Water heater:

Solar panels are fitted to your roof and collect heat from the sun, using it to warm water which is stored in a hot water cylinder that works in conjunction with your existing heating system. The remaining hot water requirement is provided by your boiler which also acts as a back up to heat the water further when solar energy is unavailable.
Advanced Maintenance Services recommend the Solar Panels as an ideal partner to the new range of condensing regular and system boilers, with different models available in both oil and gas. When used together, a boiler with solar system provides a highly efficient system to give heating and hot water comfort.
Who used up all the hot water?! Maybe  nobody. May be your electric water heater needs fixing. Once you know how it works and how to diagnose it, you can fix it and save some money.