A Pumped-storage facility uses two identical pump-turbines. When operating as pumps, these units have an efficiency of 0.82, and when operating as turbines, their efficiency is 0.86. The motor-generators installed with these pump-turbines have an efficiency of 0.85 when operating as motors and 0.91 when operating as generators. Each pump-turbine uses its own 100-m-long, 150-cm steel penstock. The gross head at the site is 45m, and draft-tube losses may be assumed to be negligible. When operating as pumps, the pump-turbines can each delivery 27 m^3/s into the reservoir, which has a useful capacity of 1.75 * 10^6 m^3. When operating as turbines, the pump-turbines each deliver 9400 kW to their generator shafts. If power is purchased at 25 mils/k Wh for pumping, at what rate must generated power be sold in order to obtain a positive net annual revenue of at least $310,000?