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Back in June David started a thread to discuss 'export to grid' v. energy usage and I mentioned then that a Smart Meter was providing me with solar ‘Export to grid’ figures, which have been recorded daily along with generation figures. Shown here is the whole set of seasonal solar generation daily averages covering both the highest and lowest generation periods of 2013 here, from July 4 to December 24.

Note that the 4kW system does NOT face the usual south, and hence only generates 3mW/year. But the following conclusions are of general applicability. (N.B. attention throughout has been paid to scheduling the washing machine etc for sunny times, albeit without realtime generation monitoring guidance.)

Export averaged 67% overall. That's a lot more than expected. Generation was still mostly being exported in the autumn, despite it being a notably poor September here. The split East-West system with totally negligable winter generation does exaggerate the seasonal pattern of export, and there's no “proportionate controller” to soak up generation for immersion switching here. But on the other hand the statistics do not include the high export days from summer solstice to July 3, and autumn afternoon generation was suppressed by tree-shading on the West array.

Conclusions from this lengthy data gathering exercise are; 

A Smart Meter is a really invaluable datasource and probably the simplest for knowing the split between how much electricity is exported to the grid on the one hand, and how much is used and hence saved by having solar PV (generation - export = usage/saving). Know your grid export figures - it really is as useful as knowing your grid consumption. (Nor does having a Smart Meter mean that you have to go onto Export metering instead of ‘deemed 50% export’.)
Experience in autumn and since was that it is a heck of a lot easier said than done to minimise export and import, and ‘use the solar yourself’. For spring, the intention is to use the morning weather forecast with a plug scheduler to solar-power (hopefully) a 700W storage heater, but the suspicion is that an accurate automated proportionate controller may ultimately be needed for it.
We are very probably all exporting a lot more than we thought - or anyone else probably thought. I find it difficult to imagine a typical south-orientated 4kW system without a proportionate controller exporting less than 50%, unless daytime consumption is particularly high.
This household's behaviour hasn't changed. Export metering instead of the ‘deemed 50% export’ has even been chosen, and still behaviour hasn't changed. At only 3p/kWh for the solar usage it's still the cheapest household energy to use. Opportunities to ‘use the solar’ are nowadays being sought more than ever in fact.
The crucial usefullness of realtime generation monitoring is now apparent!
What proportion of south-facing solar households do NOT have a proportionate controller for hot water immersion switching? In hindsight it's a pity that their solar PV installations did not have them fitted ‘as standard’, but the potential electricians' market amongst the UK's half-million solar households must be huge. Is there no speadsheet ‘payback calculator’ for immersion switching out there somewhere? And it's quite beyond me why such an effective energy efficiency appliance was not included in even the Green Deal (especially with the UK 3 years behind schedule in it's modest-only renewable energy deployments and as a result having to import yet more coal and at steadily higher prices).


Even on paltry 40kWh generation December's Export was 13%. We'd assiduously used sunny spells for the washing, etc., I'd previously bought us a Superkettle to soak up solar power, and the Good Lady had even done some hoovering, bless.

Only between Dec 21 to 24 was nothing at all exported. Basically, zero export only occured on days of generation below 1.4kWh! Although electric usage here is relatively low due to cooking on gas, this doesn't really affect the ‘big picture’ of the previous post.


None of this surprises me. Unlike filling a water butt, electricity only has value at the time it is used without expensive batteries (and lots of waste even then).

During the week (5 days out of 7 most weeks) we are out all day, so apart from leaving the washing machine on (often before the system has started generating) we use very little electricity while it is being generated and a lot when it is not.

A proportional controller is only worth the money if you have something you want to use (filling the hot water tank just to “leverage” the system isn't worth it unless it's being used).

So, all I can say is roll on the Smart Meter rollout programme so that we can be paid for as much as we really export.


But be aware that you will need to ask the nice Smart Meter fitter to enable the display of Export recording. It won't occur to the meter fitter that in the case of a PV site he'll otherwise be supplying his company with your export data but not the actual data donor! (It will indeed be recording export regardless of whether you ask for it on the display or not.)

I had to retrospectively ask for mine, post-fitting. (I'd had to get the Smart Meter for a split-rate tariff that I wanted, export recording was just an afterthought, and it hence it was only accidentally that I was drawn down this ‘export recording’ route.) Over a period of many weeks this took some badgering on the part of my ‘leccy company to the metering company. But I got home one day to find that the meter had received a remotely downloaded reconfiguration - and I found that it had been export recording all the time since fitting.

Limited electricity saving is of course the other side of the coin to significant export. Generation - export = grid saving. Having metered export-tariff payments of 3p/kWh or the modern 4.64p/kWh is neither here nor there compared to a grid electricity price of 13p/kWh, so energy-saving is still ’where the money is' on the import-export trade off. Hence my original point 4 - our household behaviour hasn't changed.

FiT tariff is nowadays only 14.9p/kWh, and systems are nowadays often sold on optimistically quoted electricity savings.

What I fear is that plenty of our Microgen data donors who've joined over the past year may have bought their systems partly on the basis of all-too-common quoted grid electricity savings of, for example, (£293)2250kWh/year on 3000kWh/year generation (i.e. only 25% export).

More likely is a (£176)1350kWh/year grid saving from a 3000kWh/year system.

The difference between the two is the difference between a quoted 6-year payback period and a more likely achievable 7-year payback - not a trivial difference. (I'm using Dennis' ‘PV Payback’ spreadsheet calculator to crunch these numbers, on a £5250 installation cost.)

The message that I'd hope our newer Microgen data donors here now have is simply this. Get your Smart Meter asap WITH IT'S EXPORT RECORDING ENABLED ON THE DISPLAY, record your Export as well as your Generation, derive from it your actual electricity savings. i.e. Check the facts yourself. If nothing else, it may show the simple explanation for why your ‘leccy bill hasn’t come down by as much as you'd expected.

Sorry to be the bearer of bad news for someone or other, but forewarned is forearmed and all that. As you say, roll on the Smart Meters rollout.


My modest contribution to the debare:

We have a nominal 4kW system in the Scottish Borders. Installed May 2010.

Although we do not have a smart meter our suppler had the network (SP Energy networks) install an export meter when it went live. So we have import, export and generation readings on weekly basis since the MCS certificate was issused (july 2010)

In 2013 we actually generated 3259 kW -this was a poor year it has ben more like 3500kW. 

Our peak generation is April, May June ~1000kW generated.

We consistently export an annual average of 60% of the generated electricty.

FiT and export payments are based on actual meter readings - so atleast we are getting paid for all we export. (For info our electricty supplier is Ecotricity)

We are self employed/work from home.

oil central heating and wood burning stove everything else electric.

Our near neighbours have a similar size system with a smart meter supplied by Scottish Gas and get almost identical readings and out turn figures to us. (Scottish Gas say they are not allowed to use the export readings from their own smart meter !)

Fianlly all our figures are provided to Jamie as an excel speadsheet monthly.


Presumably, your 4kW is south-facing? Is your neighbours? Neither south-facing DaveCBoA, CAB or Keith have a ‘Public Profile’ for me to message directly, so I can only ask here; what are your percentages?

In the summer thread, probably only those of us who participated in it were aware of it's implications. Only 40% used solar and 60% export when facing south on 3500kWh/year and with relatively high electricity usage overall is the sort of data that really is of relevance to a lot of people.

Keep these stats coming, folks.


I see that Morrisok's 4kW facing south has also exported 67% and used only 33% between July and December on about 4300kWh/year. Also interesting …


Hi Roger,

I have only just seen your request for export data. I only ever think about visiting the Solar Farm Forum when I get an email reminder for readings or a report, hence my lack of response.    

I don't mind sharing my export data. My system has been installed for about 2 and half years now.

I record both my consumption and generation to PVOutput but my consumption data is hidden so you would not be able to view it. So here is an annotated screen capture of my monthly export. For those of you not familiar with PVOutput the E vs G column shows the percentage of export against generation.

When I first had Solar PV installed I attempted to employ a white goods utilisation system (I work from home), switching in loads of washing, tumble drying, immersion, air conditioning etc. The best I every managed was to get my export down to 16% in February 2012. I soon realised that even if you are at home all day, and have the time to constantly watch your generation and house load monitors, trying to mop up generation manually is far too time consuming and a lost cause in the more productive months of the year.

In an attempt to reduce export and benefit from my generation, and after some research I installed an i2 Intelligent Immersion proportional controller in May 2012 (See this thread). This significantly helped to reduce export and produces enough hot water from April to the end of October, with the occasional top up by Gas CH if we get two or three days of poor weather. In the early days of having the i2, as my children were away at University I would switch the i2 off when I went on holiday which bumped up the export percentage in those months.

Even with the I2 installed I could still export 20% or more of my generation in summer months, as the immersion would trip out when it got up to temperature and commence exporting. To this end I put together a pumping solution (see this thread) that stops the immersion tripping out by bleeding hot water from the top of the tank to the bottom, always keeping the immersion just below its cut off point. Last year my total export was 199kw on a generation of 3,733kw which is around 5%. This has been a great success and has saved me around £100 in Gas consumption in the last twelve months. 

In an attempt to get the maximum savings and further minimse export, last month I sent my i2 back to the manufacturer for hardware and software modifications to improve performance still further (my i2 was an early unit). So far in March this has reduced the total export figure down to just 0.62% in the first 18 days.

Regards Keith


A couple of questions, Keith:

(i) The saving on gas is impressive but do you actually use all that hot water? 

(ii) Are you saying that the immersion is just below its thermostatic cutoff all the time just because of the pumping system?  I'm not sure I understand how this could happen if you are producing 20+ kWh/day (I've had 4 days at that level this month).

My immersion seems to only require 2-3 kWh to top up most days using a Solar iBoost.  Admittedly, I do have the boiler heat water for 1/2 hour in the evening, but will turn that off next month probably.


URN 215 North Yorkshire 3.96 kWp, S facing, 40 deg tilt, minor shading


Hi Steve,

"(i) The saving on gas is impressive but do you actually use all that hot water?"

Yes we do, demand for hot water is high in our household with four adults at home. 

“(ii) Are you saying that the immersion is just below its thermostatic cutoff all the time just because of the pumping system?  I'm not sure I understand how this could happen if you are producing 20+ kWh/day (I've had 4 days at that level this month).”

I wrote a detailed post about the pumping solution last year (Here) post date 03/07/2014

If like me you have Gas central heating with a hot water only option, then a 20 to 30 blast in a morning will heat you a full tank up to at least 55 degrees. In our household this would be sufficient for the morning rush, though it would more than likely need another blast in the evening for pot washing, and the evenings water based activities.

When I had Solar PV installed and discovered proportional immersion controllers, what I wanted to achieve was to have enough hot water at all times as I did with Gas water heating, without having to fire up the boiler once or twice a day for 30 minutes. I knew how much Gas was used in summer (when the heating was off) which was around 21kw per day. Though, at that time I used to have the water heating on for much longer than 30 minutes in a Morning and evening.

With an immersion heater like mine (top mounted) how much water you can heat and the Kw’s used to heat that water, depends on a number of variables. These are the diameter of your tank, if your immersion is top mounting, the length of your Immersion heater, and the starting temperature of the water.  My tank is only 18" wide but is 2 meters tall, holds over 220 liters, and my immersion is 36 inches in length.

Originally (forget pumping solutions for now) if my tank was cold (circa 18C) it took about 7-8kw of solar generation above base load for the Immersion to reach cut off point, which is set to 65C. When cut off is reached only the top third of the tank was hot, the other 2 thirds of the tank was still cold (I have four temperature displays mounted up the length of the tank). Since this is only enough water for a single bath at best, there was not enough hot water for morning, never mind trying to satisfy demands all day long. Therefore, I embarked on a quest to design a solution that would provide as much hot water as was needed, without having to use any Gas if possible. 

It soon became clear that a number of issues needed to be addressed to achieve this. First and foremost I had to produce more hot water than was just was contained in the top third of the tank. This is how the pumping solution came about. This took a while to get right and I had to go through testing a number of thermostats and configurations over quite a few months to get the system working correctly. It’s pretty easy if you don't watch out, to pump all your hot water from the top to of the tank to the bottom, and end up with a luke warm tank overall, at which point the sun goes in. Of course if this happens you have not lost the heat, but you do have to fire up the boiler to get the tank upto a useable temperature.

The other main issue to consider is that peak hot water demand can be early morning, when PV generation is low at best, or not possible due to the time of year. Therefore you need to carry over some hot water from the previous day, or modify your bathing times. To achieve this you need a very well insulated tank and piping. My tank is 20 years old plain copper with no foam insulation, with just a standard red tank jacket, and with no pipes insulated in the airing cupboard. To give you an idea if at midnight the temperature at the top off the tank was 55C by 8am the next morning it would be under 40C which is not hot enough for bathing without using the entire tank. Therefore last autumn I embarked on a major insulation project of the tank and pipes, which has reduced the 8 hour overnight loss to about 3C which is a lot better.

Therefore, in a morning most days there is usually sufficient hot water to meet demand. After which much of the tank may be at ambient temperature, with just some heat remaining at the top. Throughout the day (assuming the sun is shinning) the tank recharges slowly as the day progresses, 65C hot water is pumped from the top to the bottom of the tank, never letting the top of the tank go below 62C. As I explained above, a third of a tank takes about 7kw of generation, the other 2 thirds take another 14 kw, so even on a 20kw day i do not export. On only two occasions in July last year when I was out, when generation was over 28kw did I export a little. 

Even in winter we never have to put the water heating on every day in the morning or evening, maybe twice or sometimes three times a week for 30 minutes only once a day. Over the last fortnight in Sheffield where I live, the generation has been very good and no Gas water heating has taken place at all. All our water heating has been met by Solar Generation.

Of course in winter it is more difficult to achieve heating sufficient hot water than in summer, though there are plenty of days that reach 7kw or above. I suggest that if your system is tripping out after 2-3kw you need to knock of the Gas heating, consider a pumping solution, and in winter bath later in the day to suit the daily generation pattern. At our house I showed my adult son and daughter the four temperature gauges on the tank and explained what temperature they needed to read to get a warm bath or shower. There was moaning to begin with in winter, but now I hear them opening the airing cupboard door to check if the water is hot enough, and if it isn't they just wait a while and bath later in the day.

For those of you reading this I don't want you to think that my objective is to export nothing and to heat hot water at the expense of all else. In fact we do not actually ever think about the proportional controller and the pumping solution, it just works automatically, and 9 times out of 10 there is sufficient hot water all the time, if not we burn some Gas. We schedule our washing and ironing for sunnier days, and in the summer when it is hot, we are able to run our air conditioners also, and still generate sufficient hot water. We just get on with life and save some cash along the way.

regards Keith

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