In short. Fuel loss on generator sites is common, gradual and usually invisible in the accounts, because diesel is bought as an expense and consumption is rarely checked against the energy the set produced. The cure has four parts: know what the set should use, physically secure the fuel and batteries, run refuelling as a recorded two-person process, and reconcile every month — litres delivered against litres in the tank against kWh generated. Many sites find a gap once they start reconciling.
Who this page is for: For owners, facility managers, accounts and security teams: find out whether your generator’s fuel use is genuine, close the routes by which diesel and batteries go missing, and run refuelling as a controlled, recorded process.
Key facts
- The best single check is litres per kWh at similar loads, compared with your own baseline month by month.
- A fuel level that falls while the set is stopped is a leak or a theft unless a recorded transfer or a gauge fault explains it — check the same day.
- Short delivery is a common cause, alongside theft from the tank. Check every delivery.
- Physical security must never block cooling air, exhaust, service access or fire escape.
- Batteries and copper cables are also frequent targets, and their loss stops the set from starting.
Where fuel goes missing
| Route | How it happens | Typical sign |
|---|---|---|
| Tank drain valve | Fuel drawn off from the base tank drain | Level drops while the set is stopped; stains around the drain |
| Filler neck | Siphoning through an unlocked filler cap | Level drops overnight or on holidays |
| Fuel lines | Tapping into return or supply lines | Fuel use rises without a change in load |
| Short delivery | Less fuel delivered than invoiced, or dip readings manipulated | Level rise after delivery smaller than the invoice |
| False running records | Hours logged that the set did not run, to cover fuel drawn | Log-book hours do not match the controller hour meter |
| Day-tank and transfer pump misuse | Fuel pumped out of the bulk tank into containers | Bulk tank falls faster than the day tank uses |
| Leaks | Weeping joints, damaged hoses, overflow from overfilling | Stains, smell, soaked ground |
| Adulteration | Cheaper fuels mixed in to make up volume | Smoke, poor starting, filter blocking; density outside the normal band |
Know what your set should use
The controller on most modern sets records kWh generated and running hours. Divide the litres used in a period by the kWh produced. Compare the figure with the set’s own history at similar average load, and with the maker’s data sheet figures. Specific consumption rises at light load, so compare like with like. Our fuel consumption page explains the load effect in detail.
Worked reconciliation (illustrative)
Illustrative figures to show the method. A factory’s 500 kVA set (400 kW) for one month:
| Item | Value |
|---|---|
| Opening stock (dip, start of month) | 1,200 L |
| Deliveries invoiced | 6,400 L |
| Closing stock (dip, end of month) | 1,150 L |
| Litres used | 6,450 L |
| Running hours (controller) | 92 h |
| kWh generated (controller) | 18,400 kWh → average 200 kW, 50 % load |
| Expected at about 0.28 L/kWh for this load (from the set’s history) | about 5,150 L |
| Unexplained difference | about 1,300 L — roughly 20 % of the fuel used |
An investigation compares delivery dips with invoices, checks the log book against the controller hour meter, and inspects the drain valve and lines. In cases like this the cause is usually one of three: short deliveries, draw-off from the tank, or hours logged that the set never ran. Each points to a different fix.
Refuelling procedure
For large tanks, a calibrated dip chart (litres per centimetre) is essential — tank volume is not proportional to depth in horizontal cylindrical tanks.
Physical security measures
- Lockable filler cap and an anti-siphon filter in the filler neck.
- Drain valve: remove the handle, cap or plug the outlet and lock it.
- Canopy doors: keyed locks, with keys held by named people only.
- Battery security: locked battery box or clamp; batteries marked; battery charger alarm connected so removal is noticed.
- Cables: armoured cables, locked termination boxes, cables in covered trenches where possible.
- Lighting and CCTV covering the filler, drain and canopy doors.
- Fencing or a cage for exposed sets — designed with enough clearance for cooling air, exhaust, maintenance access and escape. A cage that starves the radiator of air causes overheating shutdowns.
- Bunding or spill containment sized for the tank, so leaks and draw-off spills are contained and noticed.
Electronic monitoring
- Fuel level sensor with remote alerts: notify a sudden drop while the set is stopped, an unexpected rise (unrecorded delivery), and a low level.
- Controller data: running hours, kWh, starts and alarms downloaded monthly — an independent record that does not depend on the log book.
- Door and battery alarms linked to the remote monitoring system.
- Delivery flow meter on bulk tanks, read at every delivery.
Rental sites
On hired sets, agree in writing who supplies fuel, how it is measured at hand-over and return (dip reading and photograph of the gauge, hour meter reading), and who is responsible for losses while the set is on site. The same reconciliation applies.
Monthly fuel register
| Date | Opening dip (L) | Delivered (L) & invoice no. | Closing dip (L) | Hours / kWh (controller) | L per kWh | Signed (two people) |
|---|---|---|---|---|---|---|
Monthly review: questions for the manager
- Do log-book hours match the controller hour meter?
- Is litres per kWh within the normal range for the load this month?
- Does every delivery show a matching rise in tank level?
- Were there any level drops while the set was stopped?
- Are locks, seals and the drain plug intact?
- Is the battery the same one that was fitted (check the marking)?
Common mistakes
- Checking fuel bills but never reconciling them against kWh generated.
- Accepting deliveries without a dip reading before and after.
- Fitting cages or covers that starve the set of cooling air.
- Keeping the only running record in a log book the operator fills in.
What to send us
Send the set’s kVA, tank size, the last three months of fuel deliveries, opening and closing dip readings, controller running hours and kWh, and a description of how refuelling is done today. Use the service and AMC enquiry, or WhatsApp the details and photographs.
Questions owners ask
How do I know if fuel is being stolen from my generator?
Reconcile monthly: litres used (opening stock plus deliveries minus closing stock) divided by kWh generated from the controller. Compare with the set’s own history at similar load. A sudden rise, or any level drop while the set is stopped, needs investigation.
What is a normal fuel consumption per kWh for a diesel generator?
At good load, modern diesel sets commonly use about 0.24–0.30 litres per kWh; at light load the figure is higher. The best benchmark is your own set’s history at similar load and its data sheet.
How do I check that the full quantity of diesel was delivered?
Dip the tank before and after, read the delivery meter, check the tanker seals, and compare the level rise with the invoice using a calibrated dip chart for your tank.
Can locking the generator cause problems?
It can if it blocks cooling air, exhaust flow, maintenance access or escape. Locks on fillers, drains and doors are fine; cages and enclosures must keep the clearances the generator needs.
Should the generator be stopped while refuelling?
Where the filler is on the generator itself or near hot exhaust parts, yes, unless the system is designed for refuelling while running. Bulk tanks remote from the set can usually be filled while it runs, under a safe procedure.
Why are generator batteries stolen so often?
They are valuable, easy to remove and resell. A missing battery is only noticed when the set fails to start in an outage, so lock them, mark them, and alarm the charger.
Related pages
- Generator Fuel Consumption
- Fuel Runtime Calculator
- Fuel-Tank Sizing
- Fuel Cleaning & Polishing
- Fuel Tank Rental
- Generator Controller Alarms Explained for Site Staff
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.