Perfect Solar Days

Friends, March may have been depressingly dull, but the start of April has been upliftingly sunny. And Tuesday April 4th was a perfect solar day.

Perfect solar days are days in which the sun shines unremittingly from dawn until dusk, with not a cloud in the sky: such days are rare: Typically there are around 10 per year and I try record the solar PV generation from each one. I do this using data from the Tesla App which records solar generation every 5 minutes via a current transformer (CT) clamp on the cable from the inverters.

The graph below shows 12 perfect (or near perfect) solar days from 2021 and 2022. The days are not evenly spaced because I can’t control the weather!

Click on image for a larger versions. Graph showing the daily hour-by-hour generation for a series of near-perfect solar days in 2021 and 2022.

Attentive readers will recall that in November 2022 I had eight extra solar panels installed on the east-facing roofs of Podesta Towers (link). I had expected that these would contribute nothing to solar generation in winter, but that generation would be significant in spring, summer and autumn.

The graphs below show the generation profile for each day for which there is reasonably well-corresponding day from before the new panels were installed. As expected the new generation is very weak in December and January, but by April has grown significantly.

As expected the extra generation from the panels on the North and East is in the morning. Happily, the maximum power looks like it will peak in summer at less than 5 kW – the maximum rating of the cable between the inverters and the consumer unit.

Click on image for a larger versions. Graph showing the daily hour-by-hour generation for a series of near-perfect solar days in 2021 and 2022.


The aim of the installing the new panels was to create extra generation in the spring and summer which would hopefully be enough to keep the house off-grid for 6 months. This goal has been hampered by the dullness of March, but things seem to be returning to normal now. However the weather is still cold enough to require the heat pump to stay running for another month or so, increasing domestic consumption.

The graphs below all have a vertical axis of kWh/day and the data are smoothed to highlight trends: They compare:

  • Domestic consumption and solar generation. One can see that solar generation is rising to be roughly equal to domestic consumption and should soon exceed it.
  • Domestic consumption and Grid Consumption. In winter, the two are practically equal, but in spring, grid use is falling as we rely more on solar energy.
  • Net Grid Consumption/Export and solar generation. As solar generation rises  we have become net exporters of electricity.

Click on image for a larger versions. Graphs variation of Daily Consumption, Daily Grid Consumption, Net Grid Consumption and Solar PV generation, all expressed in units of kW/day.



9 Responses to “Perfect Solar Days”

  1. David cawkwell Says:

    Weather in Portugal has turned nicely temperatures climbing into high 20s all last week. I had no idea there are so few cloudless days in the UK. My panels are now producing so much power I have no use for it. Time to build a hot tub!

    • protonsforbreakfast Says:

      A hot tub! Brilliant. Insulate it well and you can have an inter-seasonal heat storage device!

  2. Wintergreen Says:

    Thank you for your excellent blog. To a lapsed engineer like myself, the desire for knowledge and scientific rigour is a delight. I am looking to shake off a debilitating gas habit, and was wondering whether you had considered the merits of a dedicated heat pump cylinder as a DHW source. The issue is horribly off topic, so I am reluctant to write more here!

    • protonsforbreakfast Says:

      Dear Wintergreen, good evening.

      I am not sure what you mean. We have a 200 litre cylinder for DHW which is heated by the heat pump. Is that what you meant? Or did you have something else in mind?

      All the best


      • Wintergreen Says:

        Hi Michael. I wasn’t clear. I want to replace my gas boiler with PV/ASHP, and am pondering the most efficient arrangement. We live in a tall Victorian house, with a loft room at the top, which is uselessly warm in summer, and never really cold in winter. I want to know whether it would be more efficient to install a dedicated HP cylinder there, and leave the external ASHP (which might then be a little smaller) to cover the central heating.

        There is an obvious design advantage: the dedicated cylinder would use heat from the loft room, where it is substantially useless. There will be a small knock-on increase in heat loss for the building as a whole, but I can’t believe it will be very significant.

        This is the sort of thing I have in mind:

        I have found the COP curves, but am unable to copy them here. At 15 degrees ambient, COP for 55 degree flow is 3.5. At 10 degrees it is 3.3. Those figures would be a bit higher for for 50 degrees.

        There is a video of an equivalent Daikin product which indicates typical consumpton of 800-1200kwh per year –

        According to my researches, it seems unlikely that an external ASHP heating an internal unvented cylinder could match those numbers, particularly in the depths of winter. Similarly, this sort of arrangement would surely be more efficient than any of the PV/immersion setups that are widely acclaimed and available. These cylinders are not particularly costly either, but they seem to have no traction in the UK market. Am I missing something?

      • protonsforbreakfast Says:

        Dear Wintergreen, Good Afternoon. I trust you are enjoying the Easter sunshine.

        Yes, These standalone heat pump-storage cylinder combinations do seem very technically sweet. And I considered then for my own installation a couple of years ago. The fact that they do not seem to have much traction in the UK is probably due to the fact that if you are installing an ASHP it’s just simpler to let it do both jobs (DHW and space heating). They would be very well-matched with air-to-air heat pumps (aka air conditioning) which does not have the possibility of heating water.

        In the US these are very popular items, typically located in garages, they replace simple insulated cylinders heated by an immersion heater.

        But your description of your property and the plans you have in mind have triggered my ‘spidey-senses’ that you may not have understood how these units work. Please forgive me if this incorrect. But the mistake has been made by many others including very well-known people.

        I get the sense that you are imagining that the unit will draw heat from inside the house where it is warm. This would be a bad idea: the thermal energy that it harvested from the air would then cool your house and require to you to replace that thermal energy with energy from teh rest of the house – which would then be replaced by your main ASHP!

        In fact if you take a look at the installation manual

        Click to access 0020285063-00-arostor-200-270ltr-install-1456700.pdf

        you can see that it is designed to draw in air (using ducts) from either the outside of the building or from an unheated loft space insulated from the main part of your dwelling. The chilled air exhaust is then sent directly outside again using ducting.

        So you can see that actually in winter, it will be drawing in air at – roughly – the same temperature as the main ASHP, so there is likely to be no COP advantage. This article has some COP data for DHW heating only in October.

        That said there may be some advantages to using such a cylinder. The plumbing-in may be simpler (I don’t know) and your main ASHP won’t need to switch modes for an hour each day (for low water use) or more (for high water use).

        Do get back to me if I have still failed to understand your installation.

        Best wishes


  3. Wintergreen Says:

    Hi Michael, I had understood the operation of the cylinder correctly, hence my comment that “there will be a small knock-on increase in heat loss for the building as a whole, but I can’t believe it will be very significant.” I would put this in a loft room where there are some useless heat sources – batteries, pipes from the ASHP, valves, pumps, inverter – in addition, there is heat that migrates up the building in any event. I realise that in drawing on some of that, I am robbing Peter to pay Paul, but I can’t imagine it is significant as against the thermal mass of the building (500m2) and losses elsewhere. If it turned out to be a problem, I could arrange a hybrid whereby some of the air intake came from outside.

    More generally, I am surprised at the enthusiasm for diverting PV solar electricity into immersion heaters cylinder (COP < 1) when this sort of device would represent a far more efficient destination for those electrons (/holes).

    • protonsforbreakfast Says:

      Dear, Wintergreen, good morning. OK, so I have understood your idea correctly. I would discourage you strongly from doing this. Why?

      When operating, the heat pump will heat water with (I guess) around 3 kW of heating power. To do this it has to extract around 2 kW of heat from the air within your house. This corresponds – roughly – to cooling around 1 cubic metre of air per second by ~3 °C. My guess is that this will quickly cool down your loft room to below 10 °C and raise the spectre of condensation and mould growth. This is aside from the fact that ‘coolth’ will spread into the upper rooms of your home.

      Also, you describe the room as having heat sources, but if there are things like batteries there, these will be adversely affected by cold, and will definitely not function well in a condensing environment.

      I would suggest you try to characterise numerically the thermal conduction to the environment outside and the inside of the house. Unfortunately, I don’t quite know how to do that (!).

      Regarding PV diverters, their prevalence is a historical anomaly. They were a cheap alternative to using excess solar energy before batteries were available. The COP is exactly 1 as for all resistive heating. But using excessive PV (20%) efficiency to run a heat pump (COP ~3) gives 60% overall thermal efficiency – the same as a solar water heater.

      Best wishes


  4. Wintergreen Says:

    Thanks once again for your careful thoughts on this proposal. A couple of points I think it’s worth coming back on.

    First, I don’t think there would be any condensing in the environment. The device operates as a dehumidifier, and the is an exhaust dribble of water to the outside (like a condensing boiler).

    Second, the critical issue is the amount of “coolth” produced (good word!). These units operate at 400w. I am confident that there will be plenty of spare heat on my top floor for much of the year (in summer, the cooling effect would be welcome). If it becomes too much in the depths of winter, I could simply alter the devices intake so it drew more (or possible all) air from outside. This sort of divertible ducting would be easy to accomplish, and the only consequence would be a reduction in COP. But given they manage a COP of over 3 at 2 degrees ambient, it would still likely be outperforming an external ASHP/internal cylinder arrangement. Moreover, in the summer the COP would be up at 4, for 55 degree hot water.

    Thanks again,

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