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Diesel Storage for Backup Generators: Sizing, Fuel Quality and Compliance
A standby generator is only as reliable as the fuel beside it. The tank has to hold enough for the outage you are planning for, and keep diesel that may sit for months fit to burn.
- Run Time
- Sizing
- Fuel Quality
- Siting
A backup generator tank is sized from run time, not from the generator's size. Take the fuel consumption from the generator's datasheet at the load you expect to run, multiply it by the number of hours the site must keep running without a delivery, then add the volume the pump cannot draw and a margin for a late refill. The part that gets missed is fuel quality. Standby diesel can sit for months between runs, and diesel that sits collects water and degrades, so the storage has to be filtered, monitored and tested as well as sized. A self-bunded tank carries its own secondary containment, which is why it suits generator rooms and enclosures where building a separate bund is impractical.
Most generator failures during an outage are not engine failures. The generator starts, runs for a while, and then stops because the fuel ran out sooner than anyone expected, or because a filter blocked on sediment that had been settling in the tank since the last delivery. Both are storage problems, and both are decided long before the power goes out.
This guide covers how to size a generator tank from run time, what separates a generator tank from a general purpose diesel tank, and how to keep standby fuel usable when it spends most of its life sitting still.
Run time The number that sizes the tank. Generator output only tells you the consumption rate
600 to 10,000L The GENCUBE self-bunded range, built with provisions for genset refuelling
Months How long standby fuel can sit between runs, which is why quality matters as much as volume
Why Generator Fuel Is a Different Storage Problem
A diesel tank on a depot or a farm turns over constantly. Fuel comes in, fuel goes out, and nothing sits long enough to cause trouble. A standby generator tank is the opposite. It is filled, the generator is tested for a short run each month, and the rest of the fuel stays where it is until the day it is actually needed.
That day is also the worst possible day to discover a problem. The outage is already happening, the site is already under pressure, and the fuel delivery you would normally ring for is competing with every other site in the same outage. Generator storage has to be designed on the assumption that nobody will be able to fix it in the moment.
| What differs |
Standby power |
Prime power |
| How the generator runs |
Short test runs, then long runs only during an outage |
Continuously or for most of every day |
| Fuel turnover |
Slow. Fuel can sit for months |
Fast. The tank empties and refills regularly |
| Main storage risk |
Fuel ageing, water and sediment in a tank that rarely moves |
Running dry between deliveries |
| What sizes the tank |
The outage you must ride through without a delivery |
Consumption against the delivery schedule |
| What matters most |
Filtration, testing and monitoring |
Capacity, refill access and level alerts |
Sizing the Tank From Run Time
Generator output in kVA tells you the rate the generator will burn fuel. It does not tell you how much fuel to store. That comes from the run time the site needs, which is a business decision before it is an engineering one.
Set the run time
How many hours must the site keep running with no delivery at all? A small office might accept a few hours. A hospital, a data centre or a cold store may need days, and some sectors set a minimum for you.
Read the consumption from the datasheet
Use the generator manufacturer's fuel consumption figure at the load you realistically expect to run, not the full-load figure and not a rule of thumb. Consumption changes considerably with load.
Multiply them
Consumption per hour multiplied by run time gives the usable volume you need. This is the fuel that has to reach the generator, not the size printed on the tank.
Add what the pump cannot draw
A draw point sits above the floor of the tank so it does not pick up the sediment and water that settle there. The volume below it is not usable fuel, and it should not be counted as if it were.
Add a margin for the refill
During a wide outage, a delivery takes longer to arrive than it does on an ordinary day. Size so the tank is still carrying the load when the tanker is late, not only when it is on time.
A worked example
Suppose a generator's datasheet gives 20 litres an hour at the load the site expects to run, and the site wants 72 hours without a delivery. That is 1,440 litres of usable fuel. Allowing for the volume below the draw point and a margin for a late delivery takes the requirement past 2,000 litres, so the right tank is the next capacity up rather than the one that only just covers the arithmetic. The 20 litre figure is an assumption for the example. Use your own generator's datasheet.
What a Generator Tank Needs
Containment
Self-bunded construction
A double-walled tank carries its secondary containment with it, so there is no separate concrete or steel bund to build around it. In a generator room or a tight plant room, that is often the only practical way to contain a tank at all.
Connection
Provision for the genset
The tank has to feed the generator as well as be refilled. GENCUBE tanks are built with provisions for genset refuelling and multiple options for drawing fuel, so the connection is planned in rather than improvised on site.
Fuel quality
Filtration
A 10 micron filtration option takes out particulate and water before the fuel reaches the engine. On a tank that rarely turns over, this is protection for the moment the generator is finally asked to run for hours.
Visibility
Level monitoring
Knowing the level without opening the tank tells you whether the last test run, a leak or a theft has quietly eaten into the reserve. The day you need the fuel is not the day to find the tank half full.
A-FLO's GENCUBE generator tanks are built around this application. They are self-bunded cubes with a lockable top hatch, two fill points with camlock and cap, forklift and crane lifting points and statutory decals fitted. Capacities run from the compact 600 litre GENCUBE, which stands 810 mm high, through to the 10,000 litre GENCUBE, which is also available in a 20 foot ISO side-loader configuration. Smaller units can be supplied with a 12V pump package or a rotary hand pump where fuel also needs to be dispensed from the tank.
Keeping Standby Fuel Usable
Diesel is not indefinitely stable. Held in a tank for long periods it oxidises, it collects water as the tank breathes through warm days and cool nights, and water at the bottom of a tank is where microbial growth starts. A-FLO's guides on how long you can store diesel, water in diesel and diesel bug cover the chemistry in detail. For a generator tank, the practical answer comes down to four habits.
- Keep the tank full. A full tank has less air space, so it breathes in less moist air and collects less condensation. It is also a full reserve when the outage comes.
- Test the fuel, not just the generator. A monthly test run proves the engine starts. It says nothing about the condition of the fuel at the bottom of the tank, which is what a long run will eventually draw.
- Filter on the way to the engine. Filtration catches what has settled or grown in the tank before it blocks the generator's own filters partway through a run. Fuel filtration explained covers micron ratings and water separation.
- Clean on a schedule. Sediment and water accumulate even in a well managed tank. Diesel tank cleaning and maintenance sets out what an inspection and clean involves.
Fuel testing and tank cleaning as a service
A-FLO's service and maintenance team covers self-bunded and bulk storage tanks, fuel filtration, automatic tank gauging, tank calibration, and tank cleaning and fuel testing, with an inspection report after every service. For a standby tank that nobody looks at between outages, a scheduled service is the simplest way to make sure somebody does.
Monitoring the Reserve
A generator tank is often in a locked enclosure or a plant room that nobody visits unless something is wrong. That makes remote visibility more valuable here than almost anywhere else on a site. Tank level monitoring reports the level without anyone opening the tank, and automatic tank gauging can raise an alert when the level drops outside a test run, which is the early sign of a leak, a faulty return or fuel going missing.
Siting and Compliance
Generator tanks sit in generator rooms, plant rooms, rooftop and basement enclosures and outdoor compounds, and each of those locations brings its own conditions. Diesel is a combustible liquid, and its storage and handling is addressed in Australia by AS 1940, alongside state and territory requirements and local council conditions. What applies depends on the quantity stored and on the site itself, and an indoor installation raises questions that an open yard does not.
Some sectors add requirements of their own on top, particularly on how long critical power must be sustained. This article does not state separation distances, quantity thresholds or sector run time minimums, because they depend on the site, the sector and the jurisdiction, and they change. Confirm them against the current instruments before the tank is specified. A-FLO's guides on Australian standards for diesel tanks and on council and EPA approval for above-ground tanks set out the approval pathway.
Where Generator Storage Matters Most
- Data centres and telecommunications. Uptime is the product, and the fuel reserve decides how long a site can hold out through a grid failure.
- Hospitals and aged care. Critical power for patient care, where run time expectations are usually set well above what an ordinary business would choose.
- Cold storage and food processing. A few hours without power can write off the stock, so the generator and its fuel are insurance on everything in the building.
- Mine sites and remote operations. Where the generator is often prime power rather than backup, and the delivery schedule is the constraint.
- Construction and events. Temporary power, where a tank that can be lifted, moved and relocated matters as much as its capacity.
What Goes Wrong
- Sizing from the generator instead of the run time. A large generator with a small tank is a short outage plan, whatever the nameplate says.
- Using the full-load consumption figure without checking it. Or worse, a rule of thumb instead of the datasheet. The number drives everything else.
- Counting fuel the pump cannot draw. The volume below the draw point is sediment protection, not reserve.
- Testing the generator but never the fuel. A monthly start proves nothing about the diesel a ten hour run will reach.
- Leaving the tank half full between outages. More air space, more condensation, and half the reserve.
- No level monitoring. Leaks, faulty returns and theft all go unnoticed in a tank nobody visits.
- Assuming the tanker will come. In a wide outage, every site rings for fuel at once.
Frequently Asked Questions
How do I size a diesel tank for a backup generator?
Multiply the generator's fuel consumption at the load you expect to run, taken from its datasheet, by the number of hours the site must run without a delivery. Then add the volume below the draw point that the pump cannot reach, and a margin for a refill that arrives late. The result is the tank size, rounded up to the next capacity.
How long can diesel be stored in a generator tank?
Diesel degrades over time, and the rate depends on temperature, water, air space in the tank and the fuel itself. Standby fuel can sit for months between runs, so it should be tested periodically, filtered on the way to the engine and kept in a full tank to limit condensation, rather than assumed to be fine because the generator starts.
What is the difference between standby and prime power fuel storage?
A standby tank turns over slowly, so its main risk is fuel ageing, water and sediment. A prime power tank empties and refills regularly, so its main risk is running dry between deliveries. Standby storage leans on filtration, testing and monitoring. Prime power storage leans on capacity and refill access.
Does a generator fuel tank need to be bunded?
Diesel storage needs secondary containment, and what applies to your site depends on quantity and location under AS 1940 and state and local requirements. A self-bunded tank carries its containment in a second skin, which is why it suits generator rooms and enclosures where building a separate bund is impractical.
Can a generator tank be installed indoors?
Generator tanks are commonly installed in generator rooms and plant rooms as well as outdoor enclosures, but an indoor installation raises more questions than an open yard. The requirements depend on quantity, the building and the jurisdiction, so confirm the position against AS 1940 and local requirements before the tank is specified.
Why does my generator stop during a long run when it passes its monthly test?
A short test run draws a small amount of fuel. A long run keeps drawing until it reaches the water and sediment that have settled in the tank, which can block filters partway through. Testing the fuel, filtering on the way to the engine and cleaning the tank on a schedule address that.
Should a standby generator tank be kept full?
Yes. A full tank has less air space, so it draws in less moist air and collects less condensation as temperatures change. It also means the whole reserve is available when the outage arrives, rather than whatever was left after the last run.
What sizes does the GENCUBE generator tank come in?
The GENCUBE range runs from 600 litres to 10,000 litres, with a 10,000 litre ISO side-loader version for transport and deployment. Each is a self-bunded cube with provisions for genset refuelling, two fill points, a lockable top hatch and forklift and crane lifting points.