The most energy-effective sump pumps utilize less than 0.5 horsepower. A 1/3 HP sump pump uses about 800-1050 W when running and 13,000-4,100 W at start-up, while a 0.5 HP sump pump uses more than 1.5 times that at start-up alone. A 0.5 horsepower sewage pump costs about $30 a month during the wet season but becomes less efficient and more expensive as it ages.