There are technological developments around water heaters that have smart controls and can be directly connected to your solar PV electricity supply, as well as your other energy sources. Have a look at Mixergy and iBoost as examples, but note that we have no experience of actually using these products. Storing excess electricity as hot water is another way of maximising the electricity output from your panels. An example of a homeowner who has adopted lots of complementary technologies is presented in this video presentation. 

There are also heat batteries. As explained by the Energy Saving Trust, these store either spare heat or electricity, often generated by renewable energy systems. Heat can be stored in a material when it changes phase from a solid to a liquid. These materials are called phase change materials’ (PCM). Spare heat or electricity is used to charge the PCM inside the heat battery. When the heat is needed, the phase change material changes back into a solid with a release of heat, which is used to provide hot water.