How to size a diesel generator: kVA vs kW, standby vs prime power
In short. Add up the kW of the equipment that must run without the grid. Allow for motor starting and a margin, typically 20–25 %. Compare the result with the generator’s prime power in kW, not with standby power. For Powersan and IDEA diesel generators, kW = kVA × 0.8.
Step 1. Count only the loads that must run during an outage
List the equipment you cannot stop: machines, pumps, compressors, cold rooms, ventilation, lighting, servers, tills. The kW rating is on the nameplate or in the equipment datasheet. Add the numbers.
A shorter list means a smaller generator.
Only amps on the nameplate? Use the three-phase formula: P (W) = √3 × U × I × cos φ, where √3 ≈ 1.73 and U = 400 V. For example, a clamp meter shows 100 A per phase, cos φ = 0.8: 1.73 × 400 × 100 × 0.8 ≈ 55 kW, or about 69 kVA. Quick check for 400 V and cos φ 0.8: 1 A ≈ 0.55 kW; 1 kVA ≈ 1.44 A.
Step 2. Motors draw several times more current at start
An electric motor draws several times its running current when it starts. The exact ratio depends on the motor and the starting method; we take it into account when sizing.
If several large motors start together, the generator voltage drops. Solutions:
- start large motors one by one;
- fit a soft starter or a frequency drive;
- size for the largest motor with a margin.
Step 3. kVA is total power; kW is what your equipment gets
- kVA is apparent power: voltage × current. The alternator limits it.
- kW is active power: the part that does work, heat and light. The engine limits it.
- cos φ (power factor) is the share of kW in kVA.
Formula: kW = kVA × cos φ. Powersan datasheets state cos φ 0.8. In IDEA datasheets, kW/kVA = 0.8. So a 225 kVA generator gives 180 kW, and a 100 kW load needs 125 kVA.
If your load has a cos φ below 0.8, the generator reaches its kVA limit before its kW limit. This happens with motors that have no reactive power compensation.
Step 4. Size against prime power, not standby power
Every datasheet gives two ratings. Both are defined in ISO 8528-1.
| Rating | In the datasheet | What it allows |
|---|---|---|
| Standby | Standby, ESP | Power during grid outages, variable load. Up to 200 hours a year (ISO 8528-1). No overload (IDEA datasheets) |
| Prime | Prime, PRP | Variable load with no limit on hours a year. 10 % overload for 1 hour in every 12 (IDEA datasheets) |
| Continuous | Continuous, COP | Constant load, no hour limit. Not given in Powersan or IDEA datasheets |
The number in the model name is usually standby kVA. Under continuous work, prime power in kW is 72–73 % of that number. Outages can be long and frequent, so 200 standby hours can run out quickly.
Example. Your workshop load is, for example, 120 kW. 120 × 1.2 = 144 kW. PSYT175 (126 kW prime) is too small. PSYT225 (162 kW prime) fits.
Diesel generator sizes: which model covers your load
“Load up to” = prime kW / 1.2 (20 % margin). Power ratings are from factory datasheets.
| Site load, up to kW | Model | Prime, kW / kVA | Standby, kVA / kW |
|---|---|---|---|
| 75 | Powersan PSYT125 | 90 / 112.5 | 125 / 100 |
| 90 | Powersan PSYT150 | 108 / 135 | 150 / 120 |
| 105 | Powersan PSYT175 | 126 / 157.5 | 175 / 140 |
| 135 | Powersan PSYT225 | 162 / 202.5 | 225 / 180 |
| 200 | Powersan PSYT330 | 240 / 300 | 330 / 264 |
| 266 | IDEA IDJ450P (Perkins) | 320 / 400 | 450 / 360 |
| 400 | IDEA IDJ660B (Baudouin) | 480 / 600 | 660 / 528 |
| 605 | IDEA IDJ1000DW (Doosan) | 727 / 909 | 1 009 / 807 |
| 900 | IDEA IDJ1500P (Perkins) | 1 080 / 1 350 | 1 500 / 1 200 |
| 1 220 | IDEA IDJ2000B (Baudouin) | 1 464 / 1 830 | 2 000 / 1 600 |
We supply diesel generators from 50 to 10 000 kVA; we can bring generators under 50 kVA to order. IDEA also has sizes between these models.
In your request or tender, state prime power in kW
Lots on etender often give power as one number, for example “800 kW”, with no rating. A supplier can then offer a generator where 800 kW is standby power. Under continuous work, it gives less.
State in your request or technical specification:
- Prime power (PRP): at least … kW. If you use kVA, add cos φ 0.8.
- 400/230 V, 50 Hz, 3 phases. This is the rating in Powersan and IDEA datasheets.
- Mode: backup during outages or permanent site power, and about how many hours a year.
- Load list: large motors, pumps, compressors — they have high starting currents.
- Auto start (ATS panel) and where the generator will stand: indoors or outdoors.
FAQ
How do I convert kVA to kW?
kW = kVA × cos φ. For diesel generators cos φ = 0.8, so 100 kVA = 80 kW.
Is the kVA to kW formula different for 3 phase?
No. kW = kVA × cos φ for any number of phases. Phases matter when you calculate current.
250, 400, 630 kVA — how many kW is that?
At cos φ 0.8 — 200, 320 and 504 kW.
How many amps does a 225 kVA generator give?
At 400 V: about 325 A at standby power and about 292 A at prime power (202.5 kVA).
Prime power vs standby: which should I size for?
Size for prime. Under ISO 8528-1, standby is for up to 200 hours a year, and IDEA datasheets allow no overload at standby.
How long does a diesel generator last?
20 years with timely maintenance. Powersan warranty: 4 years or 3 000 engine hours, whichever comes first, with maintenance by certified specialists.
We will size the generator for your load
Send us your equipment list, nameplate photos or current readings. We convert them to kW and select a model by prime power. Price on request. The price includes delivery within Tashkent and supervised installation (consumables excluded).
Catalogue: Diesel generators 50–10 000 kVA · 200–500 kVA · 500–1000 kVA · from 1000 kVA · Batteries
