Tokyo Electric Power (Tepco) expects a significant slump in full-year profits after the closure of a key nuclear plant following an earthquake.
The Kashiwazaki nuclear power plant has been closed
Radiation leaks from the plant after the quake on 16 July led to it being closed in order to meet safety rules.
As a result, Tepco, the largest power producer in Asia, has cut its net profit forecast by 79% to 65bn yen (£268.9m), for the 12 months to March.
Tepco's shares have dropped around 15% since the earthquake.
The firm plans to buy other sources of nuclear power and to use a hydro power plant in order to meet peak summer demand.
Japan is a heavy user of nuclear energy and the firm's Kashiwazaki-Kariwa plant was meant to answer more than 3% of the nation's energy needs.
But the government - under pressure from the International Energy Agency to examine the plant - has demanded it should be closed until it is proved that safety requirements can be met.
Continued purchase of substitute sources of energy including expensive oil and coal are expected to add to the firm's costs.