"It doesn’t always rain when you need water, so we have reservoirs - but we don’t have the same system for electricity,” says Jill Cainey, director of the UK’s"

Actually... we do. It is called pumped hydroelectric storage [1], and it is essentially just a reservoir, a pump, and a turbine.

[1] http://energystorage.org/energy-storage/technologies/pumped-...