Solar Power in Teddington

An Accidental Installation

Slightly to my surprise, I became the owner of a solar power installation last week.

I had planned the works on my house in this order:

• Triple-glazing
• External Wall Insulation (EWI)

Wait for a Winter

• Heat pump
• Solar Panels
• Battery

This plan was partly rational. The largest carbon emissions are associated with heating, and so tackling those first – and evaluating their performance overwinter – was sensible.

But I was irrationally averse to getting solar panels because they felt like an indulgence – something I would “allow myself as a treat” after the hard work had been done. However, I changed my mind.

The main reason was that for the EWI work, I needed my neighbour’s permission to put scaffolding in the side passage by their building. My neighbour is an NHS clinic and it took several weeks to locate the person responsible and submit appropriate safety documentation. Although it was all perfectly pleasant – it was long-winded and not a process I wanted to repeat.

With that in mind, once the EWI scaffolding was erected, I called up a local solar power installer, Andy Powell from GreenCap Energy and asked whether he could install a system in the next two weeks using the EWI scaffolding. He visited the next day, and said that he could indeed use the scaffolding and that a 12-panel system would cost £4200. Importantly, he was able to install it the following week.

I had been thinking of all kinds of clever arrangements of panels, but it turns that if you want to put more than 12 panels on your roof, you need a special licence – which takes quite a bit of work – and time.

I reflected that if I installed the panels now, I would save roughly £1000 by using the existing EWI scaffolding. And that £4200 was much less than I had expected. So I put myself in Andy Powell’s capable hands and let him get on with it!

And there was one more piece of serendipity. At that point in the EWI work, it was possible to run the power cable from the panels to the main distribution board by running an armoured cable outside the house, buried underneath the EWI. This saved a lot of mess inside the house.

#1: The Components

The system consists of 12 solar panels, a device called an inverter, some isolation switches and generation meter.

• The 1.7 m x 1 m panels are from Q-cells. The choice of panels available is bewildering so I just accepted Andy’s recommendation. They look beautiful and seem to work just fine.
• Each panel generates roughly 40 V and up to 10 A amperes of DC current. They are connected in two banks to the inverter through two cables that poke through a small hole in the roof.
• The Inverter is a SOLIS 4G 3.6 kW model. It takes the DC voltage and turns it into 220 V AC that can be used around the house or exported to the grid. There is also an add-on that enables the system to be monitored from a phone.
• In my installation this AC current then goes back outside via an armoured cable buried in the wall  and then comes back inside under the floor, through a power meter, to the distribution board.

The gallery below shows some pictures of the process: click a picture for a larger version.

A pallet of panels!

Getting the panels up there

Detail of the attachment of horizontal rails. The steel bracket is screwed into a joist.

Horizontal Rail

Detail of cross section of rail.

Bracket for attaching solar panels.

The first panel.

The second panel.

The second panel being secured in place

Detail of the panel’s surface.

The Inverter and isolation switch installed in the loft.

Isolation switch and generation meter installed near the distribution board.

Generation meter detail. 30.18 kWh generated in the last 8 days.

#2: The Site

Google Maps view of my home showing the shape and orientation of the available roofs. And photographs before and after the installation. Click for a larger image.

There were two roofs available for solar panels on my house – a smaller triangular roof facing 25° east of south, and a larger roof facing 65° west of south.

My initial plan was to cram as many solar panels on the south-facing roof as possible, but being triangular, the large 1.7 m x 1 m panels do not fill up the space very efficiently. I could have squeezed 7 panels on, but in the end I opted for 6 on each roof – it seemed to look a little less ugly.

To my surprise – experimentation with Easy PV software (more details later in the article) seemed to show that the orientation wouldn’t make much difference to the overall energy generated.

The path of the Sun at the summer and winter solstices and the equinoxes. In the summer when most solar energy is generated, the sun sets up to 30° north of west, and so the west-facing panels continue generating later in the day after the south facing panels are in shadow. Having panels on both roofs allows for generation for a longer fraction of the long summer days. Click for a larger version.

I think the reason is that – as Andy Powell pointed out – no panel can generate for more than 12 hours because it doesn’t work when the Sun is behind it!

But in the summer, when most solar energy is generated, the Sun is above the horizon for up to 16 hours and at the solstice it sets more than 30° north of due west.

And so the west-facing panels continue generating later in the day after the south-facing panels are in shadow. Having panels on both roofs allows for generation for a longer fraction of the long summer days.

The path of the Sun at the summer and winter solstices and the equinoxes. The yellow zone shows sun orientations at which only the south-facing panels generate. The red zone shows sun orientations at which only the west-facing panels generate. The purple zone shows sun orientations at which both sets of panels generate.

Of course it is not just the east-west position of the Sun – the so-called azimuthal angle – that affects generation – the height of the sun in the sky – its elevation – is also important.

Sites such as this one will plot maps showing the course of the Sun through the sky on any particular date from your particular location.

The blue line shows the path of the Sun through the sky on November 11th for my location at 53° latitude and 0° longitude. 180° corresponds to due South. Click for a larger version. The shaded yellow boxes indicate the azimuthal angles at which the two banks of panels generate. And the green line shows the optimum elevation of the Sun.

From the figure above we see that the Sun is low in the sky at this time of year (Doh!) – it only rises 20° above the horizon at midday – but that even at this time of year, the generation from the west-facing panels in the afternoon prolongs the useful generation time. From initial observations the power on the two banks of panels is equal at about 1 p.m.

#3: Expected Performance

Frustratingly, working out the expected performance of a solar installation is complicated. One needs to:

• calculate the Sun path diagram for each day of the year,
• factor in the weather,
• consider each bank of panels separately.

Fortunately, approved installers such as Greencap can run standard calculations, or using software like Easy PV you can – after some messing about – come up with your own estimate.

Output from Easy PV software allows one to calculate how much electricity is likely to be generated by each bank of solar panels in a year.

For my installation, both estimates suggested that I should expect to generate roughly 3700 kWh of electricity each year. This figure is roughly how much electricity my house uses each year.

If I could capture each one of those generated kilowatt hours and use it to displace one that I buy from EDF  I would save more than £800/year. However, things are not so simple.

Looking around  one can find a few records (such as this one) of people’s generated power. They seem to indicate that in the UK I should expect roughly 5 times as much daily generation in the summer as in the Winter.

Putting that information together with the fact that I can expect 3700 kWh over the year I concocted a function (sine squared with an offset in case you care) to guide my expectations.

So in the summer I can expect the panels to generate perhaps 15 kWh/day – much more than I need – but in the winter the panels might only generate perhaps 3 kWh/day – much less than I need.

My guess at how many kWh per day I can expect from my solar panels. The average household consumption is shown is shown as a red dotted line (- – -). Click for a larger version.

#4: Actual Performance

I only have 10 days of data for the panels and these are plotted on the graph above and they seem broadly in line with my expectations.

I can already see the effect on my electricity usage. As can be seen on the graph below, my daily average use is 2.1 kWh below the average before the panels were installed.

Daily electricity usage (from a smart meter) before and after solar panel installation. Click for a larger version.

This may not sound much but even if the panels only ever performed at that level, this would prevent the emission of 73 kg of CO2 per year, and (@£0.24/unit) save me £175 per year. For those of you that are interested, that’s a 4.2% return on investment. But I expect the panel performance to be much better than this when averaged over the year.

But what is hard to capture in words is the sheer wonder of the installation. In bright November sunshine the panels generate more than 2 kW of electrical power – so I can boil a kettle and still see the smart meter read zero usage.

The inverter shows 2.34 kW of generated power on 10th November 2020!

The ‘smart meter’ shows we are taking no power from the grid.

#5: What next?

For the next few weeks I intend to let the dust settle, and try to get my head around how the system is working.

But one obvious difficultly – which will become more pressing as we move into Spring – is that when the Sun shines the panels produce kilowatts of electricity whereas the house itself generally consumes just a few hundred watts.

At the moment any excess electricity is exported to the grid – my smart meter says 13 kWh so far – as a gift to the nation!

So in the next few weeks I will sign up with a company to buy this electricity. There are several companies who will buy at between £0.03 and £0.055 per kWh.

In the longer term it may well make sense to get a battery as well. But batteries are expensive, and the more I have thought about it, the main use of a battery in my situation would not be to store solar electricity, but to switch the time at which I bought electricity from the day (when electricity is carbon intensive and expensive) to the night (when electricity is cheap and generally less carbon intensive). But that is a question for another time.