can rely on.

It depends on your total running load, the largest step load (usually a motor starting), the load character and your resilience requirement — not floor area or a single headline figure. The reliable way to get the answer is a proper load schedule, which we'll build with you.

kW is real power (the work done); kVA is apparent power (what the alternator must supply). They're related by power factor: kVA = kW ÷ pf. Generators are rated in kVA at 0.8 power factor, so a 400 kW load at 0.8 pf needs a set rated for 500 kVA.

Significantly. A motor started direct-on-line draws an inrush of typically 6–7 times its full-load current for a few seconds, and the set must hold voltage and frequency within limits while it accelerates. This transient is often the single biggest factor in sizing. Star-delta, soft starters and VSDs reduce it.

Yes. Healthcare premises are designed to HTM 06-01, which sets out resilience requirements for the electrical supply — secondary supplies, fuel autonomy, segregation of essential services and rapid changeover. Life-safety circuits often need fire-resistant cabling (BS 8519) and no-break supplies via UPS.

More than most people expect. A radiator-cooled set moves large volumes of cooling air, plus combustion air for the engine and a path for radiated heat. Inadequate ventilation raises intake temperatures and forces the set to derate — so a correctly sized generator can still underperform in a poorly designed plant room.

Specify it once. Specify it right.

Talk to engineers who size, specify and install critical power systems every week — across the South Coast and UK-wide.
Request an engineering review
Call 02381 550144

Solent Power Ltd (Company Number 12622565) is a company registered in England and Wales. Our registered office is located at Station yard, Bowes Hill, Rowlands Castle PO9 6BP. VAT No. 363504018

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