Is an oversized DG set wasting diesel, and how do I check its loading?

Often, yes. A diesel generator at a low share of its rating makes fewer kWh per litre than it does nearer its rating. To check, divide the kWh it generated by run hours times its kW rating. If the average is low, compare its kWh per litre with the maker's curve and consider a smaller set.

Work out the average load

1. Read kWh generated and run hours from the DG's energy meter and hour meter for a month.
2. Convert the rating to kW: kVA x 0.8 for most sets rated at 0.8 power factor.
3. Average load % = kWh / (run hours x rated kW) x 100.
4. Divide kWh by litres issued to the DG in the same month to get kWh per litre.

Why low load costs diesel

An engine has fixed losses, friction, cooling and auxiliaries, that do not fall with load. At a quarter load those losses are spread over few kWh, so fuel per kWh rises. Manufacturer fuel curves at 25, 50, 75 and 100 percent load show this for your model. Long running at very low load also causes wet stacking: unburnt fuel and soot build up in the exhaust, which raises maintenance and lowers efficiency further.

Options when the set is too big

Check peak demand before downsizing. A set that trips when the press motor starts saves no diesel.

• Replace it with a smaller set sized to your real peak, including the starting current of your largest motor.
• Run two smaller sets in parallel and switch one off when the load is light.
• Keep the big set for rare full-plant outages and add a small set for routine outages and night loads.
• Move non-essential loads off the DG during outages so a smaller set can carry the rest.

What this does not tell you

Average load hides peaks. A set averaging 25 percent may still hit 90 percent for a few minutes each hour. Log demand at short intervals for a week before deciding on a size.

Illustrative 250 kVA set in a small plant

• Rating: 250 kVA x 0.8 = 200 kW
• Month: 600 run hours, 30,000 kWh generated, 12,000 litres diesel
• Average load: 30,000 / (600 x 200) = 25%; output 30,000 / 12,000 = 2.5 kWh per litre
• A 100 kVA (80 kW) set would run at 30,000 / (600 x 80) = 62.5%; at an assumed 3.3 kWh per litre from its curve it needs 30,000 / 3.3 = 9,091 litres
• Diesel saved: 12,000 minus 9,091 = 2,909 litres a month
• CO2 saved: 2,909 x 2.68 = 7,796 kg, about 7.8 t CO2 a month

The 3.3 kWh per litre is assumed for illustration; read your own maker's curve, and confirm the smaller set covers peak and motor starting loads.

What Senseible does here

Senseible calculates DG CO2 from your diesel invoices at 2.68 kg CO2 per litre and keeps each month's litres in one record, so a right-sizing change shows up as fewer litres for the same outage hours.

• DG set specific fuel consumption
• Estimate DG diesel from run hours
• DG emissions per kWh vs grid
• Diesel emission factor

Sources: BEE Guide Book 3, Energy Efficiency in Electrical Utilities, Chapter 9 Diesel Generating System, 2006 IPCC Guidelines, Volume 2: Energy

Last reviewed 2026-10-18.