“How many litres per hour?” is comparable only at the same load, rating and environment. The useful purchasing question is the fuel cost per usable kWh across the real load profile, while the generator remains reliable and maintainable.
Efficiency is not the smallest brochure number. Match engine, alternator and capacity to the real load, align the test boundary and calculate annual fuel. Correct oversizing or undersizing before treating brand differences as decisive.
1. Separate L/h, g/kWh and cost per usable kWh
L/h describes fuel flow at one operating point and cannot be compared without output. g/kWh expresses specific consumption, but the measurement boundary must be known. Cost per usable kWh also includes alternator efficiency, low-load periods and idle time.
A set has different hourly and specific consumption at 25%, 50%, 75% and 100% load. One full-load number or an unsupported average is not enough for a budget.
2. Align seven conditions before comparing data
- Prime or standby rating and kW/kVA basis.
- 50/60 Hz, voltage and power factor.
- Exact engine and alternator models.
- Partial-load points or the project load profile.
- Temperature, altitude, fuel quality and cooling.
- Included auxiliary loads.
- Whether the source is a manufacturer curve, witnessed test or estimate.
3. Calculate the duty cycle, not one load point
Divide the duty into start, low load, normal load, peak and idle periods. Apply the relevant consumption and multiply by annual hours or outage events. Prime-power projects should include rotation, maintenance and refuelling losses.
Compare hourly cost, fuel cost per kWh and annual fuel budget. Missing partial-load data is a technical clarification, not a reason to invent an average.
4. Capacity matching often matters more than the marketing claim
Persistent light loading can worsen specific consumption and engine condition; insufficient capacity creates sustained overload, frequency instability and heat. Size for running load, starting steps and justified growth, then use staging or parallel sets when it improves load factor.
Do not remove necessary reserve to chase fuel savings, and do not apply an arbitrary oversizing percentage without a load list and expansion basis.
5. Six system factors change site fuel use
- Engine curve at the target load factor.
- Alternator efficiency, power factor and harmonic loads.
- Airflow resistance, recirculation and derating.
- Filters, injection condition and maintenance state.
- ATS recovery sequence, idle waiting and unnecessary runtime.
- Tank, supply/return, leakage and metering accuracy.
6. Establish a site fuel baseline after delivery
Qualified personnel should record load kW, time, starting and ending fuel, environment, frequency and voltage under a controlled test. Prefer calibrated flow or verifiable tank measurement over a single dashboard estimate.
Use the first result as a baseline. A later increase at the same load can trigger checks of air, fuel, cooling, exhaust and maintenance records.
7. Put fuel evidence into the quotation and operating plan
State engine model, rating, load-point data and source, tolerance and environmental boundary. Define target load factor, allowed idle time, rotation or paralleling, fuel quality, metering and review interval.
The decision should include annual fuel, maintenance, reliability and local support. A marginally lower brochure figure may not reduce lifecycle cost when the set is mismatched or difficult to support.