5.5 Stored energy and available power
Water held at a height represents gravitational potential energy. If m kilograms are raised through H metres, the stored potential energy in joules is given by the following simple equation in Figure 5.
Thus about 10 joules of energy input are needed to lift one kilogram of anything vertically through one metre against the gravitational pull of the Earth, but the capacities of large reservoirs are usually given in cubic metres, rather than kilograms, so we can express the equation for stored energy as shown in Figure 6.
This is therefore the energy that will be released when this volume of water falls through a vertical distance (H).
Next we will look at pumped storage and how this works to store electrical energy.