In short. A generator’s fuel system is the chain from storage tank through transfer pump, day tank, filters and water separator to the engine. Autonomy is set by usable litres divided by consumption at the actual load, not by nominal tank capacity. For standby sets in India the reliability threat is stored fuel degrading with water and microbial growth, so filtration, drain points and periodic polishing matter as much as tank size.
Diesel generator calculator
Fuel use, runtime, tank size and cost
How this is calculated
Real power is kVA × power factor. Fuel use is estimated as L/h ≈ 0.04 × rated kW + 0.22 × load kW, which follows the usual shape of modern diesel gensets: about 0.26 L per kWh near full load, rising towards 0.38 L per kWh at a quarter load because the engine's fixed losses are spread over less output.
If you enter the full-load figure from your engine datasheet, the curve is scaled to match it. Runtime is usable fuel ÷ L/h. Tank size is L/h × hours, plus the reserve margin, divided by the usable fraction.
These are planning estimates, typically within 10 to 15 percent of a healthy engine. Actual use depends on the engine model, its condition, altitude, ambient temperature and how the load varies. For a figure you can rely on, use the manufacturer's fuel curve or a metered test.
Estimates only, worked out in your browser. For a checked selection, send us your load list and site details or call 098852 01608.
The fuel chain, link by link
Diesel reaches the injectors through a series of components, and a failure anywhere in the chain stops the set as surely as an empty tank. In a typical Indian installation the chain is a bulk tank, a transfer pump with level control, a day tank or base tank, a primary filter with water separator, the engine lift pump, secondary fine filters, the injection system, and a return line carrying warm excess fuel back to the tank. Each link has a job and a failure mode.
| Component | Function | Typical failure | What to check |
|---|---|---|---|
| Bulk tank | Long-duration storage | Water layer, sludge, corrosion, leaking seams | Bottom sample, drain cock, dip for water with paste |
| Transfer pump and float switch | Moves fuel to the day tank automatically | Stuck float, failed pump, air lock | Manual run test, high and low level alarms |
| Day or base tank | Local supply, measurement, header for the engine | Overfill, sediment at pickup, blocked vent | Gauge accuracy, vent clear, pickup height |
| Primary filter and water separator | Removes water and coarse dirt before the lift pump | Bowl never drained, element blocked | Drain bowl at every visit, restriction indicator |
| Secondary filters | Fine filtration to protect injectors | Blockage, wrong micron rating, counterfeit element | Genuine element, change by hours and condition |
| Lines and return | Delivery and return path | Air ingress at joints, kinked hose, return blocked | Joints under suction, hose age, return to tank not to day-tank top |
Working out real autonomy
Autonomy is how long the set runs on the fuel that can actually reach the engine. The tank is rarely filled to the brim, the pickup sits above the bottom, and the low-level logic stops useful operation before the tank is dry, so usable volume is commonly 80 to 90 percent of nominal.
t = Vusable / (Pload × SFC)
In the formula t is runtime in hours, Vusable is usable fuel in litres, Pload is the actual electrical load in kW, and SFC is specific fuel consumption in litres per kWh at that load, which for modern diesel sets is broadly 0.2 to 0.3 litres per kWh and rises at light load. Worked example (illustrative): a 250 kVA set carries 150 kW. At an SFC of 0.24 litres per kWh it burns 36 litres per hour. A 990 litre base tank with 85 percent usable volume holds 842 litres, giving about 23 hours. The same set at 50 kW does not run three times as long, because SFC at light load may be 0.3 or more. Model-specific curves and the method are in generator fuel consumption and the fuel runtime guide.
Base tank, day tank or bulk storage
Most sets up to a few hundred kVA sold in India come on a base-frame tank sized for 8 to 12 hours at full load. For a standby set at a commercial building with a reliable DISCOM supply and daytime refuelling access, that is often enough. It stops being enough when the site is remote, when outages run for days after monsoon damage to lines, or when the set is on prime duty.
Where a larger reserve is required, a bulk tank stands outside in a bund and a day tank of a few hundred litres near the set is filled automatically by a transfer pump on float control. The day tank gives the engine a stable, measured, local source; the bulk tank gives duration. The transfer path then becomes a reliability item in its own right, so the day tank should hold enough for an operator to react to a pump failure, and a hand pump or gravity bypass is worth having. Two sets drawing from one bulk tank through one pump and one filter look like N+1 on the single-line diagram and are N on fuel; separate pickups, separate pumps and a crossover valve restore the redundancy at modest cost. Sizing against outage duration and refuelling logistics is covered in fuel-tank sizing.
Fuel quality: the standby set’s real enemy
A standby generator may burn only a few hundred litres a year while the tank holds several thousand. Diesel in a vented tank breathes humid monsoon air, condenses water on the walls and grows microbial sludge at the water interface. The sludge blocks filters at the worst moment, which is the first long outage of the season. Water also corrodes injection equipment, and in common-rail engines the tolerances are fine enough that a single slug of water can damage injectors. Practical controls are a sloped tank bottom with a drain cock at the low point, regular draining of the separator bowl, keeping tanks reasonably full to limit the air space, and periodic sampling from the bottom rather than the fill point. Fuel that has been standing for more than a year should be tested, and if it fails, polished or replaced; see fuel cleaning and polishing. Filters must be genuine and of the right micron rating; generator filters lists what we stock.
Pipework, venting and safety
Suction lines rise continuously toward the engine with no high points where air can collect. Return lines go back to the tank below the fuel surface, not into the top of a small day tank. Flexible hoses at the engine should be rated for diesel, dated and replaced on age. Every tank needs a screened vent sized for the fill rate and terminated outside. Bulk storage above the exempted quantity under the Petroleum Rules needs a PESO licence and a bund able to contain the largest tank’s volume; the threshold and the bund details must be confirmed against the current rules for your site. Spill kits, no-smoking zoning and a fire-safe separation from the set and from drains are part of the installation, and our fire and site safety guide sets out what inspectors usually look for.
Related photographs and videos
Common mistakes
- Quoting autonomy from the nameplate tank volume at full load; the real figure is usable litres at actual load, and the two can differ by 30 percent either way.
- Never draining the water separator bowl; the separator silently fills, water passes to the injection pump and the set stops under load.
- Feeding two sets from one bulk tank through one transfer pump; a single pump or float failure takes both sets off within the day-tank reserve.
- Storing a full tank for years without sampling; sludge and water accumulate and the first long outage blocks the filters.
What to send us
Send the set nameplate, the tank sizes and a sketch of how fuel moves from storage to engine, the typical and maximum load, the outage duration you must cover, and photos of the tank bottom sample and separator bowl if you have a contamination concern. For new installations add the site layout showing where a bulk tank could stand. The application engineering enquiry is the right route, and the cooling system page covers the other half of keeping a set running under load.
Questions engineers ask
How many litres per hour does a diesel generator use?
Roughly 0.2 to 0.3 litres per kWh delivered, so a set carrying 100 kW burns about 20 to 30 litres per hour. The figure depends on engine model and load fraction, and light loads are less efficient per kWh. Use the manufacturer’s consumption table at 50, 75 and 100 percent load rather than a single number.
How long can diesel be stored in a generator tank?
Six to twelve months is a reasonable planning figure in Indian conditions if the tank is kept full, drained of water and protected from heat. Beyond that, test the fuel for water, sediment and microbial growth, and polish or replace it if it fails.
Do I need a water separator if the engine already has fuel filters?
For stored fuel in a humid climate, yes. Fine filters block quickly when they meet free water and offer little protection to the injection system. A separator with a transparent bowl and drain upstream of the lift pump is cheap insurance for any set that stands idle between outages.
Does a generator fuel tank need a PESO licence?
Diesel is Class B petroleum under the Petroleum Rules, and storage above the exempted quantity requires a licence from PESO together with bunding and separation requirements. Small base tanks usually fall within the exemption; bulk tanks often do not. Confirm the current threshold for your installation before building.
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Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.