I live in a flat with a prepaid electricity meter. Buying enough electricity is a task I am faced with every month. One advantage of this system is that you can see how your electricity "units" count down, telling you how much electricity you are using.
Together with an LCD showing the amount of units that are left, the electricity meter also has an LED that flashes faster or slower according to your electricity usage. Below this light it says "1000 flashes per kWh". Kilowatt-hour is the technical term for what we call a "unit" of electricity.