TEPCO, the operator of the crippled Fukushima Daiichi nuclear plant on Japan's northeast coast, posted a bigger-than-expected annual loss of $9.67 billion on Monday and warned of tough times ahead.

Tokyo Electric Power said its 781-billion-yen net loss came in a year in which it was hit with massive costs to deal with reactor meltdowns, as well as increased imports of fossil fuels to make up for a nuclear power shortfall.

The net loss was worse than a previous prediction for a 708.0 billion yen shortfall and reflects an increase in projected compensation payouts to those affected by the world's worst atomic disaster in a generation.

Revenue was 5.35 trillion yen, down from 5.37 trillion yen a year earlier.

Naomi Hirose, the company's incoming president, warned that "unexpected situations" this summer could make its already shaky energy supply even tougher as Japan's atomic reactors remain offline.

"Even if we have power supply to barely meet demand this summer, it doesn't mean we will be fine," he told a press briefing in Tokyo.

However, the embattled firm said its loss in the current fiscal year through March 2013 would come in at a much lower 100 billion yen, while sales were set to rise to 6.0 trillion yen on an expected economic pickup and rate hikes.

TEPCO's earnings come less than a week after Japan's government confirmed it will take a controlling stake in the firm, effectively nationalising one of the world's largest utilities.

Tokyo will inject one trillion yen as part of a 10-year restructuring aimed at preventing the vast regional power monopoly from going bankrupt, a plan it said would see TEPCO come under "temporary state control."

On Monday, the firm's president Toshio Nishizawa said fuel costs are likely to jump this year after Japan shut down the nuclear reactors that once supplied about one-third of its electricity, following last year's atomic crisis.

"This increase in fuel cost is based on an assumption that we will have no nuclear power in the year (to March)," Nishizawa said.

He added that the utility expected industry to accept rate hikes, while residential rates are also set to rise, as the economy picks up pace.

"In the last fiscal year, business operators had their production levels below full capacity," Nishizawa told a press briefing in Tokyo.

"But their production levels are recovering in the second half of the year, which we expect to continue in the new fiscal year," he said.

TEPCO said the amount of electricity it sold in the latest period fell 8.6 percent as clients ushered in both mandatory and voluntary energy saving measures in the summer when demand peaked.

As the sole provider of electricity to Tokyo and a vast surrounding region in eastern Japan, TEPCO is responsible for maintaining a stable power supply to its millions of customers.

But it is staring at an enormous bill with decommissioning of four crippled reactors and the clean up of the surrounding area expected to take decades.

With public distrust in nuclear energy running high, Japan's entire pool of atomic reactors are offline, leaving TEPCO and fellow utilities with no choice but to fire up expensive fossil-fuel-powered plants.

The giant utility tried to reign in costs by chopping salaries and reducing its procurement costs, but it faced skyrocketing fuel expenses to power old thermal power plants.

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