Our Solar PV Journey

July was very slightly lower than the prediction, but still pretty close. Key statistics: Since mid-March, our energy bill has reduced to approximately 26% of what it was this time last year. We only pay for about 39% of the energy used. This is due to direct solar, stored solar, and export payments.

On Sunday, I suspended my normal "set-and-forget" policy and intervened in the natural order of things. Octopus Agile had plunge pricing and I wondered if it was possible to make use of it despite having plenty of free solar energy coming in. Well - for the first time - I managed to make money on both import and export. The average import cost was actually negative (although there was still the daily standing charge). It is a pity that it is not like that more of the time.

Again, actual generation exceeded the Global Solar Atlas prediction. 806.9 actual, 670.9 predicted. Exceeded by 20.2%: Bottom line: Compared to last year (on Standard Variable Tariff), my bill has reduced to about 30% of what it was. I am paying for only about 42% of the energy used, the rest being "free" (if you ignore my investment in plant). Another way of looking at that same statistic, is that my average cost per kWh is effectively about 10p. Of course, these figures will be considereably worse when the winter comes - but that is where we are right now. Note that these stats are for a home without gas, with a heat pump and a plug-in hybrid vehicle. Total consumption is a lot greater than the typical British household. A typical household would be able to meet a much greater proportion of its needs with a similarly sized system.

People can get very hung up on trying to calculate their "Return on Investment" and it is easy to get into a protracted discussion about how much a system is saving. I am using two measures: Since installation, my electricity bill this year has reduced by 68.0% compared to last year. This includes ALL savings: tariff change, time-shifting, solar used, export payments. The same metric can be presented as " The bill has been reduced to about 32% of what it was ". Since installation, solar energy has subsidised 57.0% of the cost of our electricity (direct solar consumption, consumption via the battery and export payments combined). Another way of presenting the same metric is: " I pay for only about 43% of the electricity used ".

May's output also good. Top: Predicted. Bottom: Actual. So the GSA prediction is accurate to within 5%.

That's better! The GSA prediction was exceeded by 8.5%:

If you are with Octopus Energy you are permitted to switch between their smart tariffs as frequently as every 28 days (correct at time of writing). This means that selecting the right tariff need not be a particularly high-risk endeavour. Even better, if you have been with Octopus for a while, then the Octopus Compare app will let you compare the effect of various tariffs on your actual smart meter data. Different types of users will have very different useage profiles. In particular, the following things have a significant effect on your PATTERN of usage. These things can make each tariff considerably more or less attractive: An all-electric vehicle (a PHEV, to a lesser extent) A heat pump Photovoltaic solar panels Thermal solar panels Battery storage Storage radiators Any electric water or space heating Before you start using Octopus Compare, there is another tool which allows you to take a quick look at all of the Octopus smart tariffs. On the energy-stats.u...

In case it has not occurred to you to do this... many browsers can be set up to start with more than one page opened. I set my copy of Chrome set up to start with the following pages open in separate tabs: The mysmartenergy.uk graph of Agile prices for today. The SolarAssistant page for my system. The BBC Weather forecast for my area. My home page (which contains a collection of useful shortcuts). This setup very much delays the opening of Chrome, but it does mean that I have pertinent information at my fingertips. And, since Chrome can be set up to sync settings across devices, I have the same information avasilable on my phone and tablet. There are even Chrome extensions which allow pre-configured displays of multiple tiled tabs to be set up (see example below).

As I said before, it is quite possible to make good use of the Octopus Agile tariff without having a high degree of automation or integration. In fact, acting exactly as if you are on Octopus Cosy while actually being on Agile would probably give substantial savings. However, there are some tools which can be helpful: As previously reccomended, the "Octopus Compare" app remains a very useful app. You may find an app called "Octopus Watch" useful. It is particularly useful for calculating savings and also has a feature to reccommend when to run an appliance expected to take a given time to run. There is a "skill" for Amazon Alexa which allows you to ask verbally when to run an appliance for the lowest cost. It is called "Agile Octopus Timing" and, once enabled, is invoked by asking, for example, "Alexa, ask Agile Electricity for a three hour slot". It is worth making a shortcut to the appropriate page on the mysmartenergy.u...

UPDATE: I have made a cost calculator spreadsheet (see link at bottom of page). I have posted this graph elsewhere. It makes it easy to estimate a notional "cost per cycle" for a LiFePO 4 battery. It can be instructive to calculate what the "cost per cycle" means when viewed as a "per kWh" value. When you calculate your "cost per cycle per kWh" don't forget to include factors such as: Charge/discharge efficiency (say 87% to 90%) Depth of discharge (obviously reduces the effective capacity) Average capacity degradation over the life of the battery (say degradation to 80%-90% at "half spent" point) It is easy to see that the cost per kWh of passing energy through your batteries can easily be in the region of 10p-15p per kWh of "delayed consumption". Knowing that figure helps to put a priority on storing the various "types" of kWh that you choose to store. It may change your perspective on exporting ...

Using the Octopus Compare app, I noticed that Agile has been beating Cosy for our usage profile. This has been the case since March, despite our usage being tailored to the Cosy tariff. I have switched to the Agile tariff and we have made some minor changes to our usage to, potentially, save even more. Points to note: The cap on Agile is technically 100p per kWh but, in practice Octopus have been capping it at 32p. This should not be a problem as long as we keep an eye on the rates. The switch to Agile was initiated on a Web page and happened very quickly (less than 24 hours, and retrospective to midnight on the day the email was received). Octopus allow you to switch frequently, but impose a 28 day limit on frequency (which seems very reasonable). Agile would be cheaper than Cosy even if we don't change our usage. Right now, Agile has long periods well below 32p. Cosy had only two 3-hour periods. Cosy sits at 32p for long periods, Agile (for now, anyway) spends a...

It seems to me, reading the forums, that there are quite a lot of people who are charging their batteries on the cheapest electricity that they can buy and exporting it back to the grid when the export rate is highest. I can see how, in principle, one might make money doing this. However I do wonder whether people have really considered all of the costs. Using round figures to keep it simple, imagine that you buy a 5kWh battery for £3000. Depending whether you get 3000, 6000, or 9000, or 12000 useful cycles out of it, this battery is costing you between £0.25 and £1.00 per charge-discharge cycle. Each charge-discharge cycle of a 5kW battery will earn me five times the export price per kWh, minus five times the import price per kwh, minus the battery cost per charge-discharge cycle*. I think that I would want to be very sure which cost-per-cycle figure was real before embarking on this idea. And that is before considering things like the "Time Value of Money", TVM - the idea...

Well, March was a bit rubbish! The prediction was for 465.5kWh but we only generated 207.7kWh. Let's hope April is better. We are more than half way to the predicted 601kWh for April (on Apr 15th) so it does look closer than March. CORRECTION!!!! The March figure was only for Mar 18th onward - so not so bad after all.

I think that, for quite a lot of folks, battery life is a major consideration. I have posted this graph about lithium ferro-phosphate (LiFePO 4 ) battery life elsewhere, so apologies if you find this repetitive. The graph is from a particular manufacturer, but it is likely to be broadly representative of any brand. You can read the graph for yourself but, broadly speaking, what it is saying is that (for the right-hand half of the graph where most of us will be operating) discharging at 1/2C instead of 1C could reasonably be expected to increase your battery life by something in the region of 40%. Reducing DOD from 100% to 90% could increase the life by about one sixth. Going from 100% DOD to 80% DOD increases it by about one half. Going from 100% DOD to 70% DOD roughly doubles it. I would guess that other factors, such as temperature, would have significant effects on battery life. Some users will be less cost-sensitive than others. I imagine that a business in South Afric...

I have been keeping an eye on competing tariffs using the Octopus Compare app. I have noticed that, just recently, Octopus Agile would be cheaper for me with my current usage patterns. Below is the chart for one month. As you can see, the chart begins with Cosy just ahead of Agile but ends with Agile clearly the winner. If you look at the pricing for a single day, it is easy to see what is happening: You don't have to be a genius to figure out why Agile might work out cheaper overall! It is cheaper on a minute-by-minute basis almost all of the time. It isn't quite as stark as this every day, but this is clearly something that I should keep an eye on. For now, I am going to stick with Octopus Cosy. Apart from anything else, the easy to remember rate periods make it easy to make judgements about setting timers or sticking on an extra load of washing or dishes. I'm not sure quite how to handle such decisions on Agile. One approach would be to act exactly as I would on Cos...

I have said elsewhere that my aim is to set up my inverter so that it does not require continuous tweaking. I know that this may be naive, but how does it look so far? Well, I think that the Octopus Cosy tariff makes all the difference. Without the afternoon cheap rate period, I don't think that it would be possible to adjust the inverter in such a way that it would work well on sunny and overcast days alike. Reading the forums, it looks like some people have set up quite elaborate automation to take advantage of weather reports, published tariff changes, etc. This is not something that I particularly want to get into. The Octopus Cosy tariff seems to work very well for me. On sunny days, despite setting "Priority Load", my batteries are reasonably full by the time 1pm rolls around. This means that the automatic charge from the grid is minimal. I am experimenting with splitting the 1pm-4pm cheap rate period into two (or even three) segments with different SoC targets. ...

While I was researching SolarPV systems, I came across the Global Solar Atlas (GSA) Website which purported to tell me (in great detail) what kind of performance I could expect to get from the system that was being proposed. The more I look at this site, the more I am impressed. So far, my actual results have agreed to a high degree with its calculations of what I could expect to generate. Another interesting detail relates to the optimum tilt angle for the panels. In most residential installations, the owner does not really have a choice of the angle at which the panels are mounted. If the panels were on a frame mounted on the ground or on a flat roof then there would be a choice. However, in most residential installations they are simply mounted on whichever roof points south-ish (north-ish in the southern hemisphere). The panels are tilted at whatever angle the roof happens to be. That does not mean that optimum tilt is not interesting though. Although you might not use data ...

I just noticed that my array was producing over 6kW. Looked outside to see the back garden list up as if there were arc lights on it. A weird glow. The sky is cloudy so that, when the sun does peek out from the clouds, it lights them up with spectacular effect! So now I have a better feel for what kWp actually means. I was seeing (briefly) 6.49kW from my 5.6kWp array (14 x 400Wp). There is an interesting article about this effect in PV Magazine. Update Since I wrote this I glimpsed 6.92 kW, albeit briefly. Anyway, it looks like, it the UK midlands 120% of the kWp value is entirely possible. It will be interesting to see what summer brings. We won't benefit from the cloud reflection as much but, in terms of power generation, of course it is the SUSTAINED power that really matters.

I have not really had a chance to explore Solar Assistant user interface very thoroughly yet, but here are my first impressions. Pros: The user interface looks good and is, by and large, intuitive to use. Screen layout works well on laptop, tablet, phone. As far as I can see, Solar Assistant gives access to ALL of the functionality of the inverter. The configuration pages are laid out in a way that will be familiar to a Sunsynk Connect user. The dashboard is updated a lot more frequently than in Sunsynk Connect. Most importantly, it gives me local access to my inverter without climbing up in the loft. It is easy to generate "View Only" accounts for interested friends and family. It supports a wide variety of inverters and batteries. For my Sunsynk inverter it seems that also connecting to the battery would not give me much additional useful information, but the option is there if I wanted to get at cell-level data. Cons: On an Android phone or ...

The sunsynk.org Website and Sunsynk Connect app give me access to my inverter statistics and configuration. Unfortunately, this access goes via a remote server which seems to go down quite frequently. Additionally, the updates to the figures only happen every five minutes. One way to solve this problem is a product called "Solar Assistant" . Solar Assistant is software which runs on a Raspberry Pi which is connected permanently to the RS-485 port on the inverter. The software monitors the inverter continuously and compiles statistics which you can view locally on a built-in web server that runs on the Pi. The Pi connects to your WiFI network.You can also make configuration changes via this web server or access it remotely when you are not at home on you home WiFi. You can buy the Solar Assistant software and run it on your own Raspberry Pi, or you can buy a pre configured Pi from Solar Assistance in South Africa. They will also sell you pre-made RS-485 cables and a 48V pow...

Right after posting about the first few weeks, I received my first electricity bill that is broken down into cheap, standard, and peak rate usage. It further confirms that time-shifting of consumption can be quite significant. Much of this is down to simply using timers, although the almost complete avoidance of the peak rate requires a battery. Interestingly, Octopus calculates the AVERAGE cost per kWh: 24.11p - almost 23% off the standard rate. This bill covers 8th February to 6th March. It probably seems very high for one month, to most people. But remember that we have removed our gas meter and are space heating and water heating using a heat pump. Even though the heat pump is about 300% efficient, it still boosts the electricity bill well above what a dual-fuel household would use.

At the time of writing, the system has been installed for a bit over a month. I got the installers back to correct some things that did not seem quite right to me - some kind of communication breakdown between the company and the guy who did the original installation. I thought that now might be a good time to look at how the system had performed and how this compared to simulations. What to compare against? Before installing SolarPV, our supplier was British Gas and we were on their Standard Variable Tariff. This provided electricity at a flat (i.e. 24hrs per day) rate. At the time of installation, the price was 33p per kWh. I have chosen to make all comparisons as if we had continued on this tariff. Another possiility would be to compare 2022-23 consumption and cost with 2023-24. I have not done this as, not only does the UK weather change dramatically from year to year, but also the tariffs have changed and the way we use our home (number of occupants, etc.) has changed. Saving...

The inverter that forms the heart of any SolarPV installation is a complex piece of equipment with numerous configuration settings. This is partly because many are designed to be used in scores of different scenarios; Single Phase, Three Phase, systems with multiple inverters, off-grid systems, etc. However, even an inverter intended only for use in run-of-the-mill domestic situations will have a lot of possible settings. If the inverter is to be installed in a relatively inacessible location, maybe an outbuilding or loft, then the ability to control it remotely becomes more important. In our installation the only location that was really practicable was the loft space. Because of this, we were careful to check that remote configuration was possible. The Sunsynk inverter comes with a WiFi dongle and can be connected to the home network. This allows all of the settings to be read and set remotely, as well as current power flows to be monitored. Although the remote monitoring and con...

No. That is just my opinion but, if you are buying a SolarPV system your savings are not really guaranteed and are relatively far in the future. I think that, overall, installation will probably be a good move. However, financing the enterprise makes it considerably more expensive and makes the it more of a gamble. If you are feeling lucky, go ahead. But don't say I didn't warn you!

For many (but not all) installations, Planning Permission is not required. You will need to investigate your own particular case, but smallish installations on "normal" properties can be done under "Permitted Development". However, if your property is listed or in certain "special" areas of the country you WILL need Planning Permission. The bottom line is, though, that you must check. My local planning department told me that I could, voluntarily, apply (and pay) to obtain a certificate of lawfulness. This is basically a document that says that the department has looked at the details of a Permitted Development and determined that it is, indeed, permitted. I am not entirely convinced of the value of doing this, lthough we did it anyway.

The system that we ultimately purchased consists of the following: 14x Longi 400Wp all-black panels on south facing roofs. Twelve on the main roof and two on a smaller roof over the garage. Bird guarding on all panels. An 8kW Sunsynk Inverter, mounted in the loft. 2x 5.32kWh Sunsynk LiFePO 4 batteries, also in the loft. Tigo optimisers for the two panels on the small roof, which are shaded for part of the day. A Tigo Access Point which communicates with the optimizers. This is connected by wire to a Wifi gateway which communicates with the network The system will stop sending power to the house in a power cut, but there are sockets (in the living room) on the essential loads (UPS) output of the inverter. A contactor deals with the situation where the grid neutral (and hence the earth) goes away in an outage. The essential loads then needs a N-E tie. The 8kW inverter is overkill for this size of instalation. It was chosen to allow future expansion. In particu...

Before long, you are probably going to hear the term "DNO Application" or "G99 Application". This refers to an application to the Distribution Network Operator (the people who manage the cables, poles, transformers, etc., that bring electrical power to your home). For any solar instalation greater than a certain size (I believe 3.68kWp at the time of writing) an application to the Distribution Network Operator must be made. This takes time (about 3 months in my case) and often money (I was charged an extra £450.00). Because of this, there is a big incentive to a salesman who wants to make a sale and move on to recommend an installation smaller than this critical size. Many users, with relatively low electricity usage, will be fine with a system of less than this size. However, it is worth being aware that this issue exists. I use the term "Dreaded G99" because some installers will avoid it because it can be a lengthy process over which they have little, ...

Many quotes will not be thoroughly itemised, so there are some "extra" costs which you will need to be aware of. "Bird Guarding" is a way of mounting a mesh at the sides of the panels to stop birds (and other critters) from getting under the panels and building homes there. If you roofs are particularly inacessible, special scaffolding arrangements may be needed. If any of your panels will be shaded then you will need to have optimizers and combiners or micro-inverters. This is necessary to stop the shaded panels from lowering the output of the unshaded ones. If you want to have power during a power cut, extra equipment is needed. Standard equipment is not suitable for life-support equipment. If you have such needs, you will require special equipment. Some installation companies are only interested in making a quick sale and moving on to the next one. If a sales person tells you that they can't install bird guarding without invalidating the war...

It is possible to lower the cost of a SolarPV system considerably by just installing panels and an inverter with no batteries. Such a system may suit some users, but to make good use of such a system you have to be able to use the solar energy as it is produced. You CAN export excess electricity to the grid and then import some later instead - but the differential in export and import rates is generally pretty high and that is what usually makes your own local storage (i.e. batteries) worth having. As to the size of battery to install, I found that the sales people did not have much expeience of homes with very large usage such as ours. I was advised that one 5.12kWh battery would be sufficient but I opted for two batteries because I had a feeling that I might need more. The fact that there is no VAT payable on batteries installed at the same time as the system also swayed me a bit. The down side of bigger batteries is that you are, in effect, making a decision to buy more batteries ...

Before we embarked on this journey I couldn't even spell tariff, let alone explain it. I was vaguely aware of "Economy7" having lived in a rental property with ancient storage radiators. I was, however, completely unaware of the plethora of different tariffs available nowadays. The reason tariffs have become so important is that energy suppliers have real difficulty in generating enough power for the peak usage periods - typically the early evening. To supply this demand is expensive because power stations have to be kept ticking over so they are ready to fire up at full power at short notice. Not only do they have to be kept ticking over, but they would not even have to be built were it not for this need. In some ways, wind power and solar power make matters worse. Although they can generate power more cheaply than gas or coal, the power is only available when the wind blows or the sun shines. Electricity suppliers can incentivise users to avoid consumption during the ...

People often think, wrongly, that having solar panels, especially with batteries, means that you will have power if there is a power cut. This is NOT the case for most SolarPV setups. In the UK, unlike somewhere like South Africa, power outages are rare and it is not necessary to go to the extra complexity and expense of installing Uninteruptible Power. The SolarPV system can be added to the wiring of the house with the minimum of alteration if the SolarPV system injects its power between the electricity meter and the consumer unit (what old timers like me call "the fuse box"). This arrangement also allows extra energy to be fed back into the grid network if desired. However, if the external power is cut the SolarPV system MUST (for safelty reasons) stop supplying power. If Uninteruptible Power is required extra measures are needed; either a system to disconnect the house wiring from the grid or a system to connect essential loads in the house to a separate output on the inv...

When you start to talk to sales people you will start to hear some terms that you might not be familiar with. Anybody with a basic understanding of electricity will probably have a rough idea of the relationship between Volts, Amps (Amperes), and Watts. However the kWp will be new to many. So here is a quick primer (apologies to any physicists out there for the simplifications): Voltage, measured in Volts is the "electromotive FORCE" that pushes electricity around a circuit. It is analagous to the "head" of water behind a dam in a river. Current, measured in Amperes (Amps), is the FLOW of electricity around a circuit. It is analagous to the flow of water over a dam or down a river. Energy, measured in Joules or Watt Seconds, is a measure of work (of sorts, but don't worry about that - you can think of it as a way of totting up power). Power, measured in Watts, is a measure of the rate of transmission of energy from one place to another or from one ...

I think that everybody knows that, before embarking on any major home-improvement project, it is advisable to get multiple quotes. What people DON'T tell you is what a difficult process this is. Apart from the rigmarole of arranging for people to come and look at the property to quote, with SolarPV systems there are a lot of possibilities to be considered. Just a few of these are: How many panels? Which roofs to fit them on? How big a capacity should the inverter have? Should batteries be fitted? If so, what capacity? Will nesting birds be an issue? Is it best to have the latest, highest-capacity panels? Will the system provide power during a mains outage? We started out knowing very little about SolarPV systems. I am an experienced electronics and software engineer, but my knowledge of SolarPV technology was virtually zero. Our initial benchmark for cost was the Costco Website. This gave us an approximate costing for several different siz...

I called this entry "revising for the exam" because that is a little bit what it felt like. Before you start getting quotes it is not a bad idea to aquaint yourself with the technology as best you can. The resources you use will depend on your existing level of knowledge of things electrical. If you are a complete beginner, then it would be a good idea to look at som "Idiots Guide..." type resources. There are a lot out there, and it would probably be a good idea to check out more than one. If you have a rudimentary idea of how a house is wired up and you know the difference between a Volt, an Amp, and a Watt then I would reccomend something intended for installers. The Sunsynk Website has some good resources. Their "Training Part 1" document gives a pretty good overview. There is a video to go with it if you like that sort of thing. Personally, I prefer to just read the materials. Sunsynk Training If you already have a smart meter then I woud recommend g...

Firstly, I should say a little about WHY we looked into installing a SolarPV (solar photovoltaic) system in the first place. In fact, before that I should probably say why we looked into solarPV and did not even consider a Solar/Thermal system (i.e. heating hot water directly from the sun). On the face of it, solar/thermal systems appear to be very attractive. The efficiency is MUCH greater than SolarPV. The reason we did not consider it is simple. We lived in California for a long time, and saw many many solar thermal systems. Almost all of them had been decomissioned. Everyody we knew there who had first-hand experience of such a system had tales of woe involving leaks. In the UK we, personally, know only one household with such a system - and it does not work properly. So, although I am certain that it MUST be possible to install such a system and benefit from it, we decided from the outset that we did not want to go down that route. As the technology continues to mature (and as en...