How much heat energy can I store in my cylinder?

By way of example — a 270litre cylinder of water heated to 65°C will hold around 14.3 kwh of energy storage. (This is equivalent to running a 1 kilowatt heater for 14.3 hours.)

The Apricus system will boost the cylinder temperature to 80°C, so a fully charged hot water cylinder will give you ample heat capacity for a typical home.