It’s been a sunny summer

Friends, it’s the 1st September: the first day of meteorological autumn. So this seems like a good time to look at solar PV generation this summer.

In case you can’t be bothered reading much further – and I would sympathise with you there – the précis is this:

  • It’s been a sunny summer.

Also comparing generation with consumption, I have devised a plan to try to increase the length of time the house is ‘off-grid’ from 4 months, to 6 months!

The Solar Installation

The 12 solar panels (340 W-peak Q-cells DUO BLK-G8) were installed in November 2020 and have been working flawlessly since.

They are installed on the sloping South and Western roofs of Podesta Towers in Teddington.

Click on image for a larger version. The arrangement of the solar cells on the roof of Podesta Towers.

2021 vs 2022

The figure below shows monthly generation for 2021 and 2022

Click on image for a larger version. Monthly generation – expressed as kWh/day – since installation in November 2020.

Looking at data above, it’s clear that (with the exception of April) generation in every month of 2022 has been larger than generation in the equivalent month in 2021

The sunny nature of 2022 also shows up in the cumulative generation data:

Click on image for a larger version. Cumulative generation in kWh throughout and 2021 and 2022. Also shown the are amounts of electricity exported.

Generation to date this year (3,140 kW) is 12% ahead of cumulative generation in 2021 (2,800 kWh). And exports to date (1,004 kWh) have already exceeded exports in the whole of 2021 (880 kWh).

For completeness, I also include the daily generation graph, but the fluctuations in this are so large that it can be difficult to interpret.

Click on image for a larger version. Daily generation in kWh/day for 2022 is shown in green. Also shown is a ±2 day running average from 20202021 and 2022. The yellow data show the expected generation based on the EU -PV sunshine database.

Analysis: Solar as a fraction of demand

The three charts below are not based upon the solar year – January to December – but the heating year July to June. Somehow this seemed more natural.

The first chart shows our typical demand for electricity through the year – an average of 9.8 kWh/day over the period July 2021 to June 2022.

Also shown (in darker green)  is the electricity used by the heat pump for space heating. This peaked in January 2022 at around 15 kWh/day making a peak demand of 25 kWh/day.

Click on image for a larger version. Average Daily electricity demand in kWh/day shown from July 2021 to June 2022. The light green section of the bars shows normally daily demand (9.8 kWh/day) and the dark green section shows electricity used by the heat pump for space heating.

The second chart shows the daily solar generation from 2021/2022. It’s clear that solar generation is irritatingly – but obviously – out-of-phase with demand.

Click on image for a larger version. Average Daily generation in kWh/day shown from July 2021 to June 2022.

The final chart shows the ratio of the two charts above showing the fraction of average demand that is met by average solar generation. This final chart is interesting.

Click on image for a larger version. The ratio of average generation to average electricity demand through the year.

First of all let’s note that this is based on just one year’s data and year-to-year variability is typically 10%. But this data shows that there are 4 months of the year (May, June, July and August) where average solar generation is able to meet average demand with more than 10% margin. And indeed with the aid of our battery, we have been off-grid for most of that time this year.

But the graph also shows that there are two more months – April and September – where average solar generation is able to meet average demand, but with a margin of less than 10%.

This means that if I could increase solar generation by even a relatively small amount, it might be possible (if the fluctuations are not too large) to take the house off-grid for a full 6 months of the year. Wow! I am getting excited at the very thought of this!


And friends that is my plan. I have asked a solar installer to add an additional 9 panels onto our array: 5 panels on the roof facing 25 °N or East and 4 on the flat roof nominal facing 25° east of south.

This addition takes the array over the 4 kW-peak installation that can be done without notifying the electricity distribution company, but the installer has told me the application is already submitted.

My hope is that over a year, the additional 9 panels will add ~1,500 kWh (167 kWh/panel) to the ~3,600 kWh (300 kWh/panel) generated by the existing 12 panels. This should be enough to raise generation in April and September above demand, and hopefully allow us to stay off grid for a whole half of a year.

Click on image for a larger version. The location of the panels in Phase#2 of the Podesta Solar Array are shown in red.

These orientations aren’t the best, but actually they are not terrible! And generating over 1 MWh per year is not negligible!

Of course, I still don’t have a date, or even an expectation of a date for doing this work.  But hopefully the panels and inverters will eventually make themselves available in the first few months of next year – hopefully before April!

Note on Embodied Carbon

I am able to afford this because although my pension lump sum is all spent, living modestly and not having to pay big bills, I have been able to save enough of my monthly pension to buy the extra panels.

And having made rough estimates of what is done with my savings, I think the best thing I can do with any resource I have available is to spend it on things that reduce carbon emissions. And there are only one or two things out there that have better ‘carbon value’ than solar panels.

Anyway: That’s the plan…

5 Responses to “It’s been a sunny summer”

  1. Ross Mason Says:

    The roof at the bottom (south) looks to me to have space for 3 more panels. And maybe one more on the S end of the west roof. Is that worth while given the orientation?

    • protonsforbreakfast Says:

      Good Evening, Ross,

      Yes, there is indeed room for extra panels on these roofs, but…

      1. Each panel generates around 40 V and are typically connected in strings of up to 6 panels which have similar illumination.
      2. It would be possible to ‘patch in’ a panel or two but my wife thinks that it would look ugly.

      We are where we are…

      As we head into winter, your solar season must be just kicking off! Good luck!


  2. Ross Mason Says:

    Ah yes. I recall a discussion about the beauty of installing floor mounted Heat Pumps. Sigh…..

  3. cclambie Says:

    Sunny indeed.
    Is certainly a tale of Long duration storage… your extra panels are going to make your mid summer Gen very high… so have you thought about ways to store the power long term in your house somehow?
    Also love to know your thoughts on this innovation:

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