The family feline thrived in its new-found economic role

by Steven Harris

Part 6

I mentioned in my last post that under government’s new Green Deal programme, your house might gain a financial identity that could outlast your brief stay with it. But what does this mean?
The principle of the Green Deal (or ‘pay as you save’, as it used to be called) is that the cost of repayments on the loan you took out to make an improvement will be less than the cost of the fuel that you would have been paying for had you not made the improvement. This principle is now what is being called the Golden Rule.

For some technologies this is really easy. When I first wrote about this topic (way back in 2007, for The ZED book) it all seemed clear. Take out a loan for (say) £12k for a solar PV system: the monthly repayments on a 25-year loan would be around £60, and the system would generate £42-worth of electric (pre-Feed-in Tariff, remember). By 2012 (I guessed) the price of fuel would have gone up so much (I got called radical for saying it would be at 8% a year) that you would now be generating £60 worth of electricity a month and so paying the loan. In every subsequent year you would start making a profit. Fantastic!

Then in 2009 came the Feed-in Tariff, at a very generous 41p per kWhr. Suddenly, even in the first year your PV system will be generating £110 worth of electric a month for your £60-a-month loan repayment.

So solar electric (PV) is easy. Put a panel in the sun – it generates electricity. Connect a system up to your house, and when you’re not using the power it will squirt back down the wires into your local grid. Whatever, there will be a use for the power and you can be pretty sure that every year you will be able to harvest roughly the same predictable amount.

The problems only crop up when you start to look at heat. You can’t squirt heat back down wires – so you have to either use it or dump it. This means that, at least until such time as every smart home is equipped with a heat storage facility, solar thermal systems are only as useful as your demand. Until we get this in place, if asked: “How do I make my solar thermal system more efficient?” one correct answer would be – have more children. Or, take more baths when it has been a sunny day.

House: Would you like to take a bath?
You: erm? No thanks. Are you saying I smell?
House: Oh go on! It’s been a lovely day!
You: Oh I see. You’ve got a tank full of hot water you’re trying to palm off on me, haven’t you, just so you can prove how efficient you are to your friends on the internet.

With the Green Deal in place this goes one step further: that bath might be the difference between meeting the Golden Rule or not. Remember last week we talked about not interfering with your home’s mission? Well, think of this. If you don’t make use of those kWhrs, and you decide instead to take a bath after a dull day with no solar heat, you might not reduce your fuel bill by enough to make the loan repayments on the solar thermal system.


House: No, you have to. I’m a little short this month and I need to balance the books.

Here’s an analogy. Walls, windows and roofs all allow heat to escape through them. The rate at which heat moves through them is called a ‘U’ value. The smaller the U’ value, the slower the heat can move. The units for ‘U’ values are Watts (of heat) per square meter (of wall/window/roof area) per degree Celsius (which, to be correct and scientific, is the same as degrees Kelvin). So the units for ‘U’ value are written as W/m2/k. If a 1m2 window has a U value of 1 (which a good window will have), this means it will allow one Watt of heat through for every degree of temperature difference there is between one side and the other.

Using my favourite unit of heat generation (and giving Katy the Editor the chance to use another cat picture), a cat gives off 16W of heat! (For comparison, a man gives off 109W and a woman 92W.)
If a window has a U value of 1W/m2/k, and that window is 1m2, then the body heat of a single cat would balance the heat lost through that window until the temperature outside was 16ºC colder than inside. If a room temperature of 20ºC was required, this would mean the cat could compensate for an external temperature down to 4ºC. Below this, you’d need additional space heating – or another cat.

This is a bit like the advice for solar thermal, about having more children to make your system more efficient, isn’t it? If you have a lot of cats, you’ll be spending less on heating due to all that body heat they are giving off (though your cat food bill will go up).

So in short (yes, really) if you add insulation to a house, you can’t be sure of your exact energy saving. Variables such as number of cats, children, parties (remember your clever fridge’s party mode) or what temperature you heat your house to will all affect the payback calculation.

When we studied the effects of cavity wall insulation on the energy consumption of people’s homes, we found that energy consumption did not drop as much as the science expected. This was because, before having the insulation installed, people had not been heating all their rooms to room temperature (and some were even getting dressed under the bedsheets), whereas afterwards they were able to keep all their rooms warm. This is called comfort take-up. Some friends of ours had this experience, after installing central heating and solar thermal in their old farmhouse: “It’s so nice now we don’t have ice on the inside of the windows, and yet the bills aren’t any more!”

So the Green Deal Golden Rule is: if you heat your house (and have just as many cats and children) just as you did before implementing the measure, you might be able to guarantee that the Golden Rule will be met. But who would monitor this?

You: It’s a bit cold in here.
House: You’ve always had it this cold in here at this time of year.
You: Yeah, but now we have insulation!
House: We don’t have anything! I have insulation that I am paying for out of the savings I make from what the fuel bills would have been without it, and without it, it would certainly have been “a bit cold” in here!

Next time, to finish Me and my clever home: WHY?