Having now had a heat pump working over a whole winter, It was time to see if in fact a heat pump was more expensive to run than a gas combi boiler. I had made many improvements to the house in addition that would reduce costs, so I did the comparison twice. Once as a straight like for like cost difference assuming the same factors as last year, then again including the cost savings through the extra solar panels and tariff prices I was able to obtain. Column J shows the standard comparison assuming things were the same as last year. It was interesting to note that the heat pump was slightly more expensive to run than a gas boiler between October and December. This was most likely due to this being the coldest dullest months when its COP and solar output would be lowest. However this effect was dwarfed by the savings in the other months, leading to a total saving of £178 over the winter period. This agreed with the measurement of the heat pumps SCOP of around 3.8 during the period which would make it cheaper than gas given the price differential between the two fuels.
Column K shows the comparison after all the changes I had made. Having extra solar panels probably didnt help that much on the dullest winter days, but getting an EV allowed access to much better overnight tariffs, which allowed the charging of the battery, hot water cylinder and general washing duties at a much reduced rate. The total saving taking all this into account was £363 over the winter period. The difference of £185 between the two analyses, shows that you dont need a lot of solar panels to make a heat pump cheaper to run than gas in the winter. The downside for me was that the small saving also didnt justify the extra expense that the new panels and battery had cost me. For a reasonable payback period, I was going to have to pin my hopes on a substantial income from electricty export during the summer months.
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After having additional solar panels installed for a year, I was now in a position to compare how the import of electricity compared to the export. The graph below, compiled using OpenEnergyMonitor, illustrates this. As expected, electricity import was low in the summer months, but picked up substantially during the winter months. Electricity export showed an opposite trend and at the end of the year I had imported 3503 kWh and exported 3325 kWh. An additional consideration was that we had purchased an EV in December, and this had consumed 769 kWh. Pleasingly, excluding the car, the house had exported more electricty than it had imported over the whole year.
We finally decided that the time was right to purchase an Electric Vehicle. Second hand prices are very low at the moment and the availability of charging points is getting better. We settled on ordering a 2020 BMW i3.
Having an EV opens up a new range of electricity tariffs available from Octopus and crunching the numbers shows some quite surprising results. Currently on Octopus Cosy and Fixed Octopus Export, I had already picked up on the fact that the cheap rate electricity price and the export price were getting very close (16p v 15p). This meant that there wasnt a great deal of financial difference whether I heated my hot water tank with excess solar via my Eddi PV diverter or did it during the cheap rate periods. One of the new tariffs I could now access with an EV is Intelligent Octopus Go. This tariff is still compatible with Octopus Fixed Export, so I could still get 15p per kWh exporting my excess solar. The idea behind this new tariff is that you hand over the control of charging your EV overnight to Octopus. You just tell them how much you want the car charged and when it needs to be done by. As a reward for this you only pay 7.5p per kWh. This price is a game changer! If I switched to this tariff, then I would obviously get very cheap electricity for my car, but it also meant that I would not get the second cheap rate period in the early afternoon that I currently have when I recharge my 5kWh house battery. Therfore if I assumed that I fully recharged my battery twice on the Cosy tariff, this would currently cost (5 x 2 x 18.13p) £1.81. If I switched to Intelligent Go this would now cost (5 x 7.5 + 5 x 30.52) £1.90. No big change. However heating my domestic hot water now becomes much more attractive during the cheap overnight period, rather than using diverted solar which could be exported instead. If I assume I use 4 kWh of hot water a day, this currently costs (4 x 15p) 60p per day in lost export. If I switched to heating it at night, it would cost (4 x 7.5p) 30p per day. This equates to about £12 per month in savings! As well as saving, I am also helping the grid stay clean by providing my green energy to them in the day when it is needed and using green energy from the grid at night when it isnt needed. So the question is, what is the point of my £500+ solar diverter now? I suppose the pricing of electricity could change in the future and it might make more sense to use my own solar again, but that seems unlikely to me. Perhaps I should look to sell it on to someone who doesnt have an EV? Now that the heating season has started, I am now able to make some comparisons between last year when I was using a gas combi-boiler and this year when I was using my heat pump.
Last year between the 26/09/22 and the 25/10/22, I used 570 kWh of gas to provide my space heating and hot water. During the same period this year, I used 197 kWh of electricity. The 2022 figure is probably an underestimate because I had recently fitted my wood burning stove last year and I know we where using it in October, whereas we hadnt used it at all this year. I also know I could reduce the 2023 figure further by not heating my water with an immersion heater, but by buying a heat exchanger and using the heat pump instead. Adding up how much electricity I used to heat my water and calculating how much I would get if I exported it instead came to about £14 for October. If I could get a COP of 2.5 for water heating, then using the immersion had acutually cost me just over £8. This should be relatively consistant over the year, so using the immersion instead of the heat pump costs about £100 a year. This would be about a 4 year payback term on buying the heat exchanger. Looking at my bills, last October cost £61.49 as I was on a good fixed tariff, at todays prices that would be £87.41, this October my bill was £38.80. So a saving of £48.61 but this included all the improvements I had made including the extra solar panels and battery. A modest saving so far compared to the costs of the improvements, so it will be interesting to how how this changes over the year and what period would be needed to recoup the investment I had made. Having been notified by Octopus that my 2 year fixed tariff was coming to an end next month, it was time to try and work out what would be the best tariff to switch to for the coming winter months. The additional solar panels were working very well and together with the existing panels could produce a total output of over 40 kWh on a good day. Taking into account water heating via my Eddi diverter and other electrical loads during the day, the amount exported was also looking very healthy. The 5kWh battery was easily picking up the slack in the evening, even if we had the central heating on as the COP for the heat pump in the warmer months was easily over 4. The unknown was how would this change once the days started drawing in and the temperature starting falling.
My gut feeling was that there would still be some export on reasonable days, but on very cloudy days I might need some assistance from the grid. Octopus have several tariffs that allow access to cheap rate electricity at certain times of the night (and day). It would be easy to set up my hot water cylinder to heat up during one of these periods and it should be possible to charge the battery in the same way. Some of these tariffs are restricted to EV owners but two, 'Flux' and 'Cosy' are open to anyone and look like good options so long as you can avoid using energy during their peak rate time (16:00-19:00). I didnt have to make a decision till late September, so I decided to monitor the situation and see what happened to my production and usage. In order to be able to get an export tariff for my solar panels, I had been attempting to transfer responsibility for my solar panel FIT payments from OVO to Octopus. While this is not actually essential to be able to do this, my previous experience with OVO regarding trying to transfer the FIT payments into my name gave me no confidence that they wouldn't cock it up.
True to form, OVO completely ignored my instructions (and Octopus's notifications) to switch. It appears that their customer service department is so understaffed that they do not bother reading emails at all! Consequently after waiting the prescribed 8 weeks, I once again referred the matter to the Ombudsman. This time as well as asking for £100 in compensation for my time, I also asked for £3 per day, which is the amount I am losing by not being paid for my exports to the grid. Hopefully this will get the ball rolling at OVO! Already having an account with Octopus, (I managed to get a good fixed rate before the energy crisis for two years) I contacted them regarding removing my gas meter. Octopus dont charge for this service at the moment and I booked the first available appointment. After that no more gas standing charge :-) Having had my Mixergy cylinder fitted for a couple of weeks now, it seemed like a good time to see how it had altered my consumption. The changeover point was very obvious in my gas usage, and pleasingly the highest electricity usage since then was under 2 kWh a day. It appeared that during Spring to Autumn we would be essentially bill free.
I also contacted Octopus about changing my deemed export FIT payments so that I could get an export meter fitted. they advised me to switch my FIT payments to them, which I did by completing a form online. How long the process would take remains to be seen. After months of communications with SSE, then OVO and latterly the Ombudsman, I finally recieved cast iron confirmation that the existing 2.66 kVp solar panel FIT payments had been transferred into my name and that I would receive the FIT payments associated with them. The panels were installed in 2012 and qualified for the highest level of FIT payment, which after the last year of rampant inflation now stands at 60.23 pence per kWh for generation. What was less impressive was the rate for deemed export which is estimated to be 50% of the total generation. That was a paltry 2.1 pence per kWh. Because of this low export payment rate and the fact that I had now had an additional 4.62 kVp solar PV array installed, I thought that it would probably be in my financial interest to investigate whether it would be better to come off the FIT deemed export and instead get an export meter fitted. This would mean that instead of getting a payment for an estimated 50% of the production on my 2.66 kVp PV array, I could get a better payment rate on the actual export of my total 7.28 kVp arrays.
The market leader in these sorts of tariffs is Octopus Energy, and although my FIT provider was still OVO Energy, that should'nt be a barrier to getting the ball rolling. Even after installing my immersion powered hot water cylinder, my daily export in kWh was looking decidedly healthy, averaging at about 10-15 kWh per day over the last month. So with the heat pump now working well it was time to get the Mixergy cylinder installed in the same small cupboard. Since it had been sitting around since last year for various reasons, I was keen to finally get it in and working. Since there were no official Mixergy installers in my area, I convinced a local plumber to have a go under the proviso that I would do all the electrical and internet connection work. Everything went smoothly and by the afternoon of the second day, the combi boiler had gone, gas was capped off and the cylinder was in and charging from the Myenergi Eddi PV Diverter for free. As it was a sunny afternoon, we had two welcome showers and a full tank of hot water from the electricity I would have previously exported.
Initially I had decided to just heat the cylinder via an immersion heater, while this is nowhere near as efficient as using the heat pump, I estimated that I only needed about 3 kWh of hot water each day and this could be supplied from my excess PV, certainly for most of the year. I was going to monitor this and if it occured that I was having to import a lot to do this, then I would reassess buying the heat exchanger needed to hook up the heat pump for domestic hot water heating. Once I had set up the OpenEnergyMonitor system, I could start to get some interesting data from the solar panels. In the above chart, the blue line shows the output from my existing 2.66 kWp array and the orange line the output from the new 4.62 kWp array. Note that the orange line goes negative at the start of the day. This is when the array is charging its DC coupled battery. The red line shows the combined output of both arrays. On this sunny April day, the maximum output touches 5kW. This chart shows the total number of kWhs produced each day.
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AuthorEx Radiographer, Information Analyst, Teacher and Self-builder. Now retired Archives
December 2023
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