Hungary's Paks nuclear plant could operate at its current reduced capacity until Monday or Tuesday, easing pressure on the electricity grid as the country faces a "critical" five days starting on Monday, Prime Minister Peter Magyar said.

However, the plant is expected to be completely powered down later this week as water levels on the Danube river, whose waters it uses as a coolant, are projected to fall further.

The 2GW Paks power plant, which normally generates about half of the country's electricity, was operating at just over 10 per cent of its capacity on Monday after the Danube fell to a record low.

"Today, and maybe tomorrow, the last turbine producing 240MW can keep working," Magyar said.

"This is very good news, as the complete shutdown of the plant can be avoided on one of the days with the highest consumption," he said.

Large parts of Europe have been hit by prolonged heat and drought, lowering river levels and raising concerns about water supplies, river transport and power generation.

"The most critical five days are starting now ... The energy grid and the water supply are under increasing strain," Magyar said in a Facebook video.

As the Paks nuclear plant is gradually powered down, Hungary will increase its reliance on imports as the heatwave is expected to boost demand for electricity.

Magyar said on Monday that voluntary cuts in power usage by households and companies made after a government appeal reduced demand by 700MW on Sunday - equal to the output of one and a half reactors in Paks - significantly easing pressure on the electric grid.

Adding to voluntary cuts by other companies, pharma company Richter said on Monday that it would reduce its electricity consumption by more than 50 percent in the next three weeks, and offered a daily 4MW of electricity produced by its solar energy park.