Rental Fuel Tanks

In short. A rental fuel tank turns litres into operating hours. Runtime is usable fuel divided by consumption at the real load, and the plan must keep a reserve so the generator never depends on a delivery arriving exactly on time. Size the base tank or external tank from the required autonomy, the expected load and the refuelling interval the site can actually support, then add bunding, filtration and a clear owner for refuelling.

Why fuel planning belongs in the rental scope

A rental generator stops when its fuel stops, however well the electrical side is engineered. Most rental sets carry a base tank sized for something like 8 to 12 hours at a typical load, which is enough for a short outage and not enough for a three-day shutdown or a month of bridging power. Once the duty runs past one shift, the questions change: how much fuel must be held on site, how it reaches the engine, who brings the next load of diesel and what happens if the tanker is late. Those are engineering questions with continuity consequences, not purchasing details.

The runtime calculation

Runtime (h) = (Usable fuel (L) − Reserve (L)) ÷ Consumption (L/h)

Usable fuel is the volume the engine can actually draw, which is less than the nameplate capacity because the suction pipe sits above the tank floor and some volume is lost to sludge and slope. Reserve is the volume deliberately held back to cover a late delivery. Consumption is the engine’s burn rate at the real average load, taken from the manufacturer’s data sheet at that load, not at full load.

A 500 kVA set running at about 60 percent load, 300 kW, might burn around 75 to 85 litres per hour (illustrative; confirm with the engine data). With a 1,000 L external tank of which 920 L is usable, and a 4-hour reserve of 320 L, the planned runtime between refuels is (920 − 320) ÷ 80 = 7.5 hours (illustrative). That figure, not “1,000 litres”, is what the site team plans around. The method is set out in more detail in the fuel runtime guide and the consumption data in generator fuel consumption.

Tank options for temporary power

Engines also return a share of the fuel they draw. When an external tank feeds a set, the return line must go back to the tank the engine is drawing from, or one tank overflows while the other runs dry.

Reserve and the refuelling window

A 24-hour operation should not be planned to reach empty at hour 24. The reserve should cover the longest realistic delay in delivery: traffic into an industrial area, a supplier who does not work Sunday nights, roads affected by monsoon rain. A useful way to set it is to ask when refuelling must start to keep the reserve intact, and then check that the supplier can meet that time every day of the job. For critical loads, many sites hold one full refuelling interval as reserve. For the wider sizing method see fuel-tank sizing.

Fuel quality, filtration and site controls

Water and dirt in diesel cause more rental breakdowns than most electrical faults. Tanks that have travelled between jobs collect condensation; deliveries from mixed sources carry sediment. Drain water from the tank sump before first use and daily on long jobs, fit a water-separating filter where the fuel is transferred, and keep spare filter elements on site. The tank should be double-walled or stand in a bund that holds at least 110 percent of its volume, placed on level ground away from drains and hot-work areas, with a spill kit and fire extinguisher beside it. Storage above certain quantities can bring licensing obligations under petroleum rules, so confirm the applicable requirement for the volume planned.

Who does what during the hire

Agree in writing who orders fuel, who receives and dips the tank, who signs for the quantity and who is called if consumption looks wrong. Record the hour meter and tank level at every refuel. Consumption that rises without a change in load points to an engine problem; consumption that falls without explanation may point to fuel going somewhere it should not. This record also gives the customer an honest measure of the energy delivered. The rental planning context is in rental and temporary power.

Common mistakes

• Planning runtime on nameplate tank capacity. The set stops with a few hundred litres still below the suction pipe.
• Using full-load consumption for every calculation. Light-load sites over-order fuel, while heavy-load sites run dry early when the figure was taken at partial load.
• Returning fuel to the wrong tank on a dual-tank arrangement. One tank overflows on the ground and the other empties.
• Setting no reserve because the supplier is usually on time. A single late tanker stops the critical load.
• Skipping water drainage on a tank that has travelled. Filters block and injectors are damaged within the first days.

What to send us

Send the generator rating and expected average load, the number of hours per day and total days the set must run, the location with access for a tanker, and how often your fuel supplier can deliver. If you already have a tank on site, add a photograph of its nameplate and the fittings. Send a generator rental enquiry and we will propose the tank arrangement and refuelling plan.

Questions engineers ask

Generators by rating

• Under 15 kVA generators
• 15–50 kVA generators
• 50–125 kVA generators
• 125–250 kVA generators
• 250–500 kVA generators
• 500–1000 kVA generators
• 1–2 MVA generators
• 2 MVA and above generators

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.