Saving Energy using Solar Panels

We have 10 off 550W solar panels installed in our garden with an 8.2kWh battery and inverter in our garage. We are with Octopus as our energy supplier on the Flux tariff for electricity and the fixed tariff for gas.

I have thought about how to use technology to further reduce our use of energy. To start with, I used the Flux tariff to write software using Python that enabled the battery to be charged in the early morning when the price was low and discharge in the evening peek period when the price is high. This has the main benefit of helping to stabilise the grid network. It also helps save energy during the winter period by ensuring the battery is charged up to service the home particularly when there is little sun; this also reduces the cost a bit. I use a Raspberry Pi to run the software. I limit the discharge to ensure that there is sufficient energy to supply the home during the night, this is about 30%.

Then I decided to use the MyEnergi eddi device to control the hot water tank. This boosts the temperature in the tank soon after the battery charge has finished in the morning and maintains the temperature when the solar panels have recharged the battery. This ensures that hot water is available for use during the day. When we started this we found that we had to replace the original 70 year old hot water tank because the immersion heater had broken and replacing that would break the old tank. This helps save energy during the warmer spring, summer and autumn periods by enabling us to turn off the gas boiler, including the pilot light, and only use the gas for some cooking purposes. Gas usage is down to about 50kWh per month (10% of normal) and the overall cost tends to be in credit or close to 0.

Any energy usage from the grid is offset by the export of excess energy from the solar panels. Ensuring the battery is charged up at the beginning of the day maximises the export of excess energy. This is offset a bit by the use of the MyEnergi eddi, which uses about 30% of the battery energy on average. Still this avoids the use of gas to maintain the temperature and is more effective. We don't yet know how well the eddi device will work in winter, maybe we will have to boost the temperature with gas in the afternoon if there is no sun.

Our energy bill for April was £69, May £6 and June £29 in credit - so it appears to be working.

  • I’m impressed by your setup. May I inquire a little? Firstly the brand and specifications of the solar panels you’ve installed? As well, am I right in thinking you have built your own battery management system? Considering the substantial savings you’ve already achieved, could you share the initial cost of your system? Sorry for all the questions, just very interesting to me. I suppose you won't yet have an estimate for the payback period based on these savings?

  • The solar panels are suntech 144 half-cell monfacial. The battery and inverter are from GivEnergy who have an API for managing it. JemEnergy here in Fleet, Hampshire UK were the installers late in 2021. The cost was £9,600. We had started saving energy in 2005 and by 2021, we had saved about £15,000  and were looking for alternative ways of saving energy, hence solar panels. We save about £1,000 per year, so if you think about payback then it will be about 10 years, i.e. 2031; although we think that we already had it saved. 

  • Then I decided to use the MyEnergi eddi device to control the hot water tank.

    PV feeding an immersion made financial sense when exports were "deemed" (e.g. 50% of generated under the old FIT arrangement), but now I think many people are on an smart export guarantee tariff, so every unit not exported means a reduction in income - it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas (depending on boiler efficiency etc)). Environmentally it's swings and roundabouts of course, but quite likely your kWh might be better spend displacing a kWh of gas fired electricity generation (at perhaps 60% efficiency)  than gas fired local hot water generation (at perhaps 90% efficiency). Depends on your gas boiler of course - if yours still has a pilot light, I guess it's not in the efficient condensing category...

    Another approach (which I've got) - is to have solar thermal panels in tandem with PVs (if you've got the roof space of course) - simple thermal panels get many more kW out of a sq m of roof than PVs, even if it's only "low grade" energy as warm water, it's perfectly useful for heating (even pre-heating) a stored hot water system. In my case it feeds a thermal store that provides space heating as well as domestic hot water - so the thermal panels make a contribution to the underfloor heating in winter too.

       - Andy.

  • Before I had the hot water tank fixed we were using gas heating to heat the hot water from 5am and again at 3pm. This enabled us to monitor what was going on. The records indicate that a months worth of gas heating for the hot water tank cost £25. Albeit with the old water tank.

    I don't yet have the same records for the eddi but one week uses 23.9kWh. This uses the battery or excess export, this cost/price of this is about 15p per kWh, which is about £3.60, which is about £15 for the whole month.

    So using the eddi is both environmentally sound and probably saves money.

    We run our gas boiler at 60 degrees centigrade. We also heat the water up to 60 degrees.

    Although we have a traditional gas boiler is still has about 20 years of life left and while it maybe less efficient than a condensing boiler I believe that it is more environmentally sound than a condensing boiler. The technicians inform me than a condensing boiler lasts on average 7 years, while a traditional boiler last about 40 years. So the environmental assessment needs to include the disposal of the previous condensing boiler and its replacement with one which had gone through design, development, delivery and installation.

    I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

  • I agree with you on standard component solutions. Although, It seems that many products are intentionally designed with a limited lifespan, which benefits manufacturers in terms of increased sales and profits.

  • I would expect a standard, non-combi, condensing boiler to last a lot longer than 7 years if you buy a decent make.  My Worcester Bosch was already in the house when I moved here 14 years ago.  The paperwork says it was installed in June 2007.

  • Just a comment:-  Heating water to 60 degrees is too hot for human use. The safety criteria we have in the US is 125 F (about 52 C) max..

    Peter Brooks

    Palm Bay   

  • Yes, but the risk of too low a temperature is bacterial growth, e.g. legionella. I set the boiler to 60 ºC, which means that it gets a few degrees hotter, and the water to about 55 ºC, which does not scald.

  • That's why I said on average. Some do last longer. My brother's lasts longer but he, as an engineer, repairs the problems. My daughter's, in London, lasts about 7 years. I was quoting what technicians told me who are involved in replacing dead condensing boilers

  • I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

    Give me a cast-iron floor-standing boiler any day, even though thermocouples burn out every few years. Ours is still going after 42 years (or thereabouts). A relay in the control system left us without hot water a fortnight ago, but the tepid shower was acceptable in the hot weather. I was able to collect a replacement from RS (Radio Spares) on the same day. I do, however, worry about the cost of the pilot light.

    it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas

    Which might alter the equation, but Andy's contribution does change my thinking. The hot water cylinder is a big battery.

    I suspect that timing is all - even our 20+ year old dish washer can be set to start at any time. So perhaps a large battery is not necessary.