TOKYO (Reuters) -JERA, Japan's biggest power generator, on Tuesday forecast its profit for the fiscal year ending next March to halve to 200 billion yen ($1.3 billion) on lower revenue from overseas power generation business and smaller gains from the time lag effect.

JERA, an unlisted company jointly owned by Tokyo Electric Power and Chubu Electric Power, posted 399.6 billion yen in profit for the year ended March 31, up from 17.8 billion yen in the year-ago period.

The power generator said results were driven by profit from overseas power generation and renewable energy business, as well as the time lag gain in the fuel price adjustment mechanism - factors whose impact it expects to be smaller this year.

The time lag effect - when a change in fuel prices is reflected in sale prices with a delay - added 250.9 billion yen to JERA's profit last year while overseas power generation and renewable energy business posted 33.7 billion yen, reversing last year's losses on improving power plant business abroad.

For the year ending in March 2025, JERA expects the time lag effect to contribute 50 billion yen to its profit with overseas power generation and renewable energy adding another 20 billion yen. In its presentation on Tuesday, the company did not explain reasons for the forecast change.

Excluding time lag effects, profit is forecast at 150 billion yen, in line with the fiscal year of 2023, JERA said.

($1 = 156.8500 yen)

(Reporting by Katya Golubkova; Editing by Sherry Jacob-Phillips and David Evans)