Solar install, part 1

I finally have a solar install, and a battery on order. So this blog is the first one on this - I don't have a lot of data yet as it just went in, and I don't have a battery yet as they are like hen's teeth, so more on both of those later.

Solar panels waiting to be installed

The basics on a solar install

Firstly, it is not a simple/quick "free electricity" solution for the current price rises. There is more to it in terms of whether it is worthwhile, and over what period, and so on. I hope I can explain some of the factors here. There are several factors.

When the sun actually came out yesterday

How much power do we use anyway?

In my case we use a lot of power. Over the winter, since I have started monitoring in more detail, we have been using around 60kWh a day. One kWh (kilowatt hour) is one "unit" of electricity, and currently that is costing me over 28p - but tariffs are a key factor here. 60kWh is a lot, and a typical household is more likely to be 10kWh/day. Bear in mind that will depend on the time of year and the type of heating and so on.

One of the first things I did was install some monitoring. This is way more useful that simply having a smart meter as it tells me much more accurately where the power usage is happening. This meant some Shelly EM modules with tasmota and current clamps - not that expensive, but did mean messing with current clamps on lots of circuits. It means I can graph usage per circuit.

It immediately highlights the key uses of power here are the hot tub (yeh, it came with the house), and the heated kitchen floor (also came with the house). Other users like tumble drier come in when used, obviously, and now we have air-con that too is a noticeable factor. Some adjustments to temperature made a noticeable difference, and I really want to find something nicer for the kitchen floor (it is also a living room). The rest of the house is gas central heating. I would say that summer is expected to be lower, but the air-con will make a difference.

So yes, we are a very high user. This is, however, a factor in terms of solar - if you make more than you need you are "selling" it as export - but getting a good tariff for that is not so simple. You make/save most with solar if you are using it yourself, as that will save you money at that huge 28p/unit rate. Exporting it may only get you 5p/unit. Oddly you can have a different company for "exported" power payment than for your "imported" power - crazy, but apparently the best deals on export are when both are the same.

The other issue here is that our usage is in bursts - we have a base of around 1kW a lot of the day, but hot tub uses an extra 3kW in short bursts, and the kitchen floor is even more, but again in short bursts and often in the morning before the sun rise in winter. So we have these high bursts of usage that will often not be covered by solar. This is where the battery comes in - being able to store the extra solar when only using 1kW and power things when they burst. (more on power monitoring)

Hot tub

The good news is the air-con is not quite so bursty the way I have it set up now, and this can more easily be covered by the solar before I even have a battery.


So why solar anyway?

One obvious factor is saving money - but this is where there is a catch - if I simply put the money in a savings account and used it to reduce my electricity bill every month, that would probably be better for the next decade - well, maybe, but certainly for a few years.

This is all a bit of a "Vimes's boots" scenario - i.e. the poor have to keep buying cheap boots, but the rich can afford to spend a fortune on boots once and they last so much longer than they end up spending way less overall. The same is true here - if you can afford a solar install you can have decades of spending way less on electricity - something from which that those who cannot afford the install would greatly benefit.

So what other factors are there?

Well, one factor for me is the stability of this (or perhaps "energy security" is a better phrase) - electricity prices are silly now and could get extra silly soon (next rise is October). If I can produce a noticeable chunk of my electricity, and have a means to reduce a lot of the rest (shut down the hot tub, etc), I can protect myself from those silly prices. It is, of course, hard to pin a figure on what this means in advance, especially if trying to plan for the next decade.

Another factor comes in with the (expensive) battery (Tesla power wall). It is able to power the house, so a short power cut (of which we have only had one so far) can be covered. Again, there are risks - the way the country is being run, and factors like Russian oil/gas, etc, could mean power cuts - who knows? This is a small risk, but it would be very nice to manage small power cuts - whether deliberate ones, or simply storm related (as last time).

Finally, it is investment in the house. We expect to be here some time - maybe "forever" from our point of view, but the solar install should improve the house value. This may be useful if ever we do want to sell, or if we raise funds on the house, or even when it comes to our kids selling it. The installation is not just throwing money away - it increases the house value some (yes, this itself is complicated as some older solar "roof rental" deals are more of a burden and can reduce value).

Of course it is also an environmental aspect - it helps reduce the burning of fossil fuels. Indeed, I seem to have earned a gold star already!

Solar and/or battery?

There are two distinct aspects - solar panels and battery. These are distinct but do help each other, so you may want solar, battery, or both, depending on your circumstances.


The Solar aspect is pretty simple - the panels make power - more in summer than winter - only during the day - more when the sun is shining than in cloud. The installer can predict how much in the year. That will allow you to work out the cost vs payback. The installer should do proper forecasts.

For us, based on 22p/unit starting point, we are looking at a little over 10 years to cover the cost of the solar system install. Over the following 10 years we are looking to save a further twice the cost of the install. This assumes some inflation on electricity prices as well as some degradation on performance.

On this basis you can see solar cost per unit over 20 years is good, better than you would get from any electricity supplier even now. On a long term plan it makes sense. It may even make sense for some people on a "loan to install" basis, but that is a risk over future interest rates.


This is where the options and costs get even more complex. It seems there are some battery systems which basically help "level out" the solar some (those 3kW spikes for hot tub, etc), and are not a lot of good as a "backup" for a power cut. These are probably a no brainer in terms of managing the solar supply from any install as the usage and the solar can vary during the day - something even a small battery can level out nicely.

There are, however, some that can back up the house, and the tesla power wall is one of those. Such systems cost a lot.

For a start, the battery helps make the most of the solar. It means you are not exporting. Without this, any time in the day we are making more than we use, we are sending that electricity to the grid. It you are lucky, and have sorted the paperwork (I have not yet, just installed), you may get 5p/unit for that. It is much better for that to go in to a battery and then get used later saving you 28p/unit.

For example, right now, on a very cloudy April morning, I am making over 2kW but only using 1.43kW. As I don't yet have all the paperwork to be on any "export tariff", I am just wasting that 0.61kW going to the grid.

A cloudy morning

In an ideal world with solar and battery, when you have enough solar in a day to cover a day's usage, you want a battery that can store enough to cover the night time, and hence never import any electricity on those days (typically in the summer). If you have usage figures then that can be quite easy to work out. Even so, you may also be able to move usage around - especially if anything (e.g. a car) needs charging as well.

But a battery has other advantages - apart from the possibility of covering a short power cut, it can also work with agile tariffs - charging from the power grid when electricity is very cheap in the middle of the night, etc. You then use that power during the day.

Indeed, just having a battery, and no solar, can end up making your electricity very cheap. We may even want a second battery at some point for this very reason.

In this case you want a battery that can charge in the short periods of cheap power (middle of the night), at the same time as perhaps charging your car, and hold enough (allowing for any solar contribution) to power your house for the rest of the day until the next cheap charging point. Again, not too hard to work out, and to decide if you want to cover winter (less solar) and summer (less battery needed).

So for some people a battery is an option - but a good battery system like the tesla power wall can end up costing a lot more than the solar install.

There are some power companies (Octopus, for example) with special tariffs where you give them control of your battery (if a Tesla battery) allowing them to feed in power when they need, and they end up charging you only for net power usage at a much lower rate. Such a tariff is extra "simple" as you don't have to do anything - it just works and you have power at a low rate. They even keep some in reserve for power cuts.


In my case, I have gone for both - though that does make it harder to work out what each separately may be saving me. It also means I need to sort some suitable tariffs. That special tariff that need control of the battery also only works if you also have a solar install and a tesla battery. Even then, there are some limits per battery on solar capacity and daytime usage, with which I may have a challenge on only one battery. This could be fun, and I am sure another blog post eventually.


The forecast payback, as I said, was a round a decade, for the solar alone, based on 22p/unit starting point.

That said, if I was to take a simple calculation of 60kWh/day usage at 28p/unit, that is £6132 a year. If I make an average of only 25kWh/day from solar, and get on a 10p/unit "they control the battery" tariff, that gets to £1277/year, saving around £4855k/year. That makes a payback of a £22k solar and battery system under 5 years. If I can make use of the battery to charge at 5p/unit middle of the night, top up with solar, and use battery for my usage, then payback would be 4 years. There is even a saving on standing charge to add to that. Now allow for inflation, or maybe an October rise to 35p/unit, and well, things get very interesting - there is a very real prospect of the whole system payback in only a few years - even Vimes would be impressed.

Of course, it could be we get a sane government, and sensible power pricing, and the payback is way longer - but I am, none the less, buying stability.

More specifics...

The supplier I have used is Green Park Power, based in Abergavenny (a short walking distance from my house). They have been very professional. I'm impressed.

The system is 31 panels, each 390W, so 12.09kW total. It is using solar edge - with per panel optimisers and an inverter in the loft. The per panel optimisers are DC/DC convertors allowing the mix of panels that have high and low light levels to all work together, even allowing for some in shade. Traditionally panels in shade can bring down the overall performance, but not so in this system.

It also allows monitoring of power per panel, and whilst there system is cloud based (which I dislike), it is very good. (I thought solar panels did not like "clouds"). I may set up some MODBUS stuff to provide some local monitoring in the long run, obviously. The inverter does connect to WiFi or, as in my case, Ethernet.

The figures in my case are roughly £12k for solar and £10 for battery, but it will very much depend on circumstances.

It really is simple

It is just working, the solar power cancels a chunk of the usage and creates some export. I mean, yes, once I have a battery, I may want to fine tune to make the best of some tariff, but in general I don't have to actually "do" anything - it just works. For now (before I get a battery) I have tweaked the settings on my hot tub, as one of the main power uses in the house, to be when the sun is shining.

But ultimately if someone bought the house they would not have to "do" anything, it would just lower their bills!

More to follow...

The solar edge monitoring if very comprehensive, and in a few days I should have some more useful data to share. Later this year I'll also have a battery to play with.

The first day, yesterday, was very gloomy, wet, and cloudy, but in the afternoon we were seeing a consistent 5kW, and at one point in the morning when the sun came out, well over 8kW. In our case, even though the system is rated to 12kW, it is 12 panels facing west and 19 facing east, so not expected to have full capacity, making 8kW rather impressive. I am going to love seeing a proper sunny day on this. But only starting stats some time yesterday, I don't even have a full (cloudy) day's stats yet.

A quick update: First whole day (all cloudy) 18kWh, second day (more sun) 43kWh so far (4:30pm). Remember the forecast was only 25kWh/day over the year, and at this rate I'll exceed that just in the summer.

Part 2


  1. Question - if you have a power cut does your house simply cut over to use the solar power already held in reserve in the battery with no down-time? Does that actually work, i.e. have you simulated power cuts by throwing the main breaker from the grid power?

    1. Once the battery is in we have a gateway to allow running off battery. This is an extra.

  2. Just got my own Victron based battery storage system. We are buying almost all energy overnight on the octopus go rate. It is fascinating to see how much Solar ends up in the battery as well. After 3 weeks, last night was the first time the batts went flat before the start of the Go cheap rate. It's all being graphed on the Home Assistant portal as well. As they say in Dutch 'meten is weten '

  3. Hi, I have a self-installed solar system plus energy monitoring via Shelly EM / Home Assistant, and I am looking at ways of enabling export limiting on my inverter, so I wondered, having come across this article, whether you'd looked more into MODBUS stuff yet? In particular, I'm after taking data from Shelly EM and feeding back to the inverter's RS485 port for the purposes of enabling export limiting, without having to buy an extra meter/CT and run another cable from consumer unit to inverter. I am struggling to find any similar scenario with my Googling.

    1. Modbus is pretty simple, but I have not done anything with it. Certainly here (UK), for connection to the grid, I had to have a suitable approved export limiting device (extra meter MODBUS connected to the inverter) to be allowed to connect.

  4. I got the approval to connect the inverter from the DNO - but as I don't have an MCS certificate it is proving difficult to persuade Octopus to sign me up for SEG, which is why I'm looking into ways of reducing how much goes out. Ultimately, I think I'd like to store any excess in a battery rather than let it go to waste, (and have to buy it back at ridiculous rates at night) but that's another project.

    I'd have had a pro install done but at the time it was proving impossible to book anyone in, and I found an "MCS approved" kit online, and (perhaps mistakenly) thought "How hard can it be?" :)


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