Storage heaters use bricks (special type) to absorb and store heat energy supplied to them by an electric element. They are also insulated to keep a large proportion of the heat in the heater. When the heater warms up to a pre-determined level the thermostat shuts off the heating element. During the day the bricks will slowly give off their heat via radiation and convetion. Usually if you want extra heat during the day you can open up a flap that will release extra heat from the bricks. If there is heat energy still in the bricks they will require less energy to heat up again.