Sizing a Factory Backup Generator (Genset) Correctly — ISO 8528, kVA vs kW & Motor Starting

A single hour of outage at a Thai factory means a stopped production line, spoiled in-process goods, and missed delivery deadlines. A backup generator (genset) is your continuity insurance — but "sizing it wrong" is the most expensive mistake: too big = wasted money + engine damage from wet stacking; too small = the motors won't start, voltage sags, the set trips. This guide covers sizing it right the first time, per ISO 8528.

kVA ≠ kW (start here)

A genset is rated in kVA at a standard power factor of 0.8:

kW = kVA × 0.8

So a 500 kVA genset = 400 kW. If your factory load is in kW, convert it back to kVA (kVA = kW ÷ PF) before comparing — picking the wrong unit is a first-step error.

ISO 8528: choose the right "rating"

The same genset comes in several ratings by usage pattern — the wrong rating voids warranty terms and risks failure:

Rating	Use when	Load	Hours/year
ESP (Emergency Standby)	Backup when the main supply fails	Variable, no overload	Limited (~200 h)
PRP (Prime)	Primary supply (no grid)	Variable, avg ≤70%	Unlimited
COP (Continuous)	Constant base load	Constant	Unlimited
LTP (Limited-Time)	Grid-paralleled top-up	Full rating	Limited/year

Most factories that already have MEA/PEA supply use ESP (backup during outages) — but if outages are frequent/long or you're off-grid, look at PRP.

Sizing: not just "add up all the loads"

The common mistake is summing every machine's rating and buying that. But the real sizing driver is usually the largest motor's starting surge.

• A direct-on-line (DOL) induction motor draws ~6× its running current momentarily at start → it demands a large kVA surge.
• If the genset is too small, when the big motor starts → voltage/frequency dips, other equipment glitches, or the genset trips.
• You must analyze the step load / transient, not just total kW.

Factors to include:

1. Total continuous load (kW → kVA)
2. Starting sequence + inrush of the largest motor
3. Non-linear loads (VFDs, UPS) creating harmonics → add margin
4. Reserve margin ~20–25% for future expansion

A way to reduce the genset size needed: use soft starters / VFDs on big motors to cut starting inrush (see choosing VFD vs Soft Starter vs DOL).

Load factor: don't oversize

Oversizing isn't "playing it safe" — a diesel genset running below ~30% load for long periods develops wet stacking: unburned fuel builds up in the exhaust/valves → black smoke, power loss, major overhaul.

• Sweet spot: run at ~50–80% of rating.
• If real load is very low at times, consider a smaller genset, or multiple sets in parallel sharing load.

Derating for Thailand

A genset's rating is quoted at a standard reference temperature (often 25–40°C) — hot, humid Thailand needs checks:

• Actual plant-room/ambient temperature (often higher than expected due to engine heat)
• For every few degrees above the reference, deliverable power drops (derate) — check the maker's derating table
• Plant-room ventilation + engine combustion air must be adequate, or power sags / overheats

ATS — automatic transfer

A genset is useless if it can't switch over in time — you need an ATS (Automatic Transfer Switch):

• Detects main-supply loss → starts the genset → transfers load to it automatically (typically ~10 seconds)
• Must be break-before-make to prevent genset power colliding with grid power (unless a purpose-built sync-paralleling system)
• When the grid returns → transfers back + lets the genset run a cool-down before stopping

Genset or BESS — how to choose

Factor	Diesel genset	BESS (battery)
Long outage (hours+)	✓ Runs as long as there's fuel	Limited by kWh capacity
Instant switchover (0 s)	~10 s (with ATS)	✓ Instant (UPS-grade)
Large load / motor starting	✓ Handles surge well	Depends on inverter kW
Quiet / no exhaust	✗ Noise + exhaust	✓ Quiet, clean
Pairs with solar	Separate system	✓ Uses midday surplus

Many factories run a hybrid: BESS covers short events / instant transfer + a genset for long outages (see the comparison in BESS for factory solar).

The sizing flow in brief

flowchart TD
  A["Sum the loads to back up
(convert to kVA)"] --> B["Find the largest motor
+ its starting inrush"]
  B --> C{"Can the genset take the
step load at motor start?"}
  C -->|"No"| D["Increase genset size
or add soft starter/VFD"]
  C -->|"Yes"| E{"Average load
at 50-80%?"}
  E -->|"Below 30%"| F["Downsize / split into sets
to avoid wet stacking"]
  E -->|"50-80%"| G["Add 20-25% margin
+ derate for Thai temps"]
  G --> H["Pick ISO 8528 rating
(ESP/PRP) + ATS"]

Other things to know

• Emissions/noise: diesel engines have DIW exhaust/noise requirements — choose a sound-attenuating canopy + an exhaust system that meets the limits.
• Fuel: size the fuel tank for the backup duration you need (liters/hr by rating).
• Routine testing: run a regular load test (prevents wet stacking + confirms readiness).

Let Sahawatthanakit design your backup power

Our engineering team handles load survey → genset sizing (including motor step load) → rating + ATS selection → installation and testing as turnkey EPC — including hybrids with solar/BESS. You get a right-sized system: no overspend, no voltage-sag risk.

• Phone: 02-096-2118 / 061-541-6939 (Khun Chin)
• LINE: @sahawatt1988
• Email: info@sahawatthanakit1988.com
• Free quote / consultation: click here

Frequently Asked Questions

How many kW does a 500 kVA genset deliver? At the standard 0.8 power factor = 500 × 0.8 = 400 kW. If your load is in kW, convert it to kVA (kW ÷ 0.8) before comparing to genset sizes.

Why shouldn't I buy an oversized genset? A diesel running below ~30% load for long periods develops wet stacking — unburned fuel accumulates in the exhaust, causing black smoke, power loss, and major repairs. The healthy operating point is 50–80% of rating.

Can I size the genset from the sum of all machine kW? Not enough — the real driver is usually the starting inrush of the largest motor (DOL ~6×). You must analyze the step load, or the motor start will sag the voltage and trip the set.

ESP or PRP rating? A factory that already has grid power and only uses the genset during outages = ESP (Emergency Standby). If it's the primary supply, outages are frequent/long, or off-grid = PRP (Prime), rated for unlimited hours.

Genset or BESS — which is better? Different strengths — a genset runs as long as there's fuel, suited to long outages / large loads; a BESS switches instantly and runs quietly, suited to short events and pairing with solar. Many factories run a hybrid of both.