Flux and immersion heater

So ... not really a battery question but all part of the same setup.

When the battery is full and the sun is shining I would make money exporting to Octopus Flux at 17p and using gas at 7p to heat the water, instead of diverting the excess solar energy to the immersion heater which is what I've been doing until the export part of the tariff came online.

So I would make money if I switched off the immersion header diverter ... but what's the carbon equation? If I export to the grid and the grid thereby has no need to burn carbon to generate that energy, whilst I at the same time burn carbon to heat my water, then that's neutral in carbon term, or even a win when efficiency is taking into consideration, as well as being a win in financial terms.

But if I'd only be exporting the extra at times when the grid was 100% renewable I'd be burning gas to heat my water that would otherwise not need to be burned. So although I'd still be making money this wouldn't be good for the world overall.

(I am not, I'm afraid, going to check the state of the grid each day and manually switch the divertor on and off accordingly.)

What/how do others decide?

Do not have an answer to your question, but you can get gas at around 4.5p a kWh if you switch your gas to Octopus tracker.
https://gcccc4f6c5ed303-gtxk2uqiyag1cro2.adb.uk-london-1.oraclecloudapps.com/ords/r/gastracker/gas-tracker/home

If it takes 7 cubic foot of gas to generate 1 kWh in a power station. How many kWh would you need to export to offset the volume of gas needed to heat your water tank?

Timbrettward
There is also the consideration of the water in the tank going stagnant or growing legionnaires.
I chose to keep diverting though i have the option of draining the tank and using the combi (which could also heat the tank if need be)

Re the legionnaires, I follow the advice in the widget's manual and do some gas heating every day. In particular there isn't enough solar to heat the water for morning showers anyway and so far I haven't bothered with trying to heat the water with the dregs of the battery.

I’ve removed the diverter and I now heat the tank up during the cheap period.

A couple of things to consider is that boilers are only around 80% efficient depending on the age vs an electric immersion being close to 100%. So even though you pay less per kW in gas, you need more of it to achieve the same temperature
Also, running the boiler and pump still uses a few hundred Watts. In the end, I don’t think the coat difference is anywhere near as much as the unit prices would suggest

That 80% is generous. You need the boiler running the boiler lower than 60deg to get the max efficiency. Then you have the temperature setting on the tank. If it's above the boiler flow temp then it will just keep burning gas during the set time slot (assuming you set a timer). Recommendation is to have the hot tank above 60 to maintain legionella deterrence. So it's a bit of a juggling act and the cost of gas compared to cheap leccy is a fair bit higher than people think.

Have this same conundrum with my oil boiler out in the country. It's about 9p/kWh to heat the tank with oil (even taking into consideration 80% efficiency) with the lowest import rate on Flux at 18p/kWh here. As the diverter only works when excess solar I could be making 20p-32p/kWh in export at those times instead of diverting, so that's a saving of 9p/kWh of oil I didn't need to use or 20p-32p export income - big difference! Until I've paid all this installation off it's hard to justify diverting.

I still use the diverter on good days (I've "used" 558 kWh with it this year, saving _£52 of oil - but that could have been at least double savings if I had just exported instead). With oil prices as they are, the price disparity is huge compared to elec on a per kWh basis. I am desperate to get off this oil ASAP though... Old house problems though so ASHP is a big risk.

I think you’re all over thinking it.

You have ‘free’ green energy that is all being used somewhere, whether it’s self use or to the grid.

Make it pay you back as efficiently as possible. You’ve vastly reduced your carbon footprint already and its cost a lot to do it.

Justification for solar diverters took a nose dive with the recent new smart tariffs, if you are on OI with 7.5p off peak and 15p export then why divert your excess solar! I’ve added a Shelly plus PM to my immersion switch and automate this in HA for high temp boost and simply time ASHP to keep everything, hot both done off peak. Gary does Solar on YouTube did some good work on solar diverter justification and cost analysis, although this was pre 15p export rates on intelligent which weakens the case further.

Jase1703

An extra thing you can do with Shelly is add a temperature sensor to the tank assuming you can find a nice exposed bit of copper.

I’ve got a scene set up so I get a boost if the hot water is below 50 and I’m exporting over 2kWs

Shatch yes I did that, included the Shelly Add On and a temp sensor, fortunately had a spare sensor slot in the control panel on the unvented cylinder. Would love to see the scene you created?

Over the summer I have been using the tank immersion heater (it's an unvented 150l cylinder) to heat the water with excess solar. I used a motor controller module to reduce the heater from 3kw to below 1kw so that I could avoid discharging the batteries when the sun goes in. I had the immersion set to 60degrees and timed from 11am to 7pm. I used Solcast to predict the days with enough spare Solar and also had the gas set but to 55degrees and only in the evening should I not have had enough Solar that day. 150l of hot water does 3 showers and there is only 2 of us so heating the tank once a day is fine. I used hardly any gas between June and August.

This is more tricky though now as the daily Solar drops. I have changed the times to 12.30pm to 5.30pm and set the 7 day timer only to come on on days predicted to give more than 25kwhr frm my 6.8kw array. Still better than gas only.

Other consideration is that I'm now on Eons 16.5p export rate. With gas at 7.5p I think it is probably better to just export it all and revert to gas to heat the water. Even if the boiler is 80% efficient there is some loss from the pipework and that the boiler heats the flow from 20degrees and leaves it at 80degrees whereas the immersion is 100% efficient and tops up from where it is. So the consideration for me now is whether I look at my own carbon neutrality (only using my electric where I can) or the wider effect of the countries carbon levels where my electric helps to keep their gas powered generating stations that little bit lower.

Windy Miller
Reducing the local air pollution (NOx) from your gas boiler might also be a consideration.

Looking at the amount of gas it took to heat my hot water last summer, compared with the amount of excess solar used to heat the tank this summer, my boiler appears to be around 70% efficient (not including the electricity consumed by the boiler itself).

Jase1703

These are a couple of scenes I have. Both of them switch off after 20 minutes

The < 43c is just so the hot water stays at a comfortable temperature for a shower, the 1kw of export plus battery is enough to cover the immersion without using anything from the grid

Legionnaires disease is of course a concern, over 60c kills it instantly but it also dies with sustained temperatures above 50c.

Shatch excellent strategy and scene, thanks for sharing.