from wikipedia http://en.wikipedia.org/wiki/Pumped_storage_hydroelectricity

The relatively low energy density of pumped storage systems requires either a very large body of water or a large variation in height. For example, 1000 kilograms of water (1 cubic meter) at the top of a 100 meter tower has a potential energy of about 0.272 kW·h. The only way to store a significant amount of energy is by having a large body of water located on a hill relatively near, but as high as possible above, a second body of water

if 1m3 at 100 meter only gets you 0.25 kWhr, that's going to be one big ass tank