Diesel Generator Cost Per Unit: The Honest Calculation, From Fuel Rate to Capital Recovery
A diesel generator's real cost per unit is the diesel rate per litre multiplied by the specific fuel consumption in litres per kWh, plus lubricant, maintenance, capital recovery and manpower, all divided by the units the set actually delivers. Fuel is typically the large majority of it, which is why specific fuel consumption — not the diesel price alone — decides whether your DG power costs somewhat more than grid power or several times more.
Most plants quote a DG cost that is only the first term, taken at full-load consumption while the set runs at a third of its rating. That understates the figure badly. What follows builds the number up term by term from your own fuel invoice, running-hour meter and maintenance records, so you can compare it to grid power on a like-for-like basis rather than against the energy charge alone.
The formula, written out
Work in symbols first and put your own values in afterwards.
Cost per unit = (F x SFC) + L + M + C + P
Where:
- F is the delivered diesel rate per litre, from your latest invoice, including transport and any applicable duty
- SFC is specific fuel consumption in litres per kWh, at the load factor you actually run
- L is lubricating oil and consumables per kWh
- M is maintenance cost per kWh, derived from cost per running hour divided by average kW delivered
- C is capital recovery per kWh — the purchase, installation and overhaul cost spread over the units the set will produce in its life
- P is manpower and readiness cost per kWh, including testing, statutory compliance and the operator's time
Every term except SFC is straightforward bookkeeping. SFC is where the engineering lives.
Why specific fuel consumption decides everything
Specific fuel consumption is the litres of diesel burned per kWh of electrical output, measured at the alternator terminals. A diesel engine is at its most efficient near its design load, and it becomes markedly less efficient as loading falls, because a fixed proportion of fuel goes into overcoming internal friction and driving auxiliaries regardless of output.
| Load factor | Typical SFC behaviour | What it means for cost |
|---|---|---|
| 90-100 per cent | Near the quoted figure, commonly around 0.25-0.28 litres per kWh for a modern set | The number your supplier quoted, rarely the one you run at |
| 70-80 per cent | Close to optimum; many sets are at their best here | The loading to design for |
| 50 per cent | Noticeably higher litres per kWh | Cost per unit rising while the diesel rate is unchanged |
| 25-30 per cent | Substantially higher, and the engine runs cool | Expensive power plus accumulating engine damage |
Treat those values as indicative and measure your own: litres consumed from the fuel register, kWh generated over the same period from the DG energy meter. If your set has no kWh meter, fix that first, because without it every DG cost figure in the plant is an estimate.
Why a lightly loaded DG is the most expensive power in the plant
Below roughly 30 per cent loading, a diesel engine runs cooler than it was designed to, combustion is incomplete, and unburned fuel and lubricating oil accumulate in the exhaust system. The symptom is a wet, sooty exhaust manifold and a black deposit at the turbocharger. The consequence is fouled injectors, glazed bores, more frequent overhauls and reduced output over time.
Our recommendation is that no DG set is sized on the connected load list alone. Size it on the load that will actually be transferred, verify with a load test, and if the set is unavoidably oversized, plan how loading will be raised during the hours it runs. We have seen plants with two sets alternate them rather than run both lightly — the correct instinct.
This is the common mistake: a 1,000 kVA set bought for an expansion that never happened, running at 250 kW every outage for eight years. More fuel per unit, more maintenance, and capital recovered over very few units.
Lubricant, coolant and consumables
Lube oil consumption is commonly a small percentage of fuel consumption by volume, and it rises as an engine ages. On its own it is a modest line item, but the associated items are not: oil filters, fuel filters, air filters, coolant and the labour of each change together form a predictable cost per running hour.
Take two years of consumable purchases for the DG, divide by running hours in that period, then divide by average kW delivered. That gives a per-kWh number you can defend.
What maintenance really costs per running hour
Maintenance cost is usually expressed per running hour, then converted. Include scheduled services, the top and major overhauls amortised over their intervals, the AMC if you hold one, and the parts that fail on calendar time rather than running time — batteries, hoses, belts, the charging alternator.
Many plants find their DG maintenance spend is driven by calendar, not hours, because a standby set deteriorates while standing. A set running 150 hours a year still needs battery replacement, fuel polishing and routine exercise, and those costs divide across very few units.
Capital recovery: paying for a machine that runs 200 hours a year
This is the term most plants omit, and it is often the one that changes the conclusion.
Capital recovery per unit is the installed cost of the set, plus the civil work, the fuel storage, the acoustic enclosure, the exhaust system and the associated electrical panels, spread across the units the set will generate in its service life. A set that runs a few hundred hours a year generates very few units, so the same capital divides into a small denominator and the per-unit figure becomes large.
Write it as: C = (installed capital + major overhauls over life) ÷ (annual kWh generated x years of life). Put your own numbers in. The result often surprises people who had decided the DG was "already paid for". Sunk capital is still capital, and when the alternative is a battery or a grid upgrade, both must be assessed on the same basis.
Manpower, testing and the cost of readiness
A standby set must be exercised, logged, fuelled and kept statutorily compliant. Someone runs it weekly. Someone maintains fuel quality in a tank that may stand nearly full for months, which makes contamination and microbial growth live risks through a humid Gujarat monsoon. Someone handles the paperwork.
None of this is dramatic, and all of it belongs in the per-unit figure if you want an honest comparison. If you are deciding between keeping a DG and moving part of its duty elsewhere, readiness cost is a genuine difference between the options.
Building your own number: a worked example in symbols
Use this as a worksheet rather than a result.
| Term | How to get it from your own records | Your value |
|---|---|---|
| F — diesel rate per litre | Latest delivered invoice, including transport and duty as billed | |
| SFC — litres per kWh | Litres from fuel register ÷ kWh from DG energy meter, same period | |
| F x SFC — fuel cost per unit | Multiply the two above | |
| L — lube and consumables per kWh | Two years of consumable spend ÷ kWh generated in the same period | |
| M — maintenance per kWh | Annual maintenance spend, including amortised overhauls ÷ annual kWh | |
| C — capital recovery per kWh | (Installed capital + life overhauls) ÷ (annual kWh x life in years) | |
| P — manpower and readiness per kWh | Annual readiness cost ÷ annual kWh | |
| Total | Add all of the above |
Two rules while filling this in. Use the same period for every row, because fuel from last month divided by kWh from last year produces nonsense. And take F from your own current invoice, because diesel pricing moves and any figure printed in an article is out of date before you read it.
How to compare DG cost against grid honestly
A fair comparison puts the same categories on both sides. Most comparisons do not, which is why DG power so often looks cheaper than it is.
| Cost element | Grid side | DG side |
|---|---|---|
| Energy | Energy charge per unit, from the tariff order in force | Fuel: F x SFC at your real load factor |
| Capacity | Demand charge on billing demand | Capital recovery on the set and its civil and electrical works |
| Losses | Transformer and cable losses on your side of the meter | Alternator, cable and auxiliary losses |
| Maintenance | Transformer and switchgear maintenance | Engine, alternator and control maintenance, plus overhauls |
| Statutory | Duties and surcharges as levied in the order in force | Pollution compliance, fuel storage compliance, testing |
| Availability | Outage exposure and what each outage costs you | Start failure risk, fuel quality risk |
Note what is not on this table: a currency figure for any of it. Every line is specific to your plant, your tariff category and your month, and the tariff components are set by order and change. Fill it in from your own bill and your own records.
A second honest adjustment: if the DG runs during your peak demand blocks, its value includes demand charge avoided, not just units supplied. That is a real benefit and it belongs in the comparison, which means you need to know how your demand charge is actually computed — the mechanism is set out in how maximum demand charge is calculated.
When a diesel generator is still the right answer
Plenty of cases. A set carrying fire pumps, emergency lighting and safe-shutdown loads is a safety asset and its cost per unit is irrelevant. A plant in an area with long outages needs sustained energy that only a fuelled machine provides economically. A process that must be brought down in a controlled sequence over an hour needs something that can run for that hour.
A DG does two things badly: running at light load for long periods, and responding fast enough to save a process from a short dip. The second is why plants with perfectly maintained sets still lose batches — the mains failure sequence has to detect, start, come up to speed and close, and the process has usually already fallen over. Put a number against that exposure before deciding where your money goes; the method is in what one power trip costs a process plant.
Where battery storage changes the arithmetic
A battery does not replace a DG set for long outages. It does something different: it takes over the short, frequent events, and it can shave the demand peaks that a DG was never intended to address.
Three effects change the comparison. Fewer DG starts mean fewer running hours, reducing fuel and extending the overhaul interval. The dips a DG cannot catch in time get caught, removing a cost that never appeared in the DG arithmetic at all. And the same asset can work on demand management for the rest of the month, which a standby DG does not.
Three things do not change. A long outage still needs fuel. A battery has its own capital recovery term, sized in both kW and kWh. And both options must be evaluated over their life rather than on first cost — the method is covered in solar and BESS payback and IRR calculation.
Our recommendation for plants with both is to stop treating it as an either-or. The useful question is which asset should serve which event, and that is a control design question that has to be answered before any equipment is ordered.
What we see on the electrical side
We build DG synchronising panels, AMF schemes and the LT boards they feed, and the same issues recur.
Load sharing is first. Two sets of different vintage sharing a bus will not share kW and kVAr correctly unless the governor and AVR characteristics are set up for it. A set carrying more than its share runs hotter and burns more fuel per unit than the logbook suggests, and nobody notices until a bearing or a winding says so. In our FAT experience the sharing is verified on a load bank before dispatch, and we push back when a client wants to skip that test to save two days — site is a bad place to discover the scheme was wrong. The control logic is set out in how a DG synchronisation panel works and shares load.
Neutral and earthing arrangement is second. When a DG source and the grid source can both feed the same board, the neutral switching decision drives the earth fault protection scheme and, done carelessly, creates parallel neutral paths that make earth fault detection unreliable. That is a decision for the single line diagram, not a site improvisation.
Third, metering. Fit a kWh meter on the DG outgoing feeder. We have lost count of the plants in Gujarat that can tell us to the litre what their DG consumed and have no idea what it generated, and without both halves there is no cost per unit — only an opinion. Our recommendation: every DG feeder built or retrofitted in our Vadodara works carries energy metering as standard, because the data outlives the panel.
Fourth, the LT assembly itself. DG incomers and the boards they feed are low voltage assemblies, built and verified to IEC 61439; where the source connects at 11 kV the applicable standards are different — IEC 62271-200 and IS 3427 in Indian practice. If a supplier cites the low voltage standard for your HT panel, that tells you something about the depth of the offer.
The governing principle
A DG set's cost per unit is not a property of diesel prices. It is a property of how you load it, how many units you spread its fixed costs across, and how honestly you count. Two identical sets in two plants can differ widely on cost per unit with the same fuel in the tank.
Fit the energy meter, log the litres, and calculate your own number for a full year. Then, and only then, decide what should replace what.
Frequently Asked Questions
How do you calculate diesel generator cost per unit?
Multiply the delivered diesel rate per litre by the specific fuel consumption in litres per kWh at your actual load factor, then add lubricant and consumables, maintenance, capital recovery and manpower, each expressed per kWh. The sum is your cost per unit. Every term should come from your own records over a consistent period, because the fuel term alone typically understates the true figure substantially.
What is specific fuel consumption for a diesel generator?
Specific fuel consumption is the litres of diesel burned per kWh of electrical output at the alternator terminals. Modern sets are commonly around 0.25 to 0.28 litres per kWh near full load, and the figure rises as loading falls because a fixed share of fuel goes into friction and auxiliaries regardless of output. Measure yours by dividing litres from the fuel register by kWh from the DG energy meter over the same period.
Why does a lightly loaded diesel generator cost more per unit?
Because fuel consumption does not fall in proportion to output. Below roughly 30 per cent load the engine also runs cooler than designed, which causes incomplete combustion and deposits in the exhaust system, leading to fouled injectors and more frequent overhauls. The result is a higher fuel cost per unit and a higher maintenance cost per unit at the same time.
Should capital cost be included in DG cost per unit?
Yes, if you intend to compare the DG against any alternative. Capital recovery is the installed cost of the set, its civil works, fuel system, enclosure and electrical panels spread across the units the set will generate in its life. A set that runs only a few hundred hours a year divides that capital into a very small number of units, which is precisely why "it is already paid for" is a misleading argument.
Is DG power always more expensive than grid power?
In normal industrial conditions it is usually several times the cost of an equivalent grid unit once every term is counted, but the comparison must include the grid's demand charges and your outage exposure on the same basis. There are hours when DG power is the only power, and then its cost per unit is not the deciding question. Compare like with like, and over a year rather than a day.
How much lubricating oil does a diesel generator consume?
Lube oil consumption is typically a small percentage of fuel consumption by volume and rises as the engine ages. The more significant figure is the total consumables cost — oil, filters, coolant and the labour of each change — expressed per running hour. Take two years of consumable purchases, divide by running hours in that period, and use the result.
Can a battery energy storage system replace a diesel generator?
Partly, not wholly. A battery responds in milliseconds and handles the short dips and brief outages that a DG cannot reach in time, and it can also work on demand management for the rest of the month. A long outage still needs fuel, so most industrial plants end up operating both, with a control scheme that decides which asset serves which event.
What load factor should a diesel generator run at?
Most sets are at their best somewhere in the region of 70 to 80 per cent of rating, and sustained running below about 30 per cent should be avoided because of incomplete combustion and deposit formation. If your set is unavoidably oversized, plan how loading will be raised during running hours, or alternate between sets rather than running several lightly. Confirm the acceptable band against the engine manufacturer's documentation for your specific set.
How do I compare DG cost with my electricity tariff?
Put the same cost categories on both sides: energy, capacity, losses, maintenance, statutory charges and availability. The grid side draws its energy and demand components from the tariff order in force and your own bill; the DG side draws fuel, maintenance and capital recovery from your own records. Comparing DG cost per unit against the grid energy charge alone, with no demand or capacity term, is the most common error.
Does running a DG during peak hours reduce my demand charge?
It can, if the set runs in parallel during the blocks that set your maximum demand and the import is genuinely reduced in those blocks. That requires a synchronising scheme with proper protection, and the saving must be weighed against a far higher cost per unit for the energy supplied. Work out both numbers before committing to a strategy of peak-hour DG running.
What is wet stacking in a diesel generator?
Wet stacking is the accumulation of unburned fuel and lubricating oil in the exhaust system of an engine that has run too long at low load. It shows up as a wet, sooty exhaust and deposits around the turbocharger, and it leads to fouled injectors, glazed cylinder bores and reduced output. The remedy is correct loading, and in persistent cases a load bank exercise under qualified supervision.
What electrical equipment does a DG installation actually need?
At minimum an AMF or synchronising panel appropriate to the duty, an outgoing feeder with protection coordinated to the rest of the system, a correct neutral and earthing arrangement for a two-source installation, and an energy meter on the DG feeder. The low voltage assemblies are built and verified to IEC 61439; where the connection is at 11 kV the governing standards are IEC 62271-200 and IS 3427 in Indian practice. The energy meter is the cheapest item on that list and the one that makes every cost discussion afterwards factual.
Related Guides
- DG synchronization panel working and load sharing
- AMF vs ATS vs DG synchronization
- 3-pole vs 4-pole ATS
- BESS with an existing DG and synchronisation panel
- Is my factory suitable for solar and battery storage
- What one power trip costs a process plant
- Product page: DG synchronization panel
- Service: Hybrid solar + BESS
Talk to Us About Your Plant
Wisdom Techno Solutions manufactures the electrical system that sits at the centre of a solar and storage installation — the LT panel, the switchgear, the metering, the protection and the control — at our works in Vadodara, Gujarat. We build DG synchronization panel, and the same engineers who design an assembly test it before it ships and commission it on site.
Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. For a solar or storage plant that covers the LT switchboard, the BESS PCC and the AC and DC distribution.
If you want to know whether your plant is a candidate, send us twelve months of electricity bills and one day's load profile. That is enough to size a system, show where the savings would come from, and tell you honestly if the answer is no.
The quickest route is our free feasibility study: send the bills and we come back with the sizing and the numbers behind it. To work out your own numbers first, use the free DG replacement calculator.
Send your single line diagram, your load list or your last twelve bills, and we will come back with the electrical scheme and the numbers behind it.
Diesel generator control, synchronisation and changeover panels are built at our LT panel manufacturer in Vadodara.