About 75% of the energy can be recovered in such a system, and very little is lost (by evaporation) even with long storage times. However, the capacity of such systems is limited by the volume of the reservoirs: the energy that can be stored is equal to mgh where m is the mass of water that each reservoir can hold, g is the acceleration due to gravity, and h is the height difference between the reservoirs.
Not only is the capacity of each individual system limited, the total potential capacity is limited by the availability of possible sites. There may also be environmental objections to the development of some new sites, or environmental reasons to abandon some existing sites. Existing rain-fed hydroelectric schemes may be convertible to pumped storage (depending on the availability of water at the bottom), which is less likely to encounter environmental objections – although if the lower water is seawater there will be environmental changes in the upper reservoir. These changes may or may not be seen as particularly deleterious. See Loch Striven Power Station for an example.
There are currently four such systems in the UK (all with fresh water in both reservoirs):
Dinorwig, which can deliver 1.7 GW for 6 hours (11.2 GWh) (en.wikipedia.org/wiki/Dinorwig_Power_Station);
Cruachan, 440 MW for 16 hours (7 GWh) (en.wikipedia.org/wiki/Cruachan_Dam);
Ffestiniog, 360 MW for 20 hours (7.2 GWh) (www.fhc.co.uk/ffestiniog.htm) and
Foyers, 300 MW for 21 hours (6.3 GWh) (www.british-hydro.org/installations/f/ foyers_fall_power_station.html).
There are potential sites for many more, some as upgrades of existing hydroelectric schemes, some as new developments. The former are likely to meet fewer environmental objections, particularly if no seawater is involved.
According to Wikipedia’s List of Power Stations in Scotland, Scotland has potential for about 500 GWh of pumped storage.
For more information see Strathclyde University’s Energy Systems Research Unit website.