Renewable Domestic Power: Conservation

Why Conserve When it's Free?

The cost of energy produced from renewable sources is, perhaps surprisingly, more expensive than energy bought from the grid. Although obtaining the energy is almost free -- the only cost is the wear on the device used to harvest it -- the fact that the energy may not appear when it is wanted means it needs to be stored. Storing energy is expensive: heat stores require lots of insulation, which makes them pricy; and electrical storage is done with batteries, which can only be discharged a certain number of times before they need to be replaced.

For this reason, even more than when connected to the grid, it is vital to conserve power.

What Can be Saved


All modern heating systems consist of a source of heat and a thermostat to control that source of heat. Older heating systems are the same, but the thermostat consists of a human to throw another log on the fire when it gets cold. For this reason, when the desired temperature is reached the amount of fuel used is equal to the amount of heat lost.

An amazing amount of heat is lost through the walls, windows, floor and roof of a house. Granite has a transmission factor (a U-value) of 3 to 5 Watt per metre per degree C, which means that if the walls are half a metre thick they will lose 6 to 10 Watts per degree of temperature difference per square metre of walls. If the walls are 38m long and average 3m high, then they will lose 350 to 1140 Watt per degree of difference. Likewise if the floor is 80 square metres, it will lose about 240 to 400 Watt per degree of difference.

Slate has a transmissivity of 2W per metre per degree, which means that a 5mm slate roof loses a whopping 40W per square metre per degree. Put that number into a roof of 150 square metres, and that is a loss of 6kW per square metre per degree. Even adding 1mm of felt (190W) and 12mm of plasterboard (35W) only reduces this figure to 17W per square metre, or 2.5kW per degree.

It is obvious that insulating the roof is liable to lead to the biggest savings. Polyurethane foam has a U-value of 0.03watt per metre per degree, so putting 75mm of polyurethane foam between the felt and the rafters is likely to reduce the roof loss to 0.39 Watts per square metre. Some insulation between the rafters, maybe only 150mm of glassfibre bats, will reduce this to the 0.35 figure required by British building regulations. This then reduces our total loss for 150 square metres of roof to just 52W per degree of temperature difference.

The walls can also be insulated, and so can the floor. Removing the floor screed, and re-pouring it with polystyrene insulation (0.03 watts per metre per degree) of 75mm thickness (0.4 watts per square metre per degree) gives 0.38W per square metre per degree - well below the 0.5W per square metre per degree needed for British regulations. Alternatively a wooden (0.14) floor may be constructed: removing the screed and replacing it with a 25mm air gap and 75mm of polystyrene between 100mm x 50mm rafters on 400mm centres with 18mm of wood flooring on top gives rafters at 0.7 and insulation at 0.38 gives an overall 0.42 beneath the floor, which is better than the 0.5 required by the building regs. It also gives a 25mm gap between insulation and floor, suitable for fitting underfloor heating. A similar scheme on the walls creates a 25mm cavity and also adds 12mm plasterboard to the insualtion, giving the walls an insulation rating of 0.38, significantly better than the regulations again. A good door and double-glazed windows should keep the wall average below 0.5. This insulation reduces the wall loss to 57W per degree of difference; and the floors to 34W per degree of difference.

Heat is lost from a house in two ways: through the walls, floor and roof; and by exchanging heated air for unheated air. The first of these is tackled with insulation, the second with draughtproofing.

Insulation can be carried to as great an extreme as we wish, but draughtproofing has an upper limit: if the house is sealed completely, the inhabitants will suffocate. By reducing draughts it is normally considered that a house will change its air three times an hour. Since a cubic metre of air has a heat capacity of approximately 630 joules per degree, that means that in a perfectly insulated house 1kW of heating per cubic metre produces a temperature rise of about 1900oC, or our 240 cubic metre house needs 126W per degree of temperature difference to overcome air circulation. Once the house is reasonably well draughtproofed, insulation is clearly the important factor.

Adding these figures together, leads to a total heat loss of about 270W per degree C of temperature difference. The average temperature in the winter months where we want to live is maybe 5oC and we like an internal temperature of 18oC: so in our house the temperature difference is likely to average 3.5kW, or 84kWh per day of heating. Without the insulation, it's likely to require an average of 2352kWh per day. The peak (with an unlikely outside temperature of, say, -7oC, is a bit less than 7kW.

It can clearly be seen that insulation can reduce the energy requirements for heating by a factor of 28: resulting in a warmer home and lower energy bills. Given that an 11m x 2m solar panel is likely to collect an average 12.5kWh per December day, it can be seen that with conservation the solar system collects about 14% of the total requirement even in the darkest month; and 20% to 25% of the requirement on most winter days. Without insulation, that would be impossible.

Here is the results summarised in a table, with the average temperatures and so heating requirements:-

  Temp Average Heat at 18oC
Jan 5oC 3.5kW
Feb 5oC 3.5kW
Mar 6oC 3.8kW
Apr 8oC 2.7kW
May 11oC 1.9kW
June 13oC 1.4kW
July 15oC 0.8kW
Aug 15oC 0.8kW
Sep 13oC 1.4kW
Oct 10oC 2.2kW
Nov 7oC 3kW
Dec 6oC 3.8kW

It's worth remembering that the people in the house are likely to contribute about 0.4kW on average, and that the electricity usage is likely to contribute another 0.5kW on average. So these figures can probably be adjusted downwards by that amount.


When I started looking at this, we were using about 22kWh per day. Now we're down to 12, which means that we only need half the size of battery. The ways we saved were:-

Replace bulbs with CFLs as they consume only 20% of the energy for lighting: about 4kWh per day.

Use the washing line, as the electric drier uses between 1 and 2 kWh per usage: about 1kWh per day.

Reduce the base load, by plugging transformers, computer and monitors into switched extension leads; and replacing a rack mount PC and two elderly Cisco switches with a dedicated ADSL and a laptop. These changes have reduced our base load from 390W to about 100W - a saving of about 5kWh per day.

At the price we're paying for electricity now, that saves about £255 per year. At the price of battery wear - maybe 16p a unit - it'll save £584 per year. That's worth it.

This page is some notes on Domestic Power from Renewable Sources, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.