Europe grapples with high energy prices, supply concerns and the prospect that climate change will lead to ever hotter summers, Bulgaria's contribution towards a solution has won it the nickname the "Battery of the Balkans".

Helped by €700 million ($783.86 million) in European Union grants, the Black Sea country has doubled its battery capacity over the last year, allowing it to store solar energy produced in the day and release it during peak hours in the evening.

That enabled a significant rise in power exports that supplied neighbouring countries when heatwaves in July and August left national grids struggling to cope with demand.

"Battery storage is critical now. It’s not a nice to have, it’s a must have," said Nikola Gazdov, chairman of industry body the Association for Production, Storage and Trading of Electricity, or APSTE for short.

European Union countries have for years sought to develop battery storage to balance the intermittent nature of solar and wind energy.

Progress has been delayed by long waits for regulatory approvals and grid connections, although adoption is expected to pick up, especially in Germany, the UK and Italy. The events of this year, when high energy prices have added to grievances that have provoked social unrest have focused minds further.

Success in storing power for when it is most needed helps to determine how much customers pay for electricity and to cushion the impact of the disruption linked to the US-Israeli war on Iran.

Bulgaria has only limited amounts of domestic oil and gas, and its electricity output is dwarfed by many other EU countries.

But it succeeded in increasing its power exports by 62% this year from last year, APSTE figures show, to 3.5 terawatt-hours after EU grants and regulatory changes helped to draw in private investment to its battery storage. Its storage capacity is now around 6 gigawatts, up from 3 GW a year ago.

Some of the exports went to Romania, which was forced to shut a nuclear reactor in August after a drought that reduced the levels of the Danube River whose waters are used to cool reactor turbines.

Apart from the exports' value, the increase in battery power also helped Romania to maintain reservoir levels by reducing its need to use hydropower, traditionally used to meet peak demand.

Bulgaria has exported as much as 50,000 megawatt-hours of electricity per day, said Dimitar Zarchev, head of Central Dispatch Management at Bulgaria's Electricity System Operator.

"To put that into perspective, that works out to an average of around 2,000 megawatts flowing out of Bulgaria every hour — roughly equivalent to the output of the Kozloduy Nuclear Power Plant.”The plant is located in northwestern Bulgaria.

The batteries are expected to have less impact in winter when reduced daylight hours mean less solar power.

Zarchev expects over the coming winter months that Bulgaria will have to rely more on conventional power plants than stored renewable power.

Further increases in battery storage also face other issues, including the need for Bulgaria to expand its grid.

The pace of the scale-up has also eroded some of the financial incentives that drew investors as price spreads have narrowed, reducing the amount of revenue to be made from the gap between the price when power is stored and when it is discharged.

"At some point, batteries may begin to cannibalise part of their revenues," Bulgaria's Energy Ministry said in a statement to Reuters. "It will then become even more important for them to participate not just in simple arbitrage between cheap and expensive hours, but also in balancing markets and system services".

Greece, known for its many months of sunshine, looked like a natural leader in the battery race.

While its solar capacity has grown significantly, more than tripling between 2020 and 2025, according to the International Renewable Energy Agency, its battery storage is lagging and the country depends on imported gas to meet peak evening demand.

Despite three rounds of auctions for standalone batteries since 2023, Greece only installed a first chunk in April. Total storage is expected to reach one gigawatt by the end of the year, a small fraction of Bulgaria's capacity, according to government figures.

Greek retail electricity excluding taxes and levies averaged €0.1967 per kilowatt-hour in the first half of the year, compared to €0.1117 for Bulgaria, data from the EU statistics service Eurostat showed.

The Greek energy ministry said in a presentation last month it was working to speed up storage with the help of EU funds to try to lower energy prices.

Officials have said another 5 GW of battery capacity is expected to come online by 2030, in addition to the GW expected to be online by the end of this year. For some that is not soon enough.

"If we had had those six gigawatts operating now, our power system would have been more balanced and this would have passed on to consumers," Manolis Karapidakis, the chief executive officer of the Greek association of power storage developers, told Reuters.