Bulgaria doubled its battery storage capacity in a year, from about 3 GW to about 6 GW. Data from the Association for Production, Storage and Trade of Electricity (APSTE) show the country exported 3.5 TWh of power, 62% more than last year.
The speed of the build-out has led people to call Bulgaria "the battery of the Balkans". The EU has put €700m into storage here, and that money, along with changes to the rules, has also drawn private investment.
The batteries store power from solar plants during the day and release it in the evening, when demand is highest. This has boosted exports to neighbouring countries, whose grids were under heavy strain in the July and August heatwaves.
The Electricity System Operator (ESO) says up to 50,000 MWh a day leaves the country. Dimitar Zarkov, head of ESO's central dispatch centre, said exports average about 2,000 MW an hour, roughly the output of the Kozloduy nuclear plant in north-west Bulgaria.
Some of the power went to Romania. In August, Bulgaria's northern neighbour shut down a nuclear reactor because of drought. The low Danube made it hard to cool the turbines, which use river water. Bulgarian power helped Romania save water in its reservoirs, because it had to run its hydropower plants less at peak hours.
Nikola Gazdov, chairman of APSTE, says battery storage is now decisive: a necessity, not a nice-to-have.
Greece has many sunny days but lags on storage. Its solar capacity more than tripled between 2020 and 2025, according to the International Renewable Energy Agency. Greece has held three auctions for stand-alone battery systems since 2023, but the first capacity was only installed in April this year. It still covers the evening peak with imported natural gas.
Eurostat figures show that in the first half of the year, a kilowatt-hour cost households in Greece €0.1967 before taxes and fees. In Bulgaria it cost €0.1117.
A plan from Greece's energy ministry, announced last month, aims to build storage faster with EU money to bring down power prices. Government figures show 1 GW is expected by the end of the year. Officials talk of another 5 GW by 2030. Manolis Karapidakis, chief executive of the Greek association of storage developers, says that if those 6 GW were running now, the Greek system would be better balanced and consumers would feel it.
Across Europe, energy prices are high, there are worries about supply, and summers are getting hotter because of the climate. The EU has for years wanted more storage to fill the gaps when the sun and wind do not produce power. Slow permits and slow grid connections stand in the way. Batteries are expected to spread faster in Germany, Britain and Italy. Bulgaria has little oil and gas of its own and produces less power than many EU countries.
High prices this year have fed discontent and unrest, and raised interest in storage. Power kept for peak hours affects final prices and softens the effect of supply disruptions linked to the US and Israeli war against Iran.
In winter, batteries in Bulgaria will play a smaller role, because shorter days give less solar power. Zarkov says Bulgaria will then rely more on conventional plants than on stored green energy. Another jump in storage will also need a bigger transmission grid.
Fast growth is already weakening the incentive for investors. The gap between the price of power in cheap and expensive hours is narrowing, and with it the profit from storing and reselling power.
The energy ministry said in a statement that at some point batteries may start to eat into their own revenue. It said it will matter more for them to take part in more than trading between cheap and expensive hours. They also need to be in the balancing market, where gaps between power produced and power used are evened out, and in system services.