Iran has restarted the Bushehr nuclear power plant after it was shut down last month, the government-linked Irib news agency reported.
State electricity company Tavanir said during the closure that essential maintenance work would be conducted at the site, leading to power cuts.
But in March, Iranian nuclear official Mahmoud Jafari said Bushehr was experiencing technical problems owing to the inability of engineers to find spare parts.
US sanctions were affecting the work of Russian contractors at the site, he said.
"After repairs the Bushehr power plant is back online, and 1,000MW of electricity is injected into the country's distribution network," Mostafa Mashhadi, a spokesman for Tavanir, told Irib.
That might not help matters much as Iran is currently in the midst of one of its worst electricity shortages and rolling blackouts are now frequent.
Iran said that electricity demand had hit a record of 65.9 Gigawatts, well above the available 55 Gigawatt production capacity.
Extremely hot summer temperatures had led to a surge in demand, mirroring the situation in neighboring Iraq.
The Bushehr reactor was commissioned by the Shah of Iran in the 1970s, but its completion by German contractors was disrupted by the revolution and the Iran-Iraq War.
Russian contractors eventually made the power plant operational and it opened in 2011.
Russia supplies the plant with enriched uranium, closely monitored by the UN's International Atomic Energy Agency.
Iran has ceased co-operation with the agency at other nuclear sites in the country. There are concerns that advanced centrifuges at Natanz are slowly building up enough medium-enriched uranium to produce highly enriched, or "weapons-grade", uranium, sufficient for a nuclear device.
In 2019, Iran began work on a new reactor at Bushehr, claiming that newly enriched uranium stockpiles would be used for the power plant.
The resumption of operations at Bushehr comes as China urged the US to rejoin the so-called nuclear deal, as negotiations between Tehran and world powers aimed at restoring the accord falter.
Wang Yi, the Chinese Foreign Minister, said the US should "completely remove all its illegal and unilateral sanctions on Iran and any third party, to meet Iran halfway and make further breakthroughs in the negotiation.”
Iran has said in recent weeks that it has no obligation to allow international inspectors full access to its nuclear sites, something the US and EU insist would be the first step to restoring confidence in a new deal.
Inspections by the International Atomic Energy Agency were a cornerstone of the deal, technically known as the Joint Comprehensive Plan of Action.
“With regard to the Iranian nuclear issue, the most important thing is that the United States should make its decision to return to the agreement as soon as possible. After 13 years of arduous negotiations, the JCPOA (Joint Comprehensive Plan of Action) is an important achievement of multilateralism and a paradigm for dispute settlement through dialogue and consultation,” Mr Wang said.