Home > home, science > The experiment on the roof: Solar PV installation

The experiment on the roof: Solar PV installation

We recently moved into a new, much bigger, house and the increase in energy bills came as a bit of a shock to the system. In our old, three bedroom terraced, house we had electric heating and a lower night time electricity tariff. This meant even with three kids, and washing machines, tumble driers and dish washers on every day that our fuel bills were less than £70 a month on average (with no gas in the house). Now we have gas heating, no off-peak rates, and with a much bigger house there are more lights (we currently have nine bulbs mounted on the living room roof, compared to one before, for example), PID detectors, an alarm etc. So our predicted energy usage based simply on the house size and number of people is almost £190 a month! Also this appears to be what the old owners were paying based on the home report we got before moving in. A bit of a shock to the bank balance. So we thought that it would be good to try and reduce this.

Solar panels on the east facing part of our roof

Having just moved in, we don’t have a great handle on our energy usage, and this is skewed somewhat by the fact that my wife has been on maternity leave (baby was born last week) since we moved in, and so has been in the house all the time. We happen to live in an area that gets a reasonable amount of help for energy saving measures, so we have had free loft insulation installed recently, which should help with the heating bills. Sadly, although we qualify for free cavity wall insulation, our walls are ‘non-standard’ cavities and so can’t be filled in, which would have helped a lot, I feel. We’ve been replacing older light bulbs and just generally trying to be sensible about turning things off. Next month we will also be getting a new boiler and thermostat system, which should further reduce our heating costs.

To address electricity costs things are less straightforward, and you really need to look at energy generation systems to get any significant gains. The obvious one is solar PV, but you might think living in the ‘North’ (we are in Fife in east central Scotland) that this a no-go. I think it should work out OK for us. Here I will go over the various things we had to consider and then look at some of our initial energy generation data. If you want a more comprehensive overview of all the different types of renewable technologies and the benefits/drawbacks and costs then I recommend the excellent Energy Saving Trust website, which also has info on Government grants and loan schemes. I should also say that part of our thinking was to try and generate electricity in a more ‘green’ fashion, and not wholly as a cost measure, but it is also nice to think we have quantum physics on the roof!


The main reason that renewables are financially viable for home use in the subsidy that is received in the form of a ‘feed in tariff‘ (FiT) for the energy that you produce. I won’t go into all the changes to the FiT in the UK at present, but we are eligible for a FiT of 21p/kWh of energy produced, payable for 25 years and index linked. Note that a kWh is one unit of energy as measured on your electricity meter. In addition to this, any energy that you produce you can then use and any excess is then fed back into the grid at an index-linked payment of 3.1p/kWh. The working assumption for current systems is that depending on the total power that your system can generate the tariffs plus useage savings will pay for your system in around 9-10 years and then you are simply generating cash. A typical return is therefore between 7.5-12% on your initial investment. This type of arrangement makes the systems fairly attractive if you can afford the upfront costs.

In our case we had about £8k to invest on our system, and we offset some of this by making use of an interest free loan of (up to) £4k that is available form the Energy Saving Trust. I don’t think this is available across the whole of the UK, only in certain areas. Our rough goal is to cover the payments for this loan (over 8 years) from the FiT cash that we generate.

We got a few quotes for systems, but we went for one with Gener8 Power who are a local company based in Dalgety Bay. Theirs wasn’t the cheapest system at just under £8k, but I was happy with their ability to answer questions about the system in a speedy fashion, and now that we have the system installed, I’ve been very happy with the work they have carried out. Also, to receive the Feed in Tariff, the installers and the equipment they install must be MCS (Microgeneration Certification Scheme) certified.

Our general circumstances are useful to know, if you want to compare with your own. We have a large 4 bedroom house and there are now six of us, 2 adults, four kids, living there. My data on energy usage is a bit patchy, but based on last weeks usage we burn through around 14 kWh per day on average of electricity.


The ideal orientation for solar panels is on a South facing roof. While the front of our house is south facing, the roof is east-west orientated. This, in addition to our Northerly location, might suggest that solar is not viable. The South facing roof is optimal, but the efficiency drop is not huge for east-wst facing. For our 25 degree inclined roof, it’s about a 20% reduction. When you get a quote for a PV system you get a design document that looks at the house orientation and roof details and combines this with geographical data, panel and inverter data and works out a prediction for the energy generated and the cash generation and energy savings.

Efficiency map for solar Pv installation as a function of angle of roof and roof orientation. Image from Energy Saving Trust: http://goo.gl/BTZkf

We looked at different scenarios, but opted for a 12 panel setup, with 6 panels on each half of the roof. Each panel feeds into a inverter which transforms the DC generated by the panels into an AC signal that can be handled by the mains system. We didn’t really have much input into the panel and inverter types, but we have 12 REC 240Wp (Watts peak) polycrystalline panels and a Aurora Power One inverter. This gives us a total solar panel area of around 19.8 m^2 and an overall system output of 2.88kWp. If memory serves me right, for an extra £1200 you could get another 4 panels added. The inverter output is then fed through a meter which monitors the generated energy and this then outputs into the mains. The installer said our (old fashioned) electricity meter should run backwards if there is feed-in into the grid, which sounds interesting to see! The predicted output for our system over the year is shown below (which impies around 5-6 kWh per day in March).

Predicted energy output from our inverter.

A technical point is that the software that the companies use to do the solar PV calculations is called PV*SOL, which is the software we use to teach on our BSc and MSc degrees in Renewable Energy at Dundee, so clearly we are using an industry standard, which is always good.


As the energy generated is available immediately and only when the sun is out, the reality will be that we only use it if we happen to be in during the day. While my wife is on maternity leave over the summer this will certainly be the case, so hopefully we will see the benefit. During the winter, maybe less so. We also need to think about putting the washing machine, dish washer etc on while the sun is at it’s highest to maximise the energy usage. It’ll be interesting to see how we can adapt to this. It might mean getting a new dish washer with a timer on it,as we had in our old house to take advantage of off peak rates, for example.

We have had the system in for two days. On day one, where we didn’t get a full days use, we generated 4.9 kWh. The day was overcast in the morning, with the sun out for most of the rest of the day. On day two we generated 8 kWh on a pretty sunny day. So 12.9 over the two days, about 45% of our total energy use in that period (assuming we used it all), and £2.71 minimum in feed in tariffs generated. So far I’m quite happy with this, and it suggests that the predicted values look reasonable when compared to this (very) limited data. We will see how it works out, and I’ll try and have an update in a few months to assess if the prediction really have been on target.

If anyone has any questions about our install just let me know and I’ll be happy to share our experiences. I’m also happy to recommend Gener8 and if you’d like a referral note I’d be happy to oblige. Obviously there are lots of other installers available.

Note (23/3/12) Day 3 was sunny all day and we got 9.5 kWh. Day 4 has started foggy and cloudy, so will give a good idea of a more average day.

  1. March 22, 2012 at 11:31 am

    I reckon your system for a good price, ours was roughly the same but for a 1.85kwp system (this was about 18 months ago).

    Cavity wall insulation can make a really big difference to heating bills but I guess it depends on what sort of non-standard cavity walls you have as to whether this is worth fighting with for you.

    When we installed our system I spent the first couple of days running up and down the loft ladder to look at the readouts – now I just do a weekly measure using the electricity meter under the stairs. If the weather with you is like it is down in Chester then you’ve had a very good couple of days. We have a single array on an East-facing roof and the output we achieved over the year was within 2% of the expected yearly output.

    • March 22, 2012 at 9:24 pm

      Yes, the prices have come down quite significantly recently, I think in part in line with the fall in FiT.

      We have a timber frame house, with an odd extension on it, so while we were eligible for free cavity wall insulation, they felt they couldn’t put it in. Solid wall insulation looks like the main alternative, but it is very expensive (say > £25k).

      Our first few days have been very good weather wise, with 9.4 kWh today, which I reckon must be about the maximum we can get for this time of year, as there was not a cloud in the sky for most of today. Our energy usage yesterday/today also fell quite a bit, so I think we used a reasonable amount of it. So a few days in I’m very happy with it. We’ll see how it pans out over the year.

      I don;t have a handy loft ladder, so am just sticking to the meter under the stairs!

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