Solar Hot Water Storage tanks are the method that is used to store the energy collected by the solar hot water system.  All solar hot water systems have a back-up heat source that can be used in the event the solar system does not produce the necessary energy.  The solar storage tanks often times have a method of back-up heat built in to them.  This can be in the form of a back-up electric element or a second heat exchanger that can be heated from a hot water boiler.  There are other options for back-up heat including on demand hot water heaters.  Here are a few pictures of solar hot water storage tanks that we have installed.