Monday 29 August 2011

Optimised power use with a PV system. Make more use of your Free to Use power!

One of the side issues of the Feed in Tariff with Solar PV is besides the wonderful 43.3p your system will earn from for each unit generated, you are able to use as much of that generated power as you can at your premises without penalty. However, most dwellings will only use around a third of all the available power generated. Most houses are big power users during hours of darkness with lights, TV, etc, PV generated power not much benefit in this case! Even if you are at home all day and running the washing machine, lawn mower, computer, etc this will only scratch the surface of peak generated power of a standard 4kWp system in the summer.

At customer’s request, we put together a test unit a while back and it’s been running faultlessly on test in a customer’s house for some 6 months or so with great effect. They have not needed to use gas to heat their water all summer, beats getting a Solar Thermal system! It will also be of considerable benefit to people who have had 'free' PV systems, only to find minimal financial gain and will be going through a period of disapointment.

The unit is fairly straightforward, and does require you to have a hot water tank with an immersion heater installed. If you have it does the job for very little outlay. There are intelligent units on the market which do the job far more elegantly, such as the EMMA and the Power Router (which we install), but as you can imagine, the purchase cost of such units are into £1000’s rather than this little control circuit which comes in at under £100 in components, or £180 + VAT to buy complete from us.

Before going any further, I strongly insist this is only assembled and installed by qualified personnel who have the knowledge and capability of recognising safe working with electricity and protection of components and wiring.

The Circuit (click on the image above to get full size)

The live output from your inverter is diverted through a Current Sense unit, which is set for a switch point optimum this application. This Current Sense unit has a built in low current relay, which is used to switch a suitable contactor. I would suggest a 10Amp contactor at the very lowest for the sort of use we are looking at.

You will see from the description below, together with reference to the circuit diagram, the ideal location for the completed unit is adjacent to your consumer unit (fuse box). As both the inverter and your immersion heater will be connected directly to this point, it really does make sense.

You can refer to the circuit/ layout diagram for the following:

1) The mains 230v AC output from the inverter needs to have the live conductor diverted through the current sense unit, but otherwise remains connected as normal.

2) The Current Sense built in relay terminal is supplied by a spare 6A MCB in the consumer unit. This relay will switch the contactor. As well as the 6A MCB, it is advisable to provide a 1A inline fuse to protect the relay and contactor coil adequately.

3) The main relay on the contactor will switch the live output from the water / immersion heater in the Consumer unit. To do this will involve diverting the existing immersion heater wiring to route through this unit. Due to regular switching this circuit will now be cycling through an inductive load, it is recommended you change the 16A MCB to a ‘C’ type 16A MCB to avoid nuisance tripping.

At the hot water tank, you will need to disconnect the immersion heater wiring from the wall switch and fit with a 110v yellow plug. Change the immersion switch to a standard switched socket. Plug a 3kVa 110v builder’s site transformer into the socket, and the newly plugged immersion heater cable into the transformer. That’s it!


Normally, your immersion heater runs at 3kW. To make the most of the free to use electricity available, we need to make the heater use less power. By far the easiest way to do this using standard and cheap to buy parts is by a 110v site transformer, available from screwfix, etc, at around £50 each. Once the transformer is in place, this will limit the power consumption of the immersion heater to around 700W. For a standard 200ltr water tank, we have found the immersion running for most of the day will heat the whole tank to around 50 degrees. This obviously assumes the day is bright and sunny all day of course!

On a unit supplied by us we would set the current switch to around 800W, so will only switch your immersion heater on when the inverter is generating more than 800W, ensuring immersion heater usage is powered by the PV system.

It must be mentioned, that although the Current Switch has a built in hysteresis to avoid relay chatter when passing current is marginally over or under the set point, on a day with patchy cloud you will experience the contactor clicking in and out regularly. This can’t really be avoided on a simple circuit as this, but has not proved to be an issue in the 6 months of operation in our test unit.

In essence, the controller is very basic and I will always welcome suggestions to improve the design.


  1. I like it. I've been toying with the idea of an EMMA device for a long while, to go with my recently installed 4kW-peak system, and am currently wondering whether to go for solar hot water instead or as well, so your suggestion is a timely alternative.

    If I understand your circuit correctly, you are simply switching a 700W immersion coil on or off according to whether the inverter is generating more than 800W, and this is without regard to how much of the inverter's output is already being used elsewhere in the house. Is the consequence therefore that when the rest of the house is using more than 100W, the immersion will be partly using imported electricity?

    Conversely, an EMMA will use only excess generated electricity (i.e. more than is already being used elsewhere in the house), and will use all of the excess up to 3kW (as opposed to using a fixed 700W). Set against that is the EMMA's much greter cost, of course.

    By my back of the envelope calculations, it makes sense that your circuit makes a useful contribution. If the specific heat capactity of water is 4.1855 J/gK, for each second that a 700W immersion coil heats the cylinder, it will increase the cylinder's mass-temperature by (700/4.1855) gK = 167 gK. In volume terms, that's 0.167 lK. In an hour, the increase will be 602 lK. In other words, 100 litres of water will increase in temperature by 6 degrees for every hour that the 700W immersion operates.

    I'll be interested to see for how many hours each day my inverter outputs more than 800W at different times of the year - so far it's only been running for a month, but I shall assess the output data as the days shorten.

    Altogether, I enjoyed your post!

  2. Hi Chris.

    I have been struggling with how to do this for a while and your post has helped so much. I am going to copy your plan but with a slight mod.

    As you know most appliances in a house take about 3 kW so few could operate at no cost when the heater is on too.

    I will add a further current controlled NC switch on the consumer unit input. This additional switch will be in series with yours and only allow the heater to operate when the load on the system is lower than 1 kW (I have a fairly high continuous electricity use). To make this work without "chatter" the heater will have to come off a separate MCB fed before the additional switch.

    The switch in line with the PV will be set to close at about 1.7 kW so all the power used by the heater will be free. Any other appliance which is switched on will switch off the heater

    Thanks for the tip on the 110 V transformer, I have analysed the day-to-day power levels from my SMA inverter and even in the summer my 4 kWp system will be marginally more "efficient" using the transformer. I expect it to win more easily in the winter!

    A silly question please, I assume the switch notation Fixed Core is for use with a single conductor while the Split Core is for use with 3 core cable. Seems logical but I haven't seen it written anywhere.

    Yours was a great post and thanks again.

  3. Just found this 1 kw immersion heater on ebay -

    Hope will be usefull to some one looking for 1 kW immersion

  4. Great concept, thanks for putting this out. Many would benefit from using up all excess power, ( I am assuming smart meters will not be here too soon !). I have been thinking up similar ideas from a slightly different angle. I offer the following input. My solar meter, ( the one I use to get readings for FIT payments), flashes calibrated amounts per kw generated. My incoming meter, ( the one they use to charge me), also flashes dependent upon how much energy I intake from the grid. Unfortunately they flash at different rates per kw, but that is not a problem technically. My plan is to stick an optical detector over each flashing light, ( no electrical connection so no legal issues), and use the output to decide if and how huch excess I am generating.I have also got from ebay, a system that can switch mains outlets via the mains wiring, so could for example activate dehumidifier, air con unit, electric heater etc as I choose. One other input that may be of use: I recall that simply placing a suitable rated diode in series with, for example an electric water heater will reduce power by 50%.