A backup power generator is only as reliable as the fuel waiting in the tank or canister. This is precisely where it surprisingly often fails in practice. The generator has been serviced, the battery is charged, the test run was successful months ago – and in an emergency, the engine still starts poorly, runs erratically, or cuts out. The cause then is often not the engine, but aged fuel.
Anyone who takes emergency power preparedness seriously must therefore not only deal with performance, connection, and switching, but also with the question: How do I store diesel or petrol so that the fuel is still usable when needed? This involves aging, bio-components, diesel bug, tank hygiene, canister material, legal requirements, and ultimately the very practical question of where new fuel will come from at all during a prolonged power outage. The relevance of this topic is also shown by official publications from the BSI and BBK: especially for fuels stored for longer periods in backup power systems, fuel quality is a real risk factor.
Why fuel does not simply "stay good for years"
Many operators mentally treat fuel as a durable reserve. This is understandable, but technically too simplistic. Fuels change over time. Oxygen, temperature fluctuations, water, metallic materials in the piping system, and biogenic components accelerate this process. It becomes particularly critical when the fuel remains in the tank for years and is only moved in small quantities during short test runs. This is precisely what the BSI (Federal Office for Information Security) explicitly points out for backup power systems: The DIN EN 590 standard assumes significantly shorter storage times for diesel in normal use, while fuel in backup power systems often stands for years.
The results from the samples analyzed by the BSI from backup power systems were clear: a large proportion of the diesel-powered systems had fuel in a critical or already unusable condition. For 60 percent of the diesel samples, the BSI found immediate or imminent unsuitability; for low-sulfur heating oil, this was significantly rarer.
Diesel: Bio-components, water, and diesel bug
The core problem with conventional diesel fuel is the FAME content, i.e., the admixed biodiesel component. The BSI explicitly describes that this component can decompose during prolonged storage. This leads, among other things, to the formation of water and acids, which in turn promote oxidation, aging, and microbial contamination. This very combination also favors what is colloquially known as diesel bug.
FAME (Fatty Acid Methyl Ester) is the biodiesel component in regular diesel fuel. This biogenic component does not improve storage stability; rather, it makes diesel more susceptible to aging, water, and diesel bug during long-term storage.
Linguistic precision is important here: not every aging process is already diesel bug. The BSI clearly distinguishes between general fuel degradation and microbiological growth. Diesel bug is strictly speaking the microbial infestation that occurs due to the water phase and suitable living conditions in the tank. The consequences are well known: slime formation, clogged filters, corrosion, deposits in pipes, and in the worst case, starting problems or engine damage.
Heat is particularly critical. The BSI points out that day tanks in backup power systems are often additionally heated, for example by proximity to the engine or returned warm fuel. This further accelerates aging. So, anyone who only looks at the large main tank and forgets the smaller day tank often overlooks the more critical point of the system.
HVO as a more storage-stable alternative?
Another topic that is increasingly appearing in fuel storage for emergency power generators is HVO. The abbreviation stands for Hydrotreated Vegetable Oil, a paraffinic diesel fuel produced from biogenic residues and waste materials. Legally relevant is that paraffinic diesel fuel of quality XTL may be placed on the market for end consumers in Germany according to § 4 paragraph 3 of the 10th BImSchV if it complies with DIN EN 15940. Since May 29, 2024, this pure fuel use has been explicitly permitted in Germany.
For operators of emergency power generators, HVO is particularly interesting because it does not contain FAME components like classic B7 diesel. According to the BSI, precisely this biodiesel component is one of the main reasons why diesel becomes chemically unstable during prolonged storage, forming water and acids, and increasing the risk of aging and microbial contamination. HVO largely bypasses this weakness. In practice, this means: better oxidation stability, lower susceptibility to diesel bug, and usually more favorable conditions for longer storage times than with conventional road diesel. This makes HVO particularly interesting for emergency power applications where the fuel remains in the tank for extended periods and is rarely used. The burdens caused by bio-components, which the BSI explicitly highlights as problematic for classic diesel, are not present in this form with HVO.
Nevertheless, HVO is not a carte blanche for arbitrarily long storage. HVO also ages if water, dirt, unsuitable container materials, high temperatures, or poor tank hygiene come into play. Tank maintenance, sampling, clean containers, and a controlled storage strategy therefore remain just as important as with other fuels. Furthermore, before use, it must always be checked whether the engine and injection system are approved by the manufacturer for EN 15940 or XTL/HVO. Especially with older generators, this is not a mere formality. Anyone who switches without approval risks technical problems and, in case of doubt, discussions regarding warranty or service.
From a technical perspective, HVO can therefore be a very interesting solution for many emergency power generators – especially where diesel is currently rarely consumed and remains in the tank for a long time. However, it does not replace the basic rules of safe fuel storage. Those who use HVO should plan the topic just as carefully as with diesel or heating oil: check manufacturer approval, evaluate tank material, avoid water, document test runs, and regularly check fuel quality.
What additives can and cannot do
Additives are not magic bullets. They can be useful, but they don't replace a clean storage strategy. The BSI explicitly states that biocides only act against microbiological growth. They do not eliminate the actual chemical aging of FAME-containing diesel. The BSI also notes that no sufficiently effective and at the same time side-effect-free additives were known for the long-term storage of diesel with FAME content that reliably stop decomposition.
In practice, this means: additives can help reduce a risk, but they don't turn normal road diesel into a stable crisis fuel for years. Those planning long-term storage primarily need a suitable fuel concept, regular checks, and, if necessary, regulated fuel replacement.
Petrol ages differently – but no less harmlessly
With petrol, the problem is slightly different. Here, microbiological effects are less prominent; rather, it's about changes in composition due to oxidation and the escape of volatile components. Precisely these volatile components are important for starting behavior and ignition readiness. If petrol is stored for too long, the fuel changes. The engine starts less easily, runs more erratically, or forms deposits in the carburetor and injection system.
Especially with small petrol-powered inverter generators, this is a typical weak point. The generator itself is often technically sound – but the old petrol in the canister or tank is no longer. In addition, petrol is more demanding to store and transport than diesel due to its higher flammability and explosive risk. The BBK explicitly points out that the storage and distribution of petrol is technically more complex than with diesel.
Storage duration: A blanket figure is only of limited help
The often-asked question "How long does diesel or petrol last?" can only be answered roughly. Crucial factors are fuel quality, container material, air exchange, water ingress, temperature, and movement within the system. For backup power systems, therefore, the simple logic of "fill it up once and forget it for five years" does not apply. The BSI recommends regular sampling and evaluation of fuel quality – regardless of the type of fuel, at least at recurring intervals.
Therefore, a practical approach is not just a calendar deadline, but a system consisting of:
- clean starting fuel,
- controlled storage,
- regular test runs,
- documented tank hygiene,
- analysis or replacement before critical conditions are reached.
Tank hygiene is not a minor issue
Many problems arise not only from the fuel, but also from the tank. Water in the tank, rust, abrasion, copper components from pipes, sludge at the bottom, or unsuitable materials significantly worsen fuel quality. The BSI explicitly points out that, especially with FAME-containing diesel, additional contamination and aging can occur due to interactions with materials. The paper recommends, among other things, oil-carrying pipes made of stainless steel or aluminum and regular quality controls.
In practice, tank hygiene means:
- avoiding water formation and condensate,
- not leaving tanks "half empty for years",
- regularly checking the tank bottom,
- checking filters and water separators,
- taking samples at a suitable sampling point,
- not paying attention to deposits only when the engine fails.
Especially stationary systems with day tank and main tank should be monitored in a documented way. Anyone relying on a system in the event of a blackout should not have to guess what the fuel condition is.
Aging of canisters and tanks
Not only the fuel ages, but also the container. Plastic canisters can become brittle over the years, mechanically weakened, or age due to UV exposure. Seals harden. Metal canisters and tanks can corrode, especially with moisture, condensation, or internal contamination. In practice, this is particularly important for reserve canisters that "go along" for years in garages, containers, workshop shelves, or trailers.
For commercial applications and public institutions, clear requirements from water and hazardous substances law apply. Section 62 of the Water Resources Act requires that installations for handling substances hazardous to water must be designed, constructed, maintained, operated, and decommissioned in such a way that no adverse changes to waters are to be expected. In addition, Section 45 of the AwSV regulates when activities may only be carried out by specialized companies, and Section 62 regulates the certification of such specialized companies.
Which regulations should one know?
For the storage and handling of fuels around emergency power generators, the following regulations are particularly relevant:
- WHG § 62: Basic requirements for installations for handling substances hazardous to water.
- AwSV §§ 45 and 62: Requirement for specialized companies and certification of specialized companies for certain installations and activities.
- WHG § 78c: Heating oil consumer installations in flood-prone and risk areas; special requirements and retrofitting obligations apply there.
- WHG § 78a: In designated flood areas, the storage of substances hazardous to water outside of installations is not permitted.
- Hazardous Substances Ordinance and TRGS 510: Requirements for the storage of hazardous substances in portable containers in commercial areas. The BAuA refers to TRGS 510 as the authoritative technical rule.
- EnergieStG § 3: Favored stationary power-generating installations. For stationary installations, the tax-related use of certain fuels is regulated separately; the current coordination with the main customs office, installation concept, and manufacturer's approval is decisive.
For existing heating oil consumer installations in flood-prone and risk areas, the state of Baden-Württemberg has clearly summarized the legal retrofitting obligations with reference to § 78c WHG and the operator and testing obligations according to AwSV §§ 45, 46, 47.
Where do I get new fuel in case of a disaster?
This is one of the crucial questions – and at the same time one of the biggest misconceptions in many emergency power concepts. Many operators assume that in an emergency, "you just refuel." This is precisely the difficult part during a widespread power outage.
The BBK clarifies that although there are generally sufficient fuel reserves in Germany, the challenge lies in storage and distribution on the last mile. Fuel depots, pumps, booking systems, filling stations, communication, and transport themselves depend on electricity, IT, and personnel. Without prepared emergency planning, fuel supply will therefore not function automatically.
The BBK therefore explicitly recommends:
- determining fuel requirements in advance,
- designating key fuel depots and key filling stations,
- providing emergency power for pumping, security, and documentation systems,
- organizing transport capacities in advance,
- establishing priorities for critical consumers.
Similarly, the BBK points out that critical infrastructure operators should be able to maintain their backup power supply for at least 72 hours without fuel replenishment. For companies, this means in practical terms: if you don't have a reliable supply of your own, you don't have a real emergency power provision.
Heating oil instead of diesel?
This is a sensitive topic and not automatically the right solution for every system. Technically, however, the BSI clearly shows that low-sulfur heating oil according to DIN 51603-1 can be significantly less critical than diesel according to DIN EN 590 for long-term storage in NEA applications – provided that the engine manufacturer approves its operation and the overall system is designed for it.
Then there's the tax aspect. In the BSI/BBK context, reference is made to its use in stationary power-generating systems in relation to energy tax law. In practice, this means that the issue must be clearly clarified with the manufacturer, specialist planner, and, if necessary, the main customs office. Simply "refilling and hoping" is not a good solution.
The better strategy: less storage pressure through hybrid solutions
The more operating hours an emergency power generator has to cover in the event of an incident, the more significant the fuel issue becomes. This is precisely why hybrid systems are becoming increasingly attractive. A combination of a PV system, battery storage, and emergency power generator can significantly alleviate the fuel problem.
The reason is simple: the storage unit handles short-term loads, load peaks, and part of the basic supply. The PV system provides additional energy during the day, if available. The emergency power generator only starts when the storage and PV are insufficient or when there are longer periods of bad weather. This reduces:
- Fuel consumption,
- Refueling requirements,
- Storage pressure,
- Engine operating time,
- Maintenance effort.
Especially for single-family homes, agricultural businesses, smaller commercial enterprises, and mobile applications on construction sites or at events, this can be a very sensible solution – both technically and economically.
Conclusion
Fuel storage for emergency generators is not a minor issue, but part of operational safety. Diesel ages, bio-components exacerbate the problem, gasoline loses volatile components, water and dirt ruin tank contents, canisters and tanks also age – and in the event of a disaster, new fuel is often much harder to get than many assume. Official publications from BSI and BBK clearly show that long-stored fuels in uninterruptible power supply systems are a real weak point.
Anyone who takes emergency power seriously therefore needs not only a good generator, but a clean overall concept of fuel choice, tank hygiene, storage strategy, legally compliant execution, and realistic replenishment planning. And in many cases, it is worthwhile to consider the whole thing as a hybrid system with battery storage and PV right from the start.
If you would like to find out which storage solution, fuel, or hybrid concept suits your application, we at SEV will be happy to advise you and recommend the right generator for your needs.