Sizing a Generator Fuel Tank

Even if a generator runs without problems, it will stop when the fuel runs out if the tank volume has not been calculated according to the outage duration and the refuelling time. Tanker deliveries can be delayed by several days, especially at weekends or in bad weather.

Tank volume is determined by how many hours the generator must run without refuelling:

Tank volume = hourly consumption × target runtime ÷ usable ratio

The calculation is simple. The hard part is choosing a realistic load factor and a realistic refuelling time.

Step 1: Find the hourly consumption

For a preliminary calculation, approximately 0.28 L/kWh at full load is used for diesel generators:

Full-load consumption (L/h) ≈ kW × 0.28 (kW = kVA × 0.8)

At part load, consumption does not fall in proportion to load. Typical ratios in our fuel consumption calculator: at 25% load ~35% of full-load consumption, at 50% load ~55%, at 75% load ~75%. Details are in our article how to calculate generator fuel consumption. The exact values are in the engine data sheet.

Step 2: Set the target runtime

Ask yourself: if the outage drags on, who brings the fuel, and how long does it take? In a city it might be a few hours. At the weekend, on a rural site or in bad weather, it can stretch to days. For homes and small businesses a few hours may be enough. In facilities such as hospitals, data centres and telecom sites, the target runtime is usually written into the project specification and is much longer.

Step 3: Don’t count the whole tank

The fuel at the bottom of the tank cannot be used. The volume below the suction pipe collects sediment and water; at the top, an expansion allowance is left. Drawing the tank right down carries sediment and air into the filters. For a preliminary calculation, approximately 85–90% of the tank volume is taken as usable:

Tank volume (L) = L/h × hours ÷ 0.85–0.90

Example table (at 75% load)

Generator	kW	Consumption (75% load, approximate)	8 hours	24 hours	48 hours
100 kVA	80	16.8 L/h	134 L	403 L	806 L
250 kVA	200	42 L/h	336 L	1,008 L	2,016 L
500 kVA	400	84 L/h	672 L	2,016 L	4,032 L
1,000 kVA	800	168 L/h	1,344 L	4,032 L	8,064 L
2,000 kVA	1,600	336 L/h	2,688 L	8,064 L	16,128 L

The table shows usable fuel. For the tank volume, divide by the usable ratio. Example: 500 kVA, 75% load, 24 hours → 2,016 ÷ 0.85 ≈ 2,370 litres of tank. Assuming full load stays on the safe side but makes the tank bigger.

Tank types

Tank type	Where?	Typical use
Base (skid) tank	Under the generator	Most canopied sets; from a few hours to a working day
Extended base tank	Enlarged base frame	Outdoor sets where long autonomy is required
Day tank	Next to the generator, small volume	Systems fed from a main storage tank
Main storage tank	Above or below ground, mostly double-walled	Long autonomy, several generators

In a system with a main storage tank, fuel is transferred to the day tank by a fuel transfer pump. The level float, overflow protection, return line and leak detection are part of the design. For canopied sets, the base tank is the most practical solution; see generator canopy.

Common mistakes on site

• The return line is connected to the wrong tank. A diesel engine sends part of its fuel back warmed up. If the return goes somewhere other than the day tank, the day tank either runs dry or overflows.
• The pump suction is not enough. The height difference or line length between the day tank and the engine exceeds the suction capacity of the lift pump. The generator stutters under load.
• The tank is bigger than necessary. On a set that runs a few hours a year, 5,000 litres of fuel is not turned over for years. Water and sediment build up, and filters clog.
• Water is never drained. Condensation collects at the bottom of the tank. A filter with a water separator and periodic draining are a must.
• No spill protection. A tank without a double wall or a spill containment bund creates an environmental problem at the smallest leak.

Regulations

Fuel storage above certain volumes is subject to fire and environmental regulations. In Türkiye, the Regulation on the Fire Protection of Buildings and local fire brigade rules are the basis for the tank’s location, volume and the precautions to be taken. The current regulations and the competent authorities should be checked at the design stage. For the generator room and layout, see our article on the generator room and installation.

Checklist

• Have the target runtime and the real refuelling time been determined?
• Has hourly consumption been checked against the data sheet at a realistic load factor?
• Have the bottom volume and expansion allowance been deducted?
• Is the base tank enough, or are a main storage tank and day tank needed?
• Are a return line, level control and overflow protection in place?
• Has a double wall or spill containment been provided?
• Have fire and environmental regulations been checked?
• Are fuel quality checks and water draining included in the maintenance plan?

Next step

Tank volumes for each model are given in the technical tables on the product pages. To work out the right tank solution for your target runtime with the Terex Diesel team, contact us.