Three energy giants Tuesday told Germany's top court that they should be compensated for the nuclear phase-out Chancellor Angela Merkel's government decided after the Fukushima disaster five years ago.

Energy giants E.ON, RWE and Sweden's Vattenfall want the Constitutional Court to rule that the move amounted to an "expropriation" of their assets.

They hope that would bolster damages claims they have already launched in lower regional courts where they are reportedly demanding a total of at least 15 billion euros ($17 billion) in separate cases.

Merkel's government decided after Japan's March 11, 2011 earthquake, tsunami and subsequent Fukushima reactor meltdowns to halt operations of Germany's eight oldest nuclear plants, and to shutter the other nine by 2022.

Today, eight of Germany's 17 nuclear plants remain in operation.

The move, shortly before key regional elections, marked a sharp reversal for Merkel. She had earlier overturned a phase-out ordered by a previous government in 2002 and extended the lifespan of Germany's nuclear fleet until 2036.

The chief of Germany's biggest power company E.ON, Johannes Teyssen, said: "Our constitutional complaint is not directed against the political decision to phase out nuclear energy. We explicitly respect this decision.

"But we cannot simply accept that parliament disregarded constitutional requirements by providing for no compensation.

"For our shareholders -- including many small stock holders who have their savings and pensions invested with us -- this creates a significant financial loss which under current law will not be compensated for. Our constitutional complaint argues against, and only against, this point."

- Billions in losses -

The energy companies have booked heavy losses amid the nuclear phase-out and shift towards green energy, much of it produced by wind and solar plants operated by households, small businesses and municipalities.

The glut of government-subsidised renewable power has led to a collapse in wholesale electricity prices.

E.ON this month said it suffered a seven-billion-euro net loss in 2015, blaming in part the massive write-downs on the value of its electricity plants.

The companies filed the first of a spate of dozens of legal cases in 2012, most in the courts of the German states where particular plants are located.

Swedish state-owned company Vattenfall is also seeking 4.7 billion euros in damages from Germany before a US tribunal.

Critics argue that the big energy companies benefited from massive state subsidies when the nuclear plants first went into operation.

Safely decommissioning all the plants and storing their radioactive parts and waste will cost around 50 billion euros, experts estimate.

Industry observers say the companies hope to use a favourable constitutional court ruling as leverage in parallel talks on state aid for decommissioning the atomic plants.

However, Environment Minister Barbara Hendricks said Tuesday that "the government will not agree to any deals".

She also defended the decision to abandon nuclear power, saying that "against the background of the decades-long controversial societal debate, there could be no more 'business as usual'" after Fukushima.

The Constitutional Court hearings run until Wednesday, and a ruling is not expected for several months.

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