Creating a relay circuit for PV immersion heater...

Thread Starter


Joined Nov 4, 2011

I recently had free (UK government) installed Photo Voltaic (PV) solar panels installed on our house. As part of the deal I can use any electricity as it is generated but the rest gets sold on by the company who installed them. Obviously I would like to get the most out of them and I thought I'd heat some water during the day so I can use it for baths / washing up at night. I already have a copper cylinder - I just need to buy a 1-1.5kW immersion heater (lower power). The system itself is capable of generating almost 3kW.

The panels and inverter still belong to the installers so I cannot touch any part of their setup before it reaches the consumer unit switchboard. Therefore, I'll have to run a cable to the immersion heater and create a circuit which checks for the amount of sun there is and switch the immersion on when it's at a certain level; which can be calibrated by comparing the power output reading on the PV system (digital readout at switchboard) to the voltage created by a mini solar panel I have (the type you get with solar battery chargers).

For example, with my mini PV panel pointing in the same direction as the main panels I can see what voltage it creates when the readout for the main panels shows 1.5kW, then I need to make a circuit which routes power through to the immersion heater whenever this voltage is reached.

This making sense? I've made circuits before but I'm no good at designing them. And I know that a relay will be required but that's about it! Any help would be much appreciated!


Joined Feb 5, 2010
If it is a system which feeds power onto 'the grid', then you should use the AC power grid to power your heater.

The only thing you need to do, as you say, is to be sure you are using it when the panels are operating at full power.

A heavy duty relay with a coil that operates from AC wall voltage will be needed.

The relay can be turned on and off with a properly designed(safe) photocell equiped electrical fixture. The type people buy to turn a light on at night and off in the day.

A dark lens(such as a pair of sunglasses might use) can be placed on the photocell to ensure that only BRIGHT daylight will toggle it.

The relay will now be controlled by the light level.

Use the relay contacts to operate a common commercial electric water heater. You WILL need to have an electric service person connect power lines thru the relay to the heater in order to maintain safety and ensure proper connections are made for conformance to building codes etc, etc, etc. You will also want to use the commercial heater because it has devices which prevent the water from boiling and creating a BOMB powered by steam. Many many many people days past died from boiler explosions and such things as water heaters are now very closely monitored and controlled for safety. Using an immersion heater in any other way (amateur built water heater) would be extremely foolish and should not be attempted even by people experienced enough to build all the required safety circuits.

Boiling a cup of water for some tea is one thing, but creating a large enough tank and heating it for uses such as bathing and showering is something best left to companies who sell such things with all the safety stickers and state approvals required.


Joined Sep 9, 2010
I think any indirect, surrogate measure such as you've described will be prone to problems. The calibration may work most of the time but break down at unexpected times. I don't know how, but it would be better in my opinion to actually measure the system performance and control your heater off that. Maybe current metering with Hall effect (non-invasive) sensors?