In short. LPG and propane generator systems store fuel on site as a pressurised liquid, which gives a gas engine the fuel independence of a diesel set without a pipeline. The engineering that comes with it is vaporisation capacity, storage licensing under PESO rules, and a fuel that in India is a butane-rich mixture with lower knock resistance than natural gas. Runtime is worked out from usable fuel mass and the engine’s consumption at the real load, never from tank volume alone.
What Indian LPG actually is
Propane and LPG are not quite the same fuel. Pure propane boils at about minus 42 degrees Celsius and has a methane number in the mid thirties. Commercial LPG in India is a blend of propane and butane whose proportions vary by supplier and season, and is commonly butane-rich. Butane boils at about minus 0.5 degrees Celsius, so it evaporates far more slowly from a tank at a given temperature, and it has a lower methane number, which means a greater tendency to knock. Both facts matter for a generator. A set rated for propane may need to be derated or re-timed on a butane-rich blend, and vapour withdrawal that was adequate for propane may starve the engine on LPG. The engine manufacturer’s fuel specification will state what blends are approved and at what rating, and the LPG supplier’s specification should be matched against it before the set is ordered. Energy content is around 46 MJ/kg for the liquid, or roughly 25 MJ per litre, and this is the figure to use for runtime rather than any gas-volume number.
Vaporisation: the part most often undersized
Liquid LPG absorbs heat from the tank walls to evaporate, and the rate at which a tank can supply vapour depends on its wetted surface area, the liquid level and the ambient temperature. A 47.5 kg commercial cylinder can supply only a modest vapour flow continuously before it frosts and the pressure collapses, which is why cylinder banks for generators are manifolded in numbers well beyond the fuel quantity alone would suggest. Bulk tanks do better because of their surface area, but as the liquid level falls so does the withdrawal rate.
| Arrangement | How gas reaches the engine | Typical fit | Watch for |
|---|---|---|---|
| Natural vapour withdrawal | Vapour drawn from the top of the tank or cylinders, heated only by ambient air | Small sets, short runtimes, warm climates | Pressure drop as tank cools or empties; frosting on cylinders |
| Liquid withdrawal with vaporiser | Liquid drawn from the tank bottom and evaporated in an electric, hot-water or direct-fired vaporiser | Sets above a few tens of kW, continuous duty, butane-rich fuel | Vaporiser capacity at full engine flow; power supply to an electric vaporiser during start-up |
| Vaporiser heated by engine coolant | Liquid evaporated by engine jacket water once the engine is warm | Prime duty sets with steady load | Cold start on vapour from the tank until the engine warms; interlock logic |
Runtime from fuel mass, not tank volume
Runtime (h) = Usable fuel (kg) / (P (kW) × SFC (kg/kWh))
Usable fuel is the stored mass that can actually be withdrawn, which for a bulk tank excludes the heel below the outlet and for cylinders excludes the residual that remains when vapour pressure falls too low. P is the average electrical load, and SFC is the engine’s specific fuel consumption in kilograms per kWh at that load, taken from the manufacturer’s curve. Worked example (illustrative): a 100 kW LPG genset carries an average 75 kW and the manufacturer’s curve gives 0.27 kg/kWh at that load, so it burns about 20 kg/h. A bulk tank with 1,000 kg of usable fuel gives about 49 hours. A bank of ten 47.5 kg cylinders holds 475 kg nominal, but if only 85 percent is practically withdrawable before pressure falls away, the usable 404 kg gives about 20 hours. Tank fill limits, usually around 85 percent of water capacity for LPG vessels, and the supplier’s delivery lead time then decide how much storage the site needs. The general method is the same as in the fuel runtime guide for diesel, with mass in place of litres.
Storage, siting and statutory requirements
LPG storage in India falls under the Petroleum and Explosives Safety Organisation. Bulk installations above the thresholds set in the Static and Mobile Pressure Vessels (Unfired) Rules require approved drawings, a licence and periodic inspection, and even cylinder installations have limits on quantity, separation distances and manifold construction. Vessels must be sited away from buildings, boundaries, ignition sources and drains, with vehicular access for the tanker, fencing, earthing, fire protection and signage to the standard required by the licensing authority. Because LPG vapour is heavier than air, generator rooms and enclosures need low-level ventilation and gas detection, and gas pipework should not pass through pits or ducts where vapour can collect. These requirements shape the site layout before the generator location is fixed, and the design must be prepared and approved by the responsible engineer; a web page is an introduction, not a substitute for that approval.
Where LPG generators fit
The case for LPG is strongest where pipeline gas is not available, where a gaseous fuel is wanted for emissions or noise reasons, where diesel storage and handling is unwelcome, such as some residential, hospitality and institutional sites, and where the duty is standby or light prime rather than continuous base load. For long hours at high load, the fuel cost per kWh generally favours piped natural gas where it exists, and for very large standby loads diesel remains the practical choice because its energy density and storage simplicity are hard to beat. Some engines approved for both natural gas and LPG allow LPG to serve as the backup fuel when pipeline supply is interrupted, and diesel engines with approved dual-fuel kits can draw LPG vapour as the substitute gas. In each case the fuel change alters rating and ignition settings and must be covered by the manufacturer’s documentation. Renewable gas routes are discussed under biomethane power, and the general approach to gas fuel assessment in the gas power systems overview.
Common mistakes
- Sizing storage by litres of tank and assuming it all converts to runtime. The engine starves on vapour long before the tank is empty.
- Specifying an engine on propane data and fuelling it with butane-rich commercial LPG. Knock, derating and plug fouling follow.
- Installing cylinders in an enclosed generator room. Heavier-than-air vapour pools at floor level and any leak becomes an explosion hazard.
- Ordering the tank before the PESO layout is checked. Separation distances force the vessel to a different corner of the site and the gas run becomes long and undersized.
What to send us
Send the electrical load list with the expected average and peak load, the required runtime between refills, and the name of your intended LPG supplier with any product specification. Add a site plan or satellite image marked with the proposed tank or cylinder area, building positions, boundaries and tanker access, and photographs of the generator location. If an engine already exists, include its nameplate and fuel system details. Use the application engineering enquiry and we will size the storage, vaporisation and engine together and flag the statutory steps.
Questions engineers ask
How many cylinders does an LPG generator need?
Two separate counts apply. The fuel quantity count comes from runtime divided by consumption in kilograms per hour. The vaporisation count comes from the continuous vapour each cylinder can supply at the lowest expected ambient, which is often a few kilograms per hour per cylinder, so that the manifold can meet the engine’s flow without frosting. Install the larger of the two numbers, usually with an automatic changeover between banks.
Does LPG give the same power as natural gas in the same engine?
Not always. LPG has a higher energy content per cubic metre of vapour, but its lower methane number limits ignition timing and compression, so manufacturers often publish a derated figure or a separate calibration for LPG. Use the rating in the engine’s fuel specification for the exact blend.
Can LPG be stored indefinitely for a standby generator?
LPG does not degrade in storage the way diesel does, which is one of its advantages for standby duty. The tank and its fittings still need the periodic inspections required by the licence, and vaporisers, regulators and shut-off valves should be exercised during the monthly test run so that the fuel system is proven along with the engine.
What approvals are needed for a bulk LPG tank at a generator site?
Bulk storage requires PESO approval of the installation drawings and a licence before the vessel is commissioned, with periodic renewal and inspection. The local fire authority and the building’s occupancy approvals may add conditions. Allow time for these in the project programme; the generator is often the quickest part of the installation to obtain.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.