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I was just reading rogerhowards smartmeter piece in the newsletter which I thought was really interesting. I tried to make  sense of the question of what that exported power would be capable of ahieving elsewhere, like in your neighbours non PV-household. Of course this kind of calculation is practically impossible to make, but it looks like your april to sept export averages somewhere in the region of my daily electricity consumption - on a back of envelope kind of calculation. Of course this is so much more complicated than this and I'm not even going to try to estimate transmission losses or anything like that, but I am curious about whether you know a pattern to this? Is there like a reasonably stable average export between certain hours or is it all over the place? Or looking at it the other way round, I know you say that you haven't changed behaviour, but is there a pattern to when you manage to use those 33% (a certain time of day I'm thinking)?

Thanks again for the piece, it set me off on a lot of thinking :-)


Hi Britta, this is a difficult one to answer. The major issue with renewables in general is the mismatch between supply and demand, and on a domestic scale this is no different. Averaging out over a whole village/town will lessen this effect, but only if the installed renewables peak power is small compared to the consumption.

Our household consumption has been logged by an OWL meter for almost three years now. It mostly behaves like a steady ‘base’ load, with consumption bursts caused by high-powered applicances superimposed. One of the observations is that the base load of the household is reasonably predictable, but also rather small compared to the heavy hitters in the household. Moreover the base load is highest in the winter, and in the evening, when solar power generation is lowest.

Ranking our high powered appliances: the electric shower (8kW), the kettle, immersion heater (3kW each), washing machine, dishwasher (2.2kW each), iron (2kW), vacuum cleaner, microwave oven (1.5kW each). The spikes in household consumption caused by switching on any of these are inherently hard to predict. Add to this the fact that renewables usually have low output power over long timescales whereas consumption occurs in high power bursts, and the mismatch is complete.

By noting that the top-5 of high power appliances are basically water heaters, it is time to get on my hobby horse again.  An immersion heater proportional controller can mitigate some of the consumption spikes. The best example is our dishwasher: changing its supply from cold to solar heated hot water reduced its electricity consumption from 1.6 to 0.5 kWh per cycle.

The price for avoiding 1.1kWh of consumption (in the evening!) is that the controller has to feed ~1.5kWh into the hot water tank to compensate, but it has got the whole next day to achieve it. In a sense, hot water now acts as a storage medium for electricity! Similarly, I expect to reduce consumption of the washing machine by feeding it from a 30 deg C thermostatic mixing valve.

As far as I can see, using solar hot water is the only cost-effective way of achieving a better production-consumption match on a household scale. And I'm not even touching on reduction of using non-renewables for water heating, but I must not get carried away here!

Hopefully this answers some of your question(s). Regards Bart


Bart, thank you, that was really interesting. I'll try and not ask too many questions… but can you walk me through what you did to get the dishwasher to use your solar heated water?

The mismatch in supply/demand is a really interesting challenge. But what I am also wondering about is how to consider the impact of PV outside the household / in a local area. You often hear people talk about export as if it was waste, or assuming the the quantities we are talking about are largely insignificant. But on the other hand some PV owners also really like the idea of ‘their’ electricity being used elsewhere.

Of course you can't quite measure this… (well, I certainly have no idea how you would), but i thought that maybe if it was possible to establish a pattern for when how much is being - let's say ‘released’ for consumption elsewhere, that might be a beginning?



Hi Britta. DECC publish a quarterly Sub-regional Feed-in Tariffs statistics spreadsheet, which is analysed down to Local Authority level - but that's of generation not export. But I'm not aware of statistics on specifically export or at neighbourhood level, other than what those of us with Export data might put onto ‘PVOutput’.

You've got me thinking now about showing my neighbours' my own PVOutput export data . Is that the ‘impact’ that you meant though?


Well, if PV could play a part in people talking more to their neighbours all over the country, then that would put a whole new meaning on the idea of ‘added benefit’ .

I suppose I'm partly being silly, questioning something that it is probably impossible to establish. But I'm just curious about how far this stuff travels. So imagine if I could put a special dye on solar electricity and see where it goes, what it ends up powering: if there is enough of power at any point in time to be contributing considerably to your neighbours baseloads or if it all just gets 'lost in transmission. So what I initially thought was along the lines of what Bart talks about as mismatch between supply and demand - how that looks if you follow the electricity beyond the point of the export meter?

(The conondrum relates a bit to the question of carbon for me. You know how your monitor tells you x amount of carbon emissions avoided. But how does the inverter know that the added generation of the PV installation has actually translated into reduced output at a power station somewhere? Obviosuly the generation will have to replace generation somewhere else before anything is avoided?!)



I agree with you that it would be good if we had the info to talk to our local folks about such things. People's understanding about paying your neighbour (i.e. us) instead of paying for more foreign coal, gas, etc might be quite a powerful focus on what renewables are all about.

My limited understanding of the pushing-pulling of AC power is extremely unreliable. But I don't think our export is pushed out at 24?volts and pulled in at slightly lower voltage very far at all - just my neighbours I'm assuming. Our technical forumites will probably put me right on this, though.

Carbon “Footprint” calculators would count your export as a form of Carbon Offsetting. (But do you know how much is your export?)


An interesting characterisation there as energy storage for the washing machine.

Going a step further, what would you chaps with immersion switching think of using the approach to feed, not a Hot Water Cylinder, but a Thermal Store? What might be the advantages and disadvantages compared to a Hot Water Cylinder?

I've no idea whether they require more or less energy (all other things being equal) than a Hot Water Cylinder. But a local solar PV householder here that I mentioned immersion switching to is thinking of using it for his Thermal Store.


One day, I hope to have an Ultimate Heating Hot Water system for the home, with a Thermal Store at the heart of it.

Thermal stores are better in every respect compared to hot water cylinders except they are 2-3 times more expensive. I am not sure why, as they are largely identical. It might just be due to the low sales volume.

I think there are plenty of websites articulating the advantages of a thermal store over a hot water cylinder much better than I ever could. Navitron is a good starting point

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