Demand for electric power was 77.54 billion kwh in July, up 12.5 percent from a year earlier.
The Federation of Electric Power Companies of Japan said in a preliminary report Friday that the increase reflects the unusually cool summer last year and record-breaking heat this year.
It said demand at the nation's 10 top power utilities increased for the seventh consecutive month.
Demand for household use rose 17.8 percent due to increased use of air conditioners. It grew 11.4 percent in the machinery industry and 8.1 percent in the chemical industry.
Combined power supply by the 10 companies hit a record 92.57 billion kwh in July, the largest amount in a single month since May 1972, when the nation's electric power industry was reorganized into the 10 utilities.
Power supply and demand do not necessarily coincide because of time differences in meter-reading.