Looks interesting, thanks for the pointer. we looked at the emma system and with the help from some of the guys on here quickly worked out a better return on investment for us was to insulate our wax tanks to a better level. But it would be interesting to hear what people think of this.

There suddenly seems to be a profusion of devices like this. Here is a list of what I know about (prices are inclusive of VAT and may not be the lowest available, particularly in the future!).

Devices that turn on a fixed load:

1. Intelligent Solar Immersion Heater Switch Mark II, http://solarimmersionheaterswitch.co.uk £229

2. SolarDivert, http://www.solardivert.co.uk/index.html £369

3. Immersion Heater Relay, http://www.rudgerenewables.co.uk/intelligent-power-management/our-immersion-heater-relay-unit £160

4. Variety of home-brew devices, e.g.:
i) Arduino-based at http://openenergymonitor.org/emon/node/217
ii) http://www.navitron.org.uk/forum/index.php/topic,16274.0.html (link from first post of jamesingram below)
iii) Another approach described in the body of the first post of jamesingram below

Devices that vary current according to excess available:

A. EMMA, http://www.coolpowerproducts.com/uk/emma.html >£1000?

B. ImmerSUN, http://www.immersun.co.uk £444

C. S&G Wooldridge Solar PV Controller, http://www.sgwooldridge.co.uk/cms/solarpvswitch £474

D. Sunny Home Manager, http://www.sma-uk.com/en_UK/products/monitoring-systems/sunny-home-manager.html - appears to be a German product allegedly launching in the UK in 2012 but haven't seen a price. And maybe this should go into the first class - haven't looked in detail at the technical capability.

E. [Allegedly forthcoming in late 2012] Add-on to Energeno's Wattson energy monitor, http://blog.diykyoto.com/2012/04/upcoming-products.html

If anyone knows of others, post below and I'll try to edit this post to add them to the list!

Apparently Chris Rudge is going to review some products "in due course", according to the editor's comment at http://www.yougen.co.uk/blog-entry/1955/Can+I+connect+solar+PV+to+my+storage+heaters'3F/

My only interest in all of this is that I'm planning to install a product next year to make use of my excess solar PV. In my view, the second class of products above - i.e. the ones that vary current to make use of all of the available excess (but no more) - is greatly preferable if available at a reasonable cost. My strong inclination at the moment is therefore towards the ImmerSUN.

I'll put a pointer to this thread on a couple of old GBF threads on this issue, to try to keep all new discussion here (those threads are at http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8653 and http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=8456 ).

I'd wait a year and they will all be loads cheaper by then.

james,
I realise you didn't come up with this, nor have you tried it yourself, but I'm wondering why you need 2 photocells. Presumably if the one looking at the red LED on your meter, only switches 'on' the immersion when the red LED is on continuously, (ie. exporting) then the presence of sun is not relevant? If that works, it's a fantastic solution - worth a shot for a tenner, even if it doesn't quite work.

Yes, thanks jamesw for the detailed list of systems out there. I agree , the ones that reduce the current to make all excess power available appear superior,and more to my liking.
I guess they work like an electronic load controller on a non grid connected water turbine, putting power not currently being 'used' into 'ballast' loads (resistive only).

We've just taken delivery of our first Immersun unit. After reviewing all the options on the market, I'm convinced these are the best, with direct monitoring of the exported generation levels, and control of the power being sent to the immersion heater so that it always remains just below the spare generated power available.

There's also a timer option so that you can ensure that the tank is hot at the end of the day via the immersion heater, and / or control a secondary heat source, and each unit has the ability to control 2 immersion heater circuits one after the other.

plus it actually looks like a well built piece of kit rather than something someone's put together in their shed after a visit to maplins.

We've previously used the relay function on the SMA inverters to trigger a contactor, but this is a right pain to install as it needs a 1-2kW Immersion heater fitting, which can be a nightmare to do, plus control cables from the inverter to the consumer unit, and customers just find it confusing to use as they have to leave the immersion switch permanently on (yes, people do seem to have a problem with this idea).

fwiw, we're offering them locally fully installed for £499 inc vat, or less if installed at the same time as a PV system, or other measure such as a voltage optimsation unit.

I do have some issues with the concept of this technology when replacing water heated by a condensing gas boiler, but the first unit at least is going in to a place that already uses the immersion heater only, and tbh we've been having so many problems making conventional solar water heating work financially without the RHI in place that I've come around to this as being a more practical and cost effective solution to that issue for a lot of people.

if we're doing links, I'll put mine up for anyone wanting more info about the unit. I won't be offended if it gets removed though.

http://www.leeds-solar.co.uk/energy-saving/immersun