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The following is from a blog by Stuart Elmes, Chief Executive of Viridian Solar

How a Feed in Tariff Loophole is Leaking Carbon Dioxide

The current design of the Feed in Tariff has created a "perverse incentive" to increase financial benefits at the expense of the environment.

The current Feed in Tariff pays 4.5p for each unit of solar electricity you generate but don't use, exporting it to the electricity grid. If you use the electricity yourself, you avoid charges from your electricity supplier nearer to 12p per unit. So, to make the greatest saving from a solar PV system you should try to use as much of your generated electricity as possible, for example by vacuuming the stairs rather than sitting outside and sunning yourself.

Unfortunately, for domestic-scale systems, this exported electricity is not measured on a meter, it is instead assumed to be 50% of whatever electricity is generated. This means that you get your money for export even if you actually export nothing.

Enter a number of companies with "Excess Energy Switching Units" - electronic gizmos to help you use up your electricity generation rather than giving it away for someone else to use. These systems turn on the immersion heater on your hot water cylinder when the PV electricity generation is higher than the electricity use. It's worth your while to use up the electricity to heat water in the hot water cylinder, even if you're saving only 3 or 4p per unit on gas heating - because you're paid as if you were exporting electricity anyway.

So what?

Well, the issue is that the electricity we use is mostly made by burning gas and coal at a relatively low efficiency in a power station. A unit of electricity is not only worth more in pence, it also cost more in carbon dioxide emissions than a unit of gas heating.

If a unit of electricity is exported from your home, it displaces a unit of electricity used somewhere else on the grid, and prevents the emission of 522 grams of carbon dioxide.

If instead, a gas-heated home installs one of these switching unit and heats water instead of exporting that unit of electricity, it is preventing the use of a gas boiler to provide a unit of heat, and saves only 212 grams of carbon dioxide.

Taken over a 25-year lifetime, a typical solar pv installation without a switching unit would save 33% more carbon dioxide and result in the emission of 5.5 tonnes fewer of carbon dioxide gas than a system with a switching unit (see below).


The Maths Bit

Let's take a 2kWp system and assume 850kWh/kWp.yr and 50% export.

If the 50% is exported, it prevents the emission of 850x0.522kg of carbon dioxide, 444kgCO2/year.

If that 50% is instead diverted to an immersion heater, and assuming a boiler efficiency of 80%, then the saving is 850x0.212/0.8 = 225kgCO2/year

(Carbon intensity in kgCO2/kWh are taken from draft SAP 2012)

The PV system with a switching unit has resulted in 220kg of extra carbon dioxide emissions each year compared to a PV system without.

Total emissions saved without switching unit: 444kg x 2 = 888kg

Total emissions saved with switching unit: 444kg + 225kg = 669kg
Increase in emissions benefit from system without switching unit: (888-669)/669 = 33%
Lifetime emissions resulting: 220kg x 25 years = 5.5 tonnes.

This is an interesting angle from a guy who manufactures both ST and PV.

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This is something that I've thought about before.

A good solution would be realistically priced export meters and a decent export rate.


The problem is that whoever dreamt up the tariff was completely incapable of doing the sums you have done, or of spotting all the tricks that people would get up to in order to optimise their financial position. It arises because governments of all political persuasions are composed of lawyers, or guys from public schools who are good at Latin and Greek and who look down their noses at scientists and engineers.

This also explains the demise of British industry because the ruling elite are technically incompetent and are concerned only with short term planning. Contrast that with Germany, Japan, or now China. It is not that UK is poor at innovation, there is no long term support for innovators from financial institutions. The banks are only concerned with making a fast buck for their shareholders and obscene amounts of money for their criminally greedy directors and CEs. They threaten the government with leaving, and the administration cannot see that we would be better without them.
The USA system of net metering makes a lot of sense... Basically what you export gets subtracted off what you import. It can even include time of day pricing so you get credit at the expensive peak daytime rate for what you generate then you can buy more electricity back at cheaper rates. It's basically like using the grid as a big battery.

I do wonder why the UK chose to go with the separate total generation meter along with a normal import meter, perhaps it was the easiest option, or perhaps they just didn't think very hard!!

DECC should fix this by allowing net metering, or by increasing the export payment to a much higher value along with smart meters when they arrive


Solar king: It is often called unexpected consequences of policy. It is only unexpected because the policy makers are stupid. They never model the activities of 'customers' of their policies. It applies just as much to education and the NHS.
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  • #9
Having had time to reflect on Stuart Elmes's article, I am not sure I totally agree with the analysis for two reasons.

Firstly, it does not take account of the de-carbonisation of the grid that will hopefully occur over the next 25 years. I would love to think that by 2037 it will be zero carbon. Whatever happens, if the UK is to meet it's carbon reduction targets for 2020 and those which should follow, then 5.5 tonnes is way too high.

The second is philosophical. We should each be taking responsibility for reducing our own carbon footprint. All of our actions should be geared to this end. Anyone who has participated in the Carbon Conversations programme will know how much changes in individual behaviour and lifestyle can radically reduce carbon emmissions. If you view optimising the use of self generated electricity in this light, you have reduced your own carbon footprint. You have also (by an infinitesimal amount) reduced demand for electricity produced by fossil fuel.

Both the argument in the original post and the one given here have validity. Your take on this will depend on your viewpoint.

Nothing is ever simple.


oil heating of course changes that perpective as well. Then fitting a PV water heater makes total sense.


I have to heat my water in the summer by electricity, my only other choice is oil. It makes absolute sense for me (and others in a similar sitiation) to use our own generated power as efficiently as we can, i.e one of these devices, otherwise I would have to buy in carbon generated electrcity. By using one of these devices, I am reducing the load on the grid, and redistributing my power consumption to generally during times of lower (domestic) usage.

Wether the export was measured or not, wether I got paid to export or not, it just makes good sense.

Bearing in mind that I can gaurantee that the 50% so eagerly agreed by the electrciy comapnies is way of estimate. MOST people are luck if they consume 25% of what they generate, you just can't balance your consumpation manually against your generation. - If they thought 50% was a bad deal, I can ausure you we'd have heard lots of squeals of 'foul play' from them, no they are getting cheap electricity and paying nearer to 2 pence / kWh for it as well (for the older systems) - otherwise why would the not have squealed when it went up to 4.5p?

Do you notice how the elctricity providers have not entered into the FiT price change discussions AT ALL, only reason - they have got to believe that they are on to a good thing - it helps them meet their CO2 redcution targets, and they don't have to do anything for it.

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