In short. Size generator fuel storage from the autonomy the facility needs, multiplied by consumption at the expected load, divided by the usable fraction of the tank, plus the operating reserve. For most Indian sites that produces a base or day tank for 8 to 12 hours and, where longer autonomy or refuelling uncertainty exists, a bulk tank with an automatic transfer system. The tank is infrastructure that delivers an operational requirement; the arithmetic is only the first step.
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.
Start with autonomy, not with tank catalogues
Autonomy is the number of hours the facility must run on stored fuel without a delivery. It is set by the consequence of running out and by how reliably fuel can be brought in, and it should be agreed with the operator before any tank is drawn. A commercial building in Secunderabad with a supplier two kilometres away may accept eight hours; a hospital, a data centre or a pharmaceutical cold store cannot. Two separate inputs are often confused here: the time the facility can tolerate before backup power is available, which drives the ATS and UPS design, and the duration for which backup must then continue, which drives fuel. The second is the fuel question.
| Facility type | Autonomy commonly specified | Usual storage arrangement |
|---|---|---|
| Hospitals and healthcare | 24 to 72 hours | Day tank plus bulk tank, duplicated transfer pumps |
| Data centres and telecom core | 12 to 48 hours, often with refuelling contracts | Bulk storage per generator group, automatic transfer |
| Manufacturing standby | 8 to 24 hours | Base tank, sometimes a bulk tank for multiple sets |
| Commercial buildings and malls | 8 to 12 hours | Base tank or room day tank |
| Prime power and remote sites | Delivery interval plus margin, often 3 to 7 days | Bulk storage sized to the delivery cycle |
Codes for the facility type, such as National Building Code provisions for hospitals and local fire authority requirements, may set minimums that override these ranges.
The sizing calculation
Storage (L) = (Autonomy (h) × Consumption (L/h)) ÷ Usable fraction + Reserve (L)
Autonomy is the required running time in hours; consumption is the litres per hour at the load expected during the outage, from the manufacturer’s curve; the usable fraction accounts for the fill limit and the unusable heel, typically 0.85 to 0.90; and the reserve is the quantity the site’s policy keeps untouched, as discussed in the fuel runtime guide.
Worked example (illustrative). A 500 kVA set serves a hospital block with an expected outage load of 300 kW, at which the data sheet gives 81 litres per hour. The hospital requires 24 hours of autonomy. Operating fuel is 24 × 81, or 1,944 litres. With a usable fraction of 0.85 the tank must hold 2,290 litres, and a 20 percent reserve of about 460 litres brings the requirement to roughly 2,750 litres. The practical answer is a 990 litre base tank under the set, which satisfies the day-tank role, fed from a 3,000 litre bulk tank outside the building through an automatic transfer system, giving a little over 24 hours plus reserve.
Base tank, day tank and bulk tank
Base tank
Most sets up to about 750 kVA sold in India come on a base-frame tank of roughly 8 to 12 hours at full load. It is convenient and compact, but it is also directly under a hot engine, it is inside the room or canopy, and its capacity is fixed by the frame.
Day tank
A day tank is a small tank near the set, usually 2 to 8 hours, fed automatically from bulk storage. It limits the quantity of fuel inside the building, gives the engine a steady suction head, and allows the bulk tank to be placed wherever access and safety dictate. Its level switches control the transfer pump, with a high-level overflow back to bulk and a low-level alarm that reaches the operator.
Bulk tank
Bulk storage holds the autonomy. Above-ground tanks in a bunded enclosure of at least 110 percent of the largest tank are the norm for Indian industrial sites; underground tanks save space but complicate inspection and water detection. Diesel is Class B petroleum under the Petroleum Rules 2002, and storage beyond the exempt quantity requires a PESO licence and a site layout that meets the separation distances; the threshold and conditions should be confirmed with the current rules and the local fire officer before the civil work starts. See generator fuel systems for the equipment side.
Transfer, return and overflow
The transfer path is the part of a fuel system that fails most often and is designed least carefully. A single transfer pump is a single point of failure for every set it feeds; duplicate pumps with automatic changeover, plus a hand pump for the last resort, are standard for critical sites. Engines return a large proportion of the fuel they draw, warmed by the injection system, so the return must go to a tank that can accept it without overflowing, and a day tank that is already full needs an overflow line back to bulk sized for the return flow. Pipework should be steel or an approved fuel-grade material with a strainer and water separator ahead of the engine filters, a shut-off valve accessible from outside the room, and a suction lift within the engine lift pump’s capability, which for many engines is only one to two metres. Where several sets share one bulk tank, one fuel-quality problem or one blocked line affects them all, and the single-line diagram should show whether that is acceptable.
Monitoring and fuel condition
A tank that cannot be read cannot be managed. Every bulk and day tank needs a local level gauge, a remote level signal to the building management system or generator controller, and low-level and high-level alarms that reach someone who can act. Provide a bottom drain valve for water, a sampling point and a fill point with a camlock coupling, a strainer and an overfill prevention valve or alarm. Stored diesel absorbs water through condensation in the monsoon and supports microbial growth; a tank sized for 48 hours of autonomy will hold fuel for months between outages, so periodic sampling and fuel polishing belong in the maintenance plan. The room and bund design are covered during the site survey and installation planning.
Common mistakes
- Sizing the tank from the generator’s full-load consumption for the whole autonomy; the tank is oversized, which costs money, holds fuel longer and still leaves the transfer path unexamined.
- Treating nominal capacity as usable; the design loses 10 to 15 percent before the first litre is burned.
- Installing one transfer pump for a hospital; a pump failure in hour three ends a 72 hour design.
- Returning hot fuel to a small day tank with no overflow; the tank vents diesel onto the floor of the generator room.
- Placing the bulk tank where a tanker cannot reach it; the storage can never be refilled in the conditions it was bought for.
What to send us
Send the generating-set rating and nameplate, the expected outage load in kW, the autonomy the facility requires and who decided it, a site plan or photographs showing where tanks could stand and how a tanker would reach them, and any existing tank details. If a code or a client specification sets a storage duration, send the clause. Send your application details and we will size the day and bulk storage, the transfer arrangement and the alarms together.
Questions engineers ask
What size fuel tank does a 250 kVA generator need?
At a typical outage load of 120 to 150 kW the set burns 35 to 42 litres per hour, so 8 hours needs about 400 litres usable, or a 500 litre tank, and 24 hours needs about 1,200 litres usable, or roughly 1,500 litres with reserve. The autonomy the site requires decides, not the kVA.
Can diesel be stored inside the generator room?
A base tank or a small day tank inside the room is normal practice, subject to the fire authority’s conditions and the quantity limits of the Petroleum Rules. Larger quantities belong in a bunded tank outside the building, with the transfer line passing through a fire-rated penetration and a shut-off valve outside the room.
Is a bigger tank always better?
No. Fuel degrades in storage, a large tank is refilled rarely and holds old stock, and the capital and licensing burden rises. Size for the required autonomy plus reserve and pair it with a credible refuelling arrangement rather than buying hours you will not maintain.
Why does the day tank overflow when the generator runs?
Hot return fuel from the engine is entering a tank that is already at its float level, and either the overflow line to bulk is missing or undersized, or the transfer pump is not stopping on the high-level switch. Check the float switches and the overflow before anything else.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.