More than 90,000 households in Chiba Prefecture remained without power as of 4 a.m. on Monday, a week after a severe typhoon.
Tokyo Electric Power Company says work to restore electricity may have to be suspended due to current weather conditions.
TEPCO is emphasizing the need for safety, as localized thunderstorms are forecast through Monday morning.
The utility said earlier that it would restore power in Chiba, Ichihara and Kisarazu cities by Monday.
It said it would restore power in southern parts of the prefecture by September 27, including the cities of Minamiboso and Tateyama.
TEPCO now says it may take longer to restore power in some districts, as workers ensure the stability of the electricity supply.
The company is warning of possible fires caused by damaged wiring as the power supply is restored. It is calling on people living in houses without power to turn the main circuit breaker off before going out or going to bed.
The utility is providing information on the power outage on its website: http://teideninfo.tepco.co.jp