Been to electric mountain in North Wales ? That station uses more electricity than it generates ! It buys electricity from the grid at off-peak times when it is cheap, this is used to pump water into a reservoir above the station giving it potential energy, then when the demand for electricity is high, the value of electricity increases, they open the valves and drain the reservoir, driving the turbines. And sell the electricity back to the grid for a nice profit. The ability to quickly and easily regulate the power output to meet the changeable energy demands of a country, make it a vital station.,(reply>), Sam Smith UIp to full output in a matter of seconds and don’t forget Ben Cruachan in Scotland.,(reply>), That's because it's a storage station, not a primary generator (if you discount the relatively minor contribution of the rain that falls into the top lake). Essentially a battery, that absorbs the excess power of always-on stations that are coasting down after providing for a peaktime burst to "charge", and then "discharges" to cover the start of the next burst whilst those stations are still spinning up. And there aren't any battery designs that are 100% efficient (never mind over-unity); you always lose some energy between charging and discharging them.<br><br>As for making profit, it's one stop short of being a nationalised public service, I expect they get paid fairly well for providing the storage service, but once the cost of the balance of input vs output energy is subtracted it might not exactly be phat lewts because all the other providers in the chain want to minimise the costs they pass on to customers in order to remain competitive...