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(With a little upfront investment).

I've blogged before about our Solar Panels and how they're performing. Liz and I have recently moved house, and decided that it made sense to get panels on our new property. Basically, we're so capitalist that we can only enjoy the warm glow of Earth's yellow sun when it is enriching our bank accounts.

burns excellent sunshine

We looked at several local installers, before settling on Sims Solar. Paul was able to produce a detailed set of measurements, understood my crazy data-logging requirements, and gave us an excellent price. As a bonus, he went to the same university as me and Liz - UEA!

It took a day to stick up the scaffolding, then a day to physically install the panels and monkey around with the wiring and fuse boxes. Then a couple of hours yesterday to set up the Internet connection to the inverter.

Here's the result:

Yup - that's our ancient electricity meter running backwards! As all that lovely sky juice gets slurped up by the silicon wafers, the photons are crushed and gently fermented into electrons, which are then organically fed back into the mains. Good people of Oxford - charge your phones on me!

Solar Panels on our roofThis is how we have set everything up...

Solar Panels - made by SunPower - eight of them on the roof.

4kW Inverter - Made by Fronius.

DataCard - allows the whole system to be connected via Ethernet or WiFi (2.4GHz only though) Fronius Datamanager - as well as a nice web front end, there is an API. Made sure it was running the latest Fronius firmware.

And this is what it all looks like when wired up in our utility closet. The solar power AC has its own isolator, meter, and RCD before being split into the house consumer unit and mains electricity meter.
Utility Cupboard
The more observant of you will have noticed the Solar iBoost nestling in that tangle of wire. The iBoost has a sensor which monitors the flow of electricity from the panels. If there is surplus electricity flowing back into the grid (i.e. the house is using less than is being generated) it sends a radio signal out at 868.3 MHz to its companion device near the hot water tank.

The iBoost switches on the electric immersion heater until the desired water temperature has been reached or the electricity load drops.
Solar iBoost
So, during a typical day, the panels may be generating 1400 Watts. But if the fridge, phone chargers, and computers are drawing less than that - the remainder is used to heat up the water. Rather neatly saving on gas costs as well.

Monitoring and the Internet of Things

As I mentioned, the inverter has Ethernet and WiFi connections. This allows the user to monitor the system via a web interface, hook it up to remote monitoring, and to programmatically get access to the device's API.

The local web interface is basic and functional - but it links up to Solar.web which allows realtime and archive access to your data.
Web Interface
There are also Solar.web apps for iOS, Android, and BlackBerry.

The documentation for the V1 API is sparse and slightly cumbersome (you can't copy & paste from the PDF). But it does work very well - spitting out JSON for just about every conceivable function.

A single API call can get realtime information about what the panels are currently generating - and how much they've generated in total.



   "Head" : {
      "RequestArguments" : {
         "DataCollection" : "CommonInverterData",
         "DeviceClass" : "Inverter",
         "DeviceId" : "1",
         "Scope" : "Device"
      "Status" : {
         "Code" : 0,
         "Reason" : "",
         "UserMessage" : ""
      "Timestamp" : "2013-12-18T09:12:06+00:00"
   "Body" : {
      "Data" : {
         "DAY_ENERGY" : {
            "Value" : 664,
            "Unit" : "Wh"
         "FAC" : {
            "Value" : 50.03,
            "Unit" : "Hz"
         "IAC" : {
            "Value" : 1.1,
            "Unit" : "A"
         "IDC" : {
            "Value" : 0.83,
            "Unit" : "A"
         "PAC" : {
            "Value" : 274,
            "Unit" : "W"
         "TOTAL_ENERGY" : {
            "Value" : 4000,
            "Unit" : "Wh"
         "UAC" : {
            "Value" : 247,
            "Unit" : "V"
         "UDC" : {
            "Value" : 326,
            "Unit" : "V"
         "YEAR_ENERGY" : {
            "Value" : 4000,
            "Unit" : "Wh"
         "DeviceStatus" : {
            "StatusCode" : 7,
            "MgmtTimerRemainingTime" : -1,
            "ErrorCode" : 0,
            "LEDColor" : 2,
            "LEDState" : 0,
            "StateToReset" : false

So, plenty to play about with. It should be fairly easy to set up something useful to do with the data.

Cost And Payback

The total system cost just shy of £8,000. That's a hefty chunk of money - but I feel it's a wise investment.

The panels - and their associated bits and bobs - are guaranteed for 10-25 years. Although the panels will degrade over their lifetime, it's not by a hugely significant amount.

According to the solar power calculators, the panels should generate an average of 3478kWh per year.

The Government's Feed In Tariff scheme will pay 14.9p per kWh generated and 4.64p per kWh exported back to the grid. They assume a uniform 50% export, meaning a total payment of 17.22p per kWh. That price rises with inflation and is guaranteed for 25 years.

Assuming my maths is correct, we'll be paid around £600 per year for generating solar power. At that rate, it will take 13 years to pay back the cost of the system.

Of course, that doesn't take into account the total savings from generating our own power. Based on our other panels, I'd expect them to generate about 2/3rds of the electricity we use. Now, the sun only shines in the day - and most days I'm out at work. But I still leave my fridge, servers, and security cameras on. I can set the washing machine and tumble dryer to come on around noon to take advantage of the solar output. On the days I work from home, my MacBook and microwave pizza will be sunshine powered.

Let's say that it will knock our electricity bills down by a third.

I'm with Ovo (switch and we both get £20) - at the moment, their electricity price is 12.15p/kWh. So, I reckon that we'll be saving at least £120/year.

Finally, there's the gas savings to consider. Again, with Ovo we pay 4.13p/kWh for gas. I've no idea how much the Solar iBoost will save as it heats up the water tank. Since having the system installed 5 hours ago, it has saved me 1.34kWh.
iBoost savings
Assuming that's the same every day - a total of £20 per year. Not a huge amount, but enough for a celebratory gin and tonic every now and again.

That takes the repay time for the panels to about 11 years (£8000 / £10+£120+£600).

All of this assumes that the price for gas and electricity doesn't rise beyond the rate of inflation. If you've been following the UK energy markets, you'll know that most of the providers raised their prices by 10% (not Ovo though!)

I suppose there's two ways of looking at it.

  • Best case scenario - there's an energy shortage and the repay period goes down.
  • Worst case scenario - cold fusion becomes a reality and electricity becomes too cheap to charge for. In which case, I'm stuck with an interesting historical anomaly on the roof. Oh well!


Expect lots of solar data geekery from this blog 🙂

At the moment, my household appliance are fairly dumb. I'd like to see if there's a way I can hook them up to an API which takes into account the current power being generated and the weather forecast to find the optimal time to switch on.

It would also be interesting to see if there's a way to get data out of the Solar iBoost. At the moment, the physical display screen will let me see how much power it has used per day / month / year - but it would be fun to get some more info out of it.

I'm going to start monitoring in earnest from the winter solstice - that's the 21st of December this year. I should be able to chart a steady rise in the amount of free money falling from the sky!

Massive thanks to Sims Solar for such a fast and professional install. If you want panels in the Oxford area, Paul is your man.

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20 thoughts on “FREE MONEY FROM THE SKY!

  1. A key question is: What does it do to the life of your roof covering and any warranty you have on same? About 5 years ago I spent a bit over $US20k on a new asphalt shingle roof with a 50 year warranty on materials and 20 year warranty on labor. Does the solar company guarantee that the roof warranty isn't compromised? Also what does the roof installation company say about installing solar with respect to leaks, shingle damage, etc? I live in Northern California where we have enough sun days to make energy harvesting pay, but I have been concerned about the consequences for roof life and integrity.

    1. Hi Art,
      I guess UK roofs are a little different. Ours have roofing tiles. Basically, one of the tiles is removed, then the fixing for the panel is screwed into the rafters, then the tile is replaced - often with a small cut made in it to fit.
      I would suggest talking to local installers in your area to see what they say.
      Enjoy the sunshine 🙂

    1. Good question. The income is tax free. So any cash paid out doesn't have to be declared as taxable income. Which is nice 🙂

  2. ben says:

    i reckon - you should think again about using pv energy to heat hot water - basically you are increasing CO2 emissions becuase you are displacing gas (low CO2 emissions, what you would have heated the water with otherwise) with electricity (high CO2 emissions).if you didnt use the iboost, someone up the road would be utilising your free low carbon electricity. as it is, they are now burning high grade electricity instead - pv water heating makes sense financially, but not in energy terms. Sorry to be a party poop, but i find the detail important : )

    1. Hi Ben,

      That's a reasonable point. I will say, however, that only a minority of the generated energy is used to heat the water - the majority of the surplus still goes back into the grid.

      If I can work out a way to get data out of the iBoost (other than just reading its display) I'll attempt to work out how much it is.


  3. says:

    its a point of much contest in the world of the installers - many have been so hard hit by the tariff drops that they try and squeeze value out of each sale as much as possible... to hell with common sense. hence these things are being promoted all the time. personally i think self consumption will become an important area in the future, but the tariff system has to catch up with this technology first

    solar thermal will generate 3 or 4 times as much energy per sq mtr of roof space, but is under incentivised in the current regime. I would urge you to use your saved £s to go solar thermal!

    1. Ben,

      You seem to have a commercial interest in selling / servicing Solar Thermal - a point you neglected to reveal in your last comment.

      I'm not so sure your numbers stack up in our case. For a few months of the year we need the heating on for a couple of hours per day - hurrah for insulation and jumpers! When it's cold, we need hot water for showers and the occasional bath. All of our washing appliances are cold-fed.

      If I take a look at the last 12 months of energy usage - in a previous flat where we used gas for cooking and had inefficient hot air heating - our usage was:

      3294.0kWh of Electricity which cost £357.42
      10172.0kWh of Gas which cost £336.73
      (Ignoring standing charges and VAT)

      If you're right and our roof could generate 3x energy of PV, and even if all of that gas was used for hot water, and all of it was offset by solar thermal, it still wouldn't make financial sense.

      And, during the summer, it would be totally useless - except for the occasional need to do some washing up. The radiators wouldn't be on and I like my showers cold. There can be no "feed in" with hot water.

      Electricity is just more useful than hot water.

      Solar Thermal obviously has its place - and can probably be used in conjunction with PV - but for a modern household it makes more sense to go with PV. Yes, there could be better incentives for hot water - but I think most people would rather have the multi-purpose nature of electricity rather than one-trick pony of hot water.



  4. says:

    hmm well I do act as a freelance consultant and designer mainly rather than "selling" solar thermal (I have been working in renewables for 10 years.) but I have no interest in driving to Oxfordshire to install or service if that is what you are getting at, I live in the Welsh borders : )

    I think you are misinterpreting my comment - I am making the same point as you. It doesnt stack up financially, and that is the problem. If you are going to use solar thermal, 4msq of roof space will generate roughly 1500kWh per year, to achieve the same you would need around 16msq of PV panels (2kW). If the main aim of energy policy is to encourage low carbon energy production, solar thermal will do this at lower environmental cost, and still leave space on many people's roofs for PV systems to generate more energy, but at higher environmental cost. Thats really all I am saying, (alongside the fact that PV water heating is ultimately damaging if you are offsetting gas usage)

    I'm not even denying that electricity is more useful - it is and I'm certainly not arguing otherwise

    The fact is that around half the UKs domestic hot water use goes through the shower head, so if you like cold showers you are certainly in the minority, and I'm willing to bet you don't actually use water @ 10 degrees in your shower, but mix in at least some hot water. A very quick sum based on average UK consumption:

    1 person showers daily @ 60litres per shower, source:

    thats about 30litres of water @ 60 degrees mixed with 30litres at 10 degrees to give a shower at about 40 degrees

    30 x 5 showers a week (reasonable?) x 52 weeks a year x number of people in your household (lets say 3 as an average?) = 32,760 llitres of water heated to 60 degrees per annum, not allowing for bathing, washing clothes, dishwashing or anything else. If that doesnt represent a useful amount of energy to tackle using a small amount of roof space I don't know what does!

    Solar thermal would roughly address that entire showering load for a family of 4 over the year, using 4msq of panels.

    1. Some very reasonable points!

      I'd be interested in seeing just how much water I do use. Although it's a power shower, we have one of those "magic" shower-heads which supposedly reduce water consumption (Free from Thames Water).

      That said, I'll check at the end of the month just how many kWh have been diverted into the immersion heater and see whether it makes a huge impact either way.

  5. says:

    so Ive got a question for you, seeing as you are a coder!

    I want to get people more interested in the performance of their solar thermal systems, but getting monitoring sorted out is, at the moment, very expensive.

    Given that many controllers have some kind of data interface in the form of a TTL/USB port, which can be used to network the controller it shouldnt be too difficult - they already sell equipment to do it. But it costs about £250 at trade price, and what they are selling you is basically a re-branded router with some fancy firmware, that probably cost about £30 to make, if that.

    So... can you think of a way of doing that much much cheaper? say using a rasberry pi or some other hobby kit? And what problems do you face taking the data output and making sense of it? there is already software for interpreting the data available for free and easily accessible:

    1. Yes, the code I've written will run on a Raspberry Pi. I've done work for other companies who have used Pi or Arduino as small servers.

      If the Inverter (or whatever) has a basic serial port, it's pretty easy to write something to interface with it and get data out.

      However, depending on how you value your time, £250 is pretty cheap to have something which "just works".

  6. says:

    Hmmm OK. The thing is it ends up being very expensive to the end user, as the installer will mark it up 25% or so, add in the installation and set up and suddenly you are at around the £400 mark

    1. Well, even a cheap Raspberry Pi is £30. Add in a decent power supply, SD card, and case - you're up to around £60. Add in a good WiFi module and that's £70 - more if you want one with an external antenna (useful if it's installed in a loft).
      Hardware wise, that's probably all you need (plus a cheap USB or serial cable).

      So, now you need to write some robust software which will continually log - and recover in the case of power failure. Also needs to have a good looking front end. Will need regular updates to deal with emerging security threats. Oh, better make sure the case is hermetically sealed so all the spiders in the loft don't make their home in there, die, and short out the circuits.

      Finally, you'll need a web server for external logging - which, again, will need constant development and maintenance.

      All of a sudden, that £400 looks fairly reasonable for a "plug and play" box with good set of cloud services.

  7. Markus says:

    Hi Terence,

    I have also a Fronius Converter with the Datamanager.
    Can you help me setting up your script so that I can display the current produced W on my blog? The Data should be pulled out of the service.
    Thank you very much and best regards,

  8. Markus says:

    Hi Terence,

    it could work also with the access to the local API of the fronius itself runnin in my network, without the solarweb.
    Do you have a tutorial with steps what I need to install to get the values out of the fronius inverter ?
    thanks a lot,


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