Data centres in Bulgaria: Backup power – diesel, gas and batteries

Backup power is one of the first technical decisions on an AI data centre project, and one of the first legal ones. A campus of 100 MW or more may need dozens of generator units, fuel storage, battery systems or a gas-fired plant. Each option brings its own permitting route in Bulgaria, and the choice affects the environmental assessment, the grid connection and the commitments a developer can give to an offtaker.

Background

In our previous updates, we outlined the EU and Bulgarian legal framework for data centres and the requirements that AI offtakers set for developers. This update looks at backup power. It sets out the rules that apply to diesel generators, gas-fired generation and battery storage in Bulgaria, and the points developers should agree with offtakers before applying for permits.

Diesel generators

Diesel generators remain the default form of backup power for data centres. In the projects we see, they are installed as modular units of 2 to 4 MW each, so a large campus can reach an aggregate rated thermal input of several hundred megawatts. The legal regime depends on the size of each unit and of the installation as a whole:

1. Medium combustion plants. Combustion plants with a rated thermal input of 1 MW or more and less than 50 MW are subject to the Ordinance on limiting emissions of certain pollutants into the air from medium combustion plants, which transposes Directive (EU) 2015/2193. A new plant must be entered in the register kept by the competent Regional Inspectorate of Environment and Water (RIOSV) under the Clean Ambient Air Act and cannot operate before registration. Plants that operate no more than 500 hours a year, calculated as a rolling average, may be exempted from the emission limit values. The exemption is not automatic: it is granted by the Minister of Environment and Water on application, supported by a declaration from the operator, who must keep a record of operating hours. The 500 hours relieve the plant from emission limits, but do not cap the hours it may run.
2. Integrated permit. Where the total rated thermal input of the combustion installations on a site reaches 50 MW, the site may require an integrated permit under the Environmental Protection Act, which implements the Industrial Emissions Directive. In that case, registration as a medium combustion plant is not required, but the permitting process is longer and imposes best available techniques, monitoring and reporting obligations.
3. Emissions trading. A large bank of generators may bring the site within the EU Emissions Trading System, which covers combustion installations with a total rated thermal input above 20 MW, subject to exclusions for small units. The site operator would then need a greenhouse gas emissions permit and would have to monitor, report and surrender allowances for its emissions.
4. Fuel storage, noise and testing. On-site fuel tanks are subject to fire safety, environmental and excise requirements. Regular testing of generators is a source of noise and emissions, and developers should assess its effect on nearby receptors at the planning stage.

The number and size of the generators should be fixed early. A change in the backup configuration after the environmental screening decision may require a new assessment and may move the site across one of the thresholds above.

Practice across the EU shows two further points that developers in Bulgaria should take into account. First, regulators tend to count operating hours for the installation as a whole, so the installation is in operation when any one generator runs, including during tests. Second, the way units are aggregated depends on the regime: some thresholds count every unit on the site, while others disregard units below a set size. The choice between a smaller number of large units and a larger number of small units can change the permitting route. Regulators in other Member States treat data centres as one installation where they are technically connected, whoever owns them, and set caps in the permit of between 72 and 360 operating hours a year, including tests. The capital cost of diesel backup is in the order of EUR 1,000 per kW of generator capacity, and many operators have switched their generators to hydrotreated vegetable oil (HVO), which can be used in modern diesel engines without modification and reduces lifecycle emissions.

Gas-fired generation

Some AI campuses replace diesel with gas turbines or gas engines, either as backup or as a source of on-site generation that operates in parallel with the grid. This option reduces local emissions and can shorten the time to power where the grid connection is delayed, but it raises additional legal questions:

1. Gas connection. The site needs a connection to the gas transmission or distribution network, with its own application, technical conditions and connection contract with the relevant network operator.
2. Combustion plant rules. Gas turbines and gas engines of 1 MW or more are subject to the medium combustion plant regime, and installations of 50 MW or more to the large combustion plant rules and the integrated permit regime. Under the large combustion plant rules, the emission limit values do not apply to gas turbines and gas engines used for emergencies that operate no more than 500 hours a year, provided the operator records the operating hours.
3. Generation licence. A plant that supplies the data centre and does not sell electricity to third parties raises different questions from one that exports to the grid or takes part in the balancing market. Where the plant sells electricity, the licensing and market registration rules under the Energy Act apply, and developers should assess these at the design stage.
4. Direct supply. Where the gas-fired plant is owned by a third party and supplies the data centre through a dedicated line, the arrangement must comply with the rules on direct lines and on the supply of electricity under the Energy Act.

The choice between diesel and gas depends on whether the plant will serve as backup or will cover part of the load as well during the period before full grid capacity is available.

Regulators in other EU markets have started to treat on-site generation as a condition for grid access. In Ireland, new data centres must provide dispatchable generation or storage on or near the site, matching their import capacity, taking part in the wholesale market and not limited in its running hours. A diesel plant that relies on the 500-hour exemption cannot meet this requirement. Bulgaria has no such requirement, but a similar approach would fit the tighter connection rules that the Energy and Water Regulatory Commission is considering.

Batteries and UPS systems

Battery systems serve two functions on a data centre site: uninterruptible power supply (UPS) that bridges the seconds before generators start, and larger battery energy storage that can carry part of the load, shift consumption or provide services to the grid. Bulgaria has one of the fastest-growing battery storage markets in the EU, and its rules are favourable to this option:

1. No licence for storage. Under the Energy Act, electricity storage is recognised as a separate activity and is not subject to licensing.
2. Grid connection. Where a battery is added to an existing consumption site and increases its connected capacity, the amendments to Ordinance No. 6 require a guarantee of EUR 25,564.59 (formerly BGN 50,000) per MW of additional connected capacity. The guarantee must be provided within three months of ESO's opinion on the connection conditions, failing which the opinion ceases to be valid.
3. Grid services. A battery system that is large enough may take part in the balancing market through ESO, either directly or through an aggregator. Revenue from these services can offset part of the cost of the backup system, but the arrangement must not reduce the availability the data centre has committed to its offtaker.
4. Fire safety. Lithium-ion UPS and battery systems are subject to the fire safety rules for buildings and require approval of the design and acceptance on completion. Fire safety requirements for battery rooms are one of the items that can delay the use permit.

Batteries can reduce the number of hours that generators run and support the case for flexible connection to the grid that offtakers and the network operator look for. They do not, as a rule, replace generators where an offtaker requires extended autonomy.

Water is a further form of storage in Bulgaria. The country's pumped storage hydropower plants, the largest of which is Chaira, store energy at grid scale and help the system operator balance supply and demand. A data centre cannot use pumped storage as on-site backup, but it can benefit from it through a supply contract or power purchase agreement with a hydropower generator, and through the resilience it adds to the grid. Pumped storage complements on-site batteries and generators but does not replace them.

Environmental assessment and offtaker requirements

Backup power forms part of the project for the purposes of the environmental impact assessment. The screening application should describe the number, type and rated thermal input of the generators, the fuel storage and the expected operating hours, including testing. A description that understates the backup configuration may lead to a new procedure once the design is fixed.

Offtaker requirements are moving in two directions. Facilities designed to a concurrently maintainable standard, such as Uptime Tier III, treat on-site generation as the primary source of power, which points to a full set of generators. At the same time, some offtakers accept reduced or no generator backup for AI training halls, where a short interruption affects a computing task rather than a customer service, and keep full backup for inference and storage halls. In our experience, the backup scope should be agreed for each data hall with the target offtaker before the developer applies for permits, as the configuration determines which of the regimes above will apply.

Autonomy is a further point to agree. Offtakers that treat a facility as critical infrastructure expect it to run on backup power for at least 24 hours, and in many cases for 48 to 72 hours, which determines the volume of fuel stored on site. The revised EN 50600 standard, in force since 2025, sets stricter requirements on redundancy, the switchover time between the grid and backup supply, and the documentation of tests. Where the workload allows it, operators reduce generator hours by shifting computing tasks in time or between sites, and offtakers may accept this for training workloads.

What is next

With offtakers reviewing their backup requirements, the network operator expected to favour flexible connections and the EU rating scheme applying from 2027, the backup configuration has become a strategic choice for AI data centre projects in Bulgaria, and not a technical detail. Developers should agree the backup scope with the target offtaker, calculate the aggregate rated thermal input of all combustion units on the site, and reflect the full configuration in the environmental screening application before applying for permits.

For further information and to stay up to date on forthcoming developments, contact your usual CMS contact or our local CMS Sofia experts, Ivan Gergov and Anna Philcheva.