China's power output, a bellwether for economic activity, posted its first annual decline in more than four years in August, adding to evidence that the world's second-largest economy is losing momentum after a brief rebound in the second quarter. Power output in the world's top consumer fell 2.2 percent to 495.9 billion kilowatt hours (kWh) in August from a year earlier, data showed on Saturday. While the annual fall was in part due to the high reading last summer, when many cities were struck by a record heat wave, overall electricity production also posted its first fall in three months - a sign of slackening demand from major industrial users.