How to use your Solar PV system to heat your hot water

For many of those who have a Solar PV system installed but don’t use all the electricity it generates there are devices on the market which can now use the electricity you would normally send back to the grid to heat your hot water.  There are three types of system which can be installed which will reduce your hot water heating bills the first two require you to have a hot water tank with an immersion heater.

Smart Switches

Smart switches are  devices which detect how much electricity you generate over and above how much you are consuming.  They do this usually by way of a clamp meters one on the cable carrying the electricity your PV system is generating and one on the main incoming supply cable.  These switches are programmable so for example if you are exporting in excess of 1kW you can program the switch to turn something on and then when your export drops it will then switch the device off.  These switches are purely on off switches and are not really suitable for using with an immersion heater.  Typically an immersion heater is 3kW (which you are unlikely to be exporting even with a 4kWp system) so if your smart switch is set to come on when you hit 1kW of export it will switch your 3kW immersion on which will then be drawing 2kW of electricity from the grid – which will cost you money.  Therefore we do not recommend a smart switching system to utilise your Solar PV to heat your hot water.



The Immersun device ( one of numerous units on the market) is the most cost effective way of heating your hot water from your Solar PV array.  The ImmerSun differs from a smart switching device in that it will only send the surplus electricity you are generating to the immersion heat rather than turning it on and it drawing the full 3kW.  The ImmerSun ramps up or down the power it sends to the immersion heater in 1% steps depending on the available electricity.  Again the ImmerSun works via a series of clamp meters but the key difference is that it can trickle charge the immersion heater and its not just purely an on / off switch.




The third way to utilise the electricity your Solar PV installation is producing to heat your hot water is with an Ecocent or similar  unit.  An Ecocent is a replacement for your hot water tank and will generate your hot water all year round using a minimum of electricity.  An Ecocent is a combined hot water heat pump and cylinder.  The Ecocent unit use 670w of electricity and if you were to schedule when it comes on to coincide with daylight hours your Solar PV array would generate the electricity the unit requires to run.  You could also combine an Ecocent with an ImmerSun.  The installation of an Ecocent is more disruptive and expensive but it does involve the replacement of your existing cylinder and is a very economical way of heating your hot water, especially if you are also generating excess electricity.