Posts Tagged ‘Solar Panels’

A Year of Solar Energy

November 8, 2021

Friends, it’s just coming up to the anniversary of the installation of solar photo-voltaic (PV) panels at Podesta Towers in Teddington. So I thought it might be interesting to see what a year of generation has brought.

First I’ll describe the installation; then explain what I expected (or at least hoped for); and then outline what has actually happened together with a discussion of the role of our domestic battery.

And finally I’ll remind you – and myself – of how solar panels fit into to my efforts to reduce carbon dioxide.

But in case you’re short of time, here are the salient points.

  • 12 solar panels generated just over 3500 kWh of electricity which is close to last year’s total domestic consumption ~3700 kWh.
  • This will have avoided emission of ~0.8 tonnes of carbon dioxide this year.
  • On the sunniest days the system generated ~25 kWh/day and in mid-winter average generation was ~ 2kWh/day which compares with around 10 kWh/day of non-heating electrical use.
  • When used with a battery, we were substantially off-grid for around 4 months.

1. The Installation

I described the installation in some detail here, but briefly it consists of 12 panels from Q-Cells, each 1.0 m x 1.7 m in size with a nominal generating power of around 340 W. I think this year’s versions are already more powerful!

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

The installation cost £4,230 pounds which did not include the cost of scaffolding which was already up on the house at that point.

I chose Q-cells panels because I liked their completely black appearance; they seemed adequately specified; they were readily available; and they were cheap – around £130/panel if I recall correctly.

I later found out that they have pleasingly low embodied carbon dioxide, less than 1.6 tonnes for this installation, and so the embodied carbon dioxide payback time will be around 2 years. Q-cells also score highly for the not producing toxic waste.

2. What did I hope for?

The quotation from local installer GreenCap Energy included an estimate of the expected output – 3780 kWh/year.

But rather than trust the installer I downloaded data from an EU project that cleverly allows one to estimate how a particular solar installation with an arbitrary location and orientation would have performed hour-by-hour over the entire period from 2005 to 2016. This data suggested I might reasonably expect 3847 ± 173 kWh/year.

Click for a larger image. Estimates from an EU re-analysis project of what my solar panels WOULD have generated over the years 2005 to 2016. Year-to-Year variability appears to be about 5%.

I then averaged these 11 years of hour-by-hour data to yield my estimate for the expected monthly performance. They are shown as yellow dots on the graph below.

Click for a larger image. Expected generation in kWh per day. The yellow dots are the monthly averages of the estimated generation from 2005 to 2016. The green dotted line is a crude guess based on a ‘sine-squared’ function. The red dotted line shows our typical average daily electricity consumption ~10.5 kWh/day.

One feature of the simulated data is that the peak generation is expected to occur from April to July – a range which is not centrally arranged around the longest day (June 21st).

3. What happened?

Solar generation was broadly in line – but a little lower – than expectations. But the day-to-day and week-to-week variability was much greater than I had appreciated.

This variability makes it hard to plot readable graphs because they look chaotic! So let me introduce the data one stage at a time.

Looking at the monthly averages (below) we see that most months were close to expectations, but April was especially sunny, and August was a bit disappointing. So far, November has been a brighter than normal. Please note, the December data is from December 2020, because the data from December 2021 is not yet available.

Click for a larger image. Comparison of monthly averages of actual solar generation (green dots) with expected generation in kWh per day. (yellow dots). The black error bars show the standard deviation of that months daily data.

Now let’s additionally plot the daily data.

Click for a larger image. Similar to the graph above but now additionally showing the actual daily solar generation (kWh/day)

A few features of the daily data are really quite remarkable.

  • Firstly, the day-to-day variability is large. This means that the solar generation on a given day is almost no indication of the likely generation on the next day.
  • Secondly, the peak generation of around 25 kWh/day can occur in either April or July despite the substantial differences in day length and solar path.
  • Thirdly, even in mid-summer there can be runs of several utterly miserable days with very little solar generation.

If we average the daily data over a week, (see below) then we still see variability – deviations from the nominally-expected generation – which deviate from the expected generation consistently over periods of up to 3 weeks

Click for a larger image. Similar to the graph above but now additionally showing the ±3 day average of the generation as pink or purple lines.

Another way to look at the data which emphasises the trend more strongly than the variability, is to show cumulative generation through the year.

Click for a larger image. Cumulative generation (kWh) shown as a blue line against nominal expected generation.

Actual generation (just over 3500 kWh) is about 8% lower than I expected, but I am not especially surprised.

It could be that 2021 was a ‘a bit dull’ over the summer months when most generation takes place, or because I had incorrectly allowed for losses at the inverter – which converts DC electricity into AC mains electricity.


The PV panels were installed in November last year and to our surprise they immediately made a measurable difference – reducing our use of electricity from the grid by around 2 units per day.

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

However, it was not till our battery was installed in March 2021 that the transformational power of solar became apparent. Within a couple of days, the household went ‘off-grid’ and remained off-grid for around 80 days.

Click image for a larger version. Daily electricity usage (from a smart meter) since solar panel installation. Install the panels led to a small reduction in grid use. Consumption rose over Christmas. Consumption began to fall as we entered spring, and then fell to zero once we installed the battery. As we enter Autumn and Winter grid consumption is rising because since July we are using electricity to power a heat pump which heats the house and provides hot water. The bold green line shows the daily consumption averaged over ± 1 week.

As we enter autumn and winter, the solar cells still contribute significantly – 48% of our electricity in October was solar. But generation will be just around 2 kWh/day through November, December and January.

Since July, the ASHP installation has been using ~1.5 kWh of electricity a day to provide ~4 kWh of domestic hot water.

Now (November) the ASHP is providing space heating within the house, and in the coldest weather (~0°C) I expect  this will require an additional 15 kWh/day of electricity to provide ~50 kWh/day of heating. Most of this electrical energy will be downloaded at cheap rates overnight.


The PV and battery system has been installed to reduce carbon dioxide emissions from the house. They are the part of a suite of measures to reduce heating demand, eliminate gas use, electrify heating and increase the use of renewable energy.

Together they have dramatically reduced the running costs of the house.

The graph below shows expected household carbon dioxide emissions (not including consumption or travel) over the period up to 2040.

Click image for a larger version. Anticipated household carbon dioxide emissions (not including consumption or travel) over the period up to 2040. The red line shows what would have happened if I had made no changes. The green line shows the expected outcome.

In the short term, all the actions I have taken have made things worse!

The embodied carbon dioxide in the solar panels, insulation, batteries, and heat pump amounts to ~11.5 tonnes, and this ‘debt’ will not be re-paid until the end of 2023.

I would love to add more solar panels, but I am resolved to hold off until my existing carbon dioxide debt is re-paid.

Overall, I hope you can see that the solar panels are central to the plan to reduce anticipated emissions by 60 tonnes by 2040


Back Down to Earth

June 22, 2021

Friends, at the end the last article I wrote:

The combination of 12 solar panels and a Tesla Powerwall battery has been sufficient for us to be practically off-grid for the last 3 months. And that will probably continue for another 3 months. feels astonishing to be sustaining a good quality of life powered entirely by the Sun.

As we approach the summer solstice, I feel like I have reached apogee in a solar-powered rocket, and I am briefly floating weightless.

A week of miserable weather has brought me firmly back down to Earth.

After 87 days drawing no electricity from the grid, as the chart below shows, we have had to re-connect.

Click for a larger version. The graph shows daily electricity drawn from the grid (kWh) since November last year. After the battery installation, this fell to almost zero. Also shown is daily electricity used from the battery and solar panels (kWh). This has risen recently because electricity is now being used for air conditioning, cooking and domestic hot water.

We have now switched the mode of operating the battery so that it charges itself at night using off-peak electricity.

Solar Statistics: Summer Solstice Review

The summer solstice is probably a good point to review the performance of the solar panels installed last November 2020.

The £4200 system consists of 12 Q-Cells Duo BLK-G8 panels tilted at 40°. Six panels facing 25° East of South and six facing 65° West of South. A fuller description can be found here.

Click for a larger version. The graph shows daily solar generation (kWh) versus day of the year along with a 5-day running average. Also shown are two estimates for expected generation (kWh)alongside typical daily consumption.

The last 5 days have seen very poor generation. Last Friday 18th June, generation was just 2.3 kWh – more typical of mid-winter than mid-summer! And a battery with 13.5 kWh capacity is not big enough to see us through this dip.

Click for a larger version. The graph shows cumulative solar generation (kWh) versus day of the year along with a cumulative exports (kWh). Also shown are lines showing the estimated annual and semi-annual generation as specified by the installer.

Total generation so far this year is 1780 kWh – very close to 50% of the installer’s annual estimate.

The system has exported 590 kWh, my benevolent contribution to the grid, and I have used around 1200 kWh saving me around £250 compared to the situation without solar panels and batteries. If the panel’s performance is similar in the second half of the year, this would give a modest 3.5% return on my investment.

Carbon dioxide emissions 

Some fraction of this generation will have displaced gas generation which would have given rise to 0.45 kgCO2 per kWh, and some fraction will have displaced a typical generating mix which would have given rise to roughly 0.2 kgCO2 per kWh.

So depending on the assumptions made, my electricity generation has probably avoided emissions of between 350 kg and 800 kg of carbon dioxide so far this year, and will probably have avoided between 0.7 and 1.6 tonnes of CO2 by the end of the year.

The bigger plan 

The installation last week of the Air Source Heat Pump, a Vaillant Arotherm plus 5 kW model, together with a domestic hot water cylinder, marks the end of my investments in reducing carbon dioxide emissions from the house.

The ‘magic’ of the heat pump is that it uses 1 kWh of electrical energy to extract typically 2 kWh of thermal energy from the air, yielding around 3 kWh of heating.

This is central to reducing my carbon dioxide emissions. It has allowed me to replace the polluting gas boiler.

To compare carbon dioxide emissions with what what would have happened if I had made no changes, I have made a month-by-month estimate of household carbon dioxide emissions over the next 20 years.

These calculations are still preliminary, but the figure below shows their general form. It charts the anticipated carbon dioxide emissions if I had done nothing, alongside the anticipated carbon dioxide emissions in my plan.

Click for a larger graph. This chart shows month-by-month calculations of anticipated household carbon dioxide emissions based my current plan, or the do nothing alternative.

The green line shows an initial rise due to the 10.5 tonnes of carbon dioxide emitted during the manufacture of:

  • External Wall Insulation Boards (1.6 tonnes)
  • External Wall Mortar (1.0 tonnes)
  • Argon Triple Glazing (1.9 tonnes)
  • Solar Panels (1.6 tonnes)
  • Battery (1.4 tonnes)
  • Heat Pump (1.5 tonnes)
  • Air Conditioning (1.5 tonnes)

The green line then shows a much lower slope. The calculations indicate a break-even in terms of carbon dioxide by the end of 2023, and the non-emission of around 60 tonnes of carbon dioxide by 2040 when compared with the ‘do nothing‘ alternative.


It’s disappointing to be back ‘on grid’ for a few days, but overall the solar panels are performing pretty much as anticipated, already avoiding the emissions of hundreds of kilograms of carbon dioxide.

And they are just one part of the plan. The installation of the Air Source Heat Pump is the last part of the plan, and I will now monitor the house to see if my expectations are fulfilled.

Solar Power in Teddington

February 20, 2011


New houses being built on Railway Road, Teddington

New houses being built on Railway Road, Teddington

Some new houses are being built on Railway Road, Teddington, at the back of my home on Church Road. The houses are being built  on a site which used to house garages, which themselves were put there in the place of houses destroyed by bombs in World War II. Searching on line I found this photograph which shows houses around 5 doors down from the bomb/building site. So much for the local history.

What I actually wanted to comment on was the fact these not very luxurious houses ALL have large expanses of solar panels built in.


Solar Panels on the roof of new houses in Teddington

Solar Panels on the roof of new houses in Teddington

Now these panels face west and this is not a superb site for solar panels. But I just wanted to comment on the fact that it is now part of normal building practice to put solar panels on new houses. Fifty years ago, when solar  panels were developed for spacecraft, only a zealot or a visionary would have suggested that this would become the orthodoxy. But it has, and we are living in that future now – where people build solar panels into modest houses in Teddington. And I mention this just to reinforce my faith that things do change. And that the world tomorrow can be different and better than the world today.  And who knows what will become the orthodoxy in 50 years from now?

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