How to size a standby generator for your facility
Size a standby generator to your measured peak demand plus roughly 20-25% headroom, then check that the unit can absorb the starting current of your largest motor without the voltage dipping below what your equipment tolerates.
Getting generator sizing wrong is expensive in both directions. Oversize it and you pay for capacity you never use, run the engine at a low load where it wet-stacks, and take a worse fuel-consumption figure for the life of the asset. Undersize it and the set trips exactly when you need it.
What information do suppliers need from you?
Suppliers cannot quote sensibly without a load picture. Before you request offers, collect:
| Input | Why it matters | Where to get it |
|---|---|---|
| Measured peak demand (kW) | The base number every calculation starts from | Utility bill demand charge, or a logged meter reading |
| Largest motor rating and starting method | Starting current can be several times running current | Equipment nameplates |
| Power factor | Converts kW to the kVA the generator is rated in | Meter reading, or assume 0.8 for mixed loads |
| Load type mix | Motors, UPS and variable-speed drives behave very differently | Facility single-line diagram |
| Required run time | Sets fuel tank size, not generator size | Your continuity requirement |
| Ambient temperature and altitude | Both derate the engine's usable output | Site location |
How much headroom should you allow?
Around 20-25% above measured peak demand is the usual working range. It covers metering error, load growth, and the fact that a diesel engine should not sit at 100% load continuously. Going much beyond that starts to cost you: sets running below roughly 30% load for long periods suffer incomplete combustion and carbon build-up.
Why does starting current decide the answer so often?
A direct-on-line motor can draw six to eight times its running current for a few seconds. The generator has to absorb that surge without the voltage or frequency dipping past what your sensitive equipment accepts. On facilities with one large motor, this — not the total kW — is frequently what sets the size. A soft starter or a variable-speed drive on that motor can let you buy a materially smaller set.
What derating applies at your site?
Manufacturer ratings are quoted at reference conditions. Hot ambient temperatures and altitude both reduce usable output, which matters across the Gulf where summer design temperatures are high. Ask every supplier to state output at your site conditions, not at reference conditions, so the offers are comparable.
How should you compare the offers you get back?
Put the quotes side by side on total cost of ownership, not headline price:
- Output at your site conditions, in kVA and kW
- Fuel consumption at 50%, 75% and 100% load
- Alternator and engine make, and local parts availability
- Warranty scope and duration, and what voids it
- Scheduled maintenance intervals and the cost of each
- Delivery, installation, commissioning and load-bank testing
- Emissions compliance and noise level at a stated distance
Ask for the fuel-consumption figures in writing. Over ten years, fuel and maintenance usually dominate the purchase price.
Frequently asked questions
- Is it safer to just buy a bigger generator?
- No. A set that habitually runs below about 30% of its rating suffers incomplete combustion and carbon build-up, and you pay for capacity you never use. Size to measured demand plus modest headroom instead.
- Do I need a load study before requesting quotes?
- A logged measurement is much better than an estimate, because nameplate ratings assume every device runs at full load simultaneously, which almost never happens. A short metering exercise usually pays for itself in the size you avoid buying.
- Should the fuel tank size affect the generator size?
- No. Tank size sets how long you can run; generator size is set by the load. Decide them separately.
Sources
- ISO 8528-1 — Reciprocating internal combustion engine driven alternating current generating sets
- IEEE 3002.8 — Recommended practice for conducting motor-starting studies