UKH

I'm in the process of researching a solar PV system (including battery) for our house.  I pretty much get the ins and outs of the various technologies involved, but one thing that's puzzling me is the diverter which some recommend - a gizmo for sending excess solar generated power to a hot water immersion heater rather than exporting it.  I don't doubt that the things work, but I'd like to understand the principles of operation better in order to understand how it would fit in with our current system boiler (setting to one side the fact that the future for that beastie is also definitely questionable.)

The diagrams and explanations I've found so far (e.g. this one https://www.youtube.com/watch?v=1yAwgye-NVQ&t=139s) make sense up to a point, but I suspect that they over-simplify the device's operation, and how it fits in to the overall PV+battery system.  In that video, for example, the presenter talks somewhat glibly about the diverter detecting power being sent to the grid and sending it to the immersion heater instead.  What it doesn't explain is what happens when the water in the cylinder is up to its set temperature and the immersion heater switches off, using good old 20th century technology to do so i.e. a thermostat.  You can't send excess power down an open circuit...

I presume that the diverter must have some way of detecting when that happens, and letting the excess power go to the grid anyway, but it's not mentioned (which I think is a fairly obvious oversight) and I'd be interested to know how it's done.  I probably don't really need to know, but being an ex-engineer, I'd really like to know before deciding to lay out money on it.  Not to mention the grief I'd get from my beloved other half if it turned out to be a lemon...

They have a cable clamp type ammeter that detects when current is leaving the house. If the tank is hot then the system will just export power as normal. Its only worth getting one if you are being  paid a feed in tariff and you have a non smart meter which won't be the case for a new installation will it?

We had panels and battery fitted last summer and have used very little grid electricity since that. Even in the middle of winter a sunny day will fully charge the battery. 

We'd an Eddi fitted 2 weeks back, they were out of stock when the main system was installed. It basically has a clamp on the power cable going out of the house and as soon as the amount detected via the clamp goes above a set amount (you can control this amount) it will send that energy to our immersion. 

Ours is set up so that the battery gets charged first, and once that's charged any surplus heats our water. A sunny day in February easily gets more than enough solar to heat our water and we're north of Inverness. 

You definitely won't get a FIT with a new installation as you're stuck with the miserly smart export guarantee (SEG) rates.  

In order to get anything back with a new installation you need a smart meter or a dumb meter that has the ability to give both import/export readings.  

We have neither, just a dumb meter. With the Eddi running we'll lose out on about £60-70 a year from the SEG but have decided that until forced to have a smart meter or the SEG rates improve a lot we'll just keep the dumb meter. 

Hi There,

We have solar panels and a "diverter" which sends any excess produced electricity to the immersion heater of our water tank. Once the tank is up to temperature, any excess is then sent to the grid. Our hot water tank is used in combination with gas boilers too.

In the summer, we generally do not need to use any gas at all to heat the hot water. Even on sunny winter days, the water comes up to a reasonable temperature.

Regardless of feed in tariffs, etc... I'd certainly recommend such a device.

Have fun,

Simon...

Are you on mains gas?  If so, and if you aren't planning to get rid of the gas supply, a diverter is probably pointless.  You will be able to sell that electricity back to the grid for more or less the same as the import cost of a unit of gas.

They only ever made financial sense for those who were on deemed (ie unmetered) export, or those off the gas grid.

We also have an Eddi.  This heats my water when there is excess generated power, rather than having to use the oil boiler.

It was straightforward to fit and last summer, we managed to not use the oil boiler at all for about three months.

That sounds good. We have only just had the Eddi fitted but I'm rather looking forward to seeing what our gas usage drops to over the summer. 

Thanks, I hadn't thought of it that way.  I'm with Octopus and AFAICT the export rate on their Fixed Outgoing tariff is actually about 50% more than the current subsidised/capped gas price - in our area at least; I believe it does vary by region (you can get even more if you get on one of their more innovative tariffs, but we're just taking it one step at a time at the moment).  OTOH, using solar electricity rather than burning hydrocarbons to generate hot water would be better for the planet.  Depends on one's balance of priorities I guess.  Then again, one might wonder how much CO2 does making a diverter involve?  Somehow these things seem never to turn out as simple as they first appear...

If the diverter can be retro-fitted to the solar PV+battery system then we can possibly put that decision off until the initial 'heavy lifting' (literally, in the case of the PV panels) has been done.

I guess the answer to my original question is that a diverter should be smart enough to detect when the immersion thermostat has gone open circuit and basically go to sleep until it clicks on again.

Yes, it's very dependent on your particular combination of import and export prices.  If you can get more for exporting a kWh of electricity than you'd pay for a kWh of gas then the diverter is actually going to cost you money... until very recently that scenario was unthinkable but the markets have changed so dramatically.

Ah. I was avoiding this point, but - it's basically true to say that the UK grid always involves burning gas in a CCGT plant.  Times when there is so much renewable power available that all of our CCGTs are switched off are, at the moment, very rare.  So if you are displacing a kWh of gas in your own boiler with a kWh of electricity that could otherwise have been exported, then you are in effect forcing a CCGT somewhere to add an extra kWh.  Carbon wise it's basically neutral.

I'm fairly certain they can be retrofitted if you decide later.

So, to try to answer your original question - my understanding is that it's effectively the same logic that diverts export power to a battery if you have one.  And that is smart enough to detect that the battery is full and therefore push the power back to the grid.

looks like some good advice on here - am interested also, but could I ask anyone for advice on decent solar panel companies (buy and fit) in (West) Cumbria without derailing the thread too much!

Thanks

Others have explained.

We have one and a deemed feed in tariff.  It's great. It has a second divert option that we send to a heater in the cellar in winter. 

^^^^^ this as well, anyone got any recommendations for a company in West Yorkshire. Tried 6 now; one quote, 2 replies saying too busy and 3 radio silence.... Anyone? Thanks....

Energy Smart Group based in Rotherham did both my PV array and ASHP. Very professional and knowledgeable, but often busy.

Thank you for that. I'll give them a try.