Electricity Consumption

Our first attempt at reducing our CO2 emissions was to replace our electrical appliances, as the need and opportunity arose, with the most efficient available. We thus now have A rated lights, AAA rated washing machine, A rated heat pump tumble drier and an air frier. We also try to use our appliances as efficiently as possible… washes are at 20C, drying is on a line whenever possible and nothing is left on standby. Our household electricity consumption is about 1340 kwh per year.

Solar Panels

The bungalow has an almost exactly north-south roof ridge alignment. This makes each panel’s output less than if it were east-west but allows more panels as they can be put on both halves of the roof. We have 32 panels… 16 facing east and 16 facing west generating about 8000 kwh per year… far more than we use. It makes no difference to CO2 emissions who uses the electricity our panels generate. If we generate 8000 kwh, a gas fired power station somewhere will throttle back by that amount reducing the amount of CO2 it emits. The arithmetic of this situation is that installing our panels reduced CO2 emissions by about 3.7 tonnes per year.

House Insulation

The house was built with cavity walls having a brick outer leaf, a 65mm empty cavity and a concrete block inner leaf. A previous owner had had the cavity filled with polystyrene beads which is about the best thing you can do with such a cavity. They had also replaced all the windows and doors with 40mm double glazing. The loft insulation was however, at best, 100mm so I removed all boarding, redundant water tanks and general junk and had another 200mm of insulation added on top of what was already there. The floors however are not so good being uninsulated, suspended wood which have quite high heat losses. I have plans to get them insulated but these haven’t come to fruition yet because of the difficulties involved.

Both calculation and measurement suggest that the house as it now stands needs about 160 watts for each degree C it is warmer than the outside. Thus, if it is 0 outside and 20 inside it needs 3,200 watts to keep the temperature steady. The house (again as it now stands) was heated by a condensing gas boiler, this had an annual gas consumption of about 10,000 kwh implying that 9000 kwh of heat is needed to keep the house at 20C 24x7x52.

Heat Pump

We then had the gas boiler replaced by an 8.5kw Ecodan air source heat pump. The heat pump has worked well. It is extraordinarily quiet outside and very quiet inside the house. Outside you can only hear the unit if there is no wind, no traffic, no lawn mowers, no dogs and no birds singing. Any of these and the noise made by the heat pump is completely drowned out. Inside, only in the kitchen and loo can any noise be heard at all. In all other rooms you simply cannot tell whether the system is running or not. The kitchen has a fan assisted radiator as there was no room for an ordinary radiator. The noise it makes is far less than that made by kitchen appliances (microwave, air frier, kettle, tumble drier) and does not interfere with conversation. In the loo, it is possible to hear the circulation pump but this is not any sort of a problem.

The heat pump has had not the slightest difficulty maintaining room temperatures in the coldest weather we have had. It’s not likely to ever have a problem because, even if the temperature dropped to minus 20, there should still be enough capacity in the heat pump and radiators to cope.

Last year our heat pump used 2674 kwh of electricity and produced 9492 kwh of heat giving a Coefficient of Performance (COP) of 3.55. These figures include the electricity needed to operate the circulation pump and other ancillary equipment. This implies a 38% reduction in CO2 emissions even if all the extra electricity demand it created came from burning gas. The heat pump is more efficient heating the house (COP = 3.9) than it is heating the water (COP = 2.4).

This COP makes it marginally cheaper to run than a gas boiler even without the benefits of also having solar panels or electric car tariffs. Having both solar panels and a heat pump means that some of the electricity for the heat pump, particularly in the spring and autumn, is self generated and thus very cheap. Last year, the income from our solar panels (£828.68) exceeded the charges for electricity (£708.38) and gas (£0.49) (but not alas the standing charges).

Electric Car

We have recently bought an electric car and are now on a tariff which has very cheap nighttime electricity. This will further reduce the financial cost of running the heat pump. It is also, of course, very much cheaper than buying diesel. Data collection for this is in its early stages.

Further Information

I have collected quite a lot of data on solar panel and heat pump performance and may thus be able to answer questions on their performance in service.

The Future

Insulating the floors is a possibility but the efficiency of the heat pump makes the case for doing so much weaker.

Another 16 solar panels and a battery are a possibility but the job needs a contractor willing to accurately measure the roof (rather than looking at aerial photos) and who can install Victron equipment. Currently the case for batteries does not appear to be very strong either from a CO2 or financial point of view.

Food CO2 emissions is something I have looked into a little but the issue is complex and requires knowledge that I currently lack. On paper, changing from cow’s milk to oat milk saves emissions but that doesn’t mean that pasture land (poor land) can be used to grow oats (arable land).

The house can be visited all the year round, by appointment. Please email dorsetgreenerhomes@gmail.com