Re: I've been pointing this out for years.
Visit Sunamp.co.uk. They are making thermal batteries to store excess electrical energy from PV panels as heat for hot water and home heating.
These are more commonly known as "thermal stores" and have been around for decades. The basic principle is similar to a standard hot water cylinder, with a couple of additions:
First, the stored water is not used directly, rather you take the heat out using a heat exchanger. This means that you can store it at a much higher temperature (90 - 95C is possible) therefore storing more energy in the same physical volume.
Input can be from any number of sources. In the same way that a "solar" cylinder will have two coils, one for the solar collector and one for the boiler, a thermal store could have more and be able to accept heat from perhaps a log burner or heat pump as well. Immersion heaters are easy to fit and can take any surplus from PV.
Because of the use of heat exchangers, you can take as much or as little heat out as you need, reducing the need for blending valves.
I've been looking at one from Newark Copper Cylinders for a rebuild project we are trying to start. They will build you a cylinder to your exact specifications and it's not as expensive as you might think.