UPDATE 1-China Sept power output growth at 3-month low

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SHANGHAI Oct 18 (Reuters) - China's power output in September rose 1 .5 percent from a year ago, with the pace of growth cooling to the lowest in three months as a slowing economy dampened electricity demand from industrial users.

Power production in September was 390.7 billion kilowatt-hours (kWh), down 10 percent from August and the lowest since May, data from the statistics bureau showed on Thursday.

Annual growth in power output has slowed markedly from the double-digit rates posted most of last year, as demand from core industrial users in the steel, cement and smelting sectors weakened on the back of China's economic slowdown.

China's GDP grew 7.4 percent in the third quarter from a year earlier -- in line with forecasts from economists polled by Reuters -- the first miss of the official target since 6.5 percent growth in the first quarter of 2009.

In further evidence of slack industrial demand, power consumption rose just 2.9 percent from a year earlier in September, slowing further from the previous month to the lowest in eight months.

Tepid demand from energy-intensive industries has caused domestic coal prices to drop around 20 percent since January. (Reporting by Fayen Wong; Editing by Ed Davies)