Now after ten months of operation and a lot of adjustments, we are getting a clearer idea of the running costs in terms of increased electricity.
The base case, before the heat pump started running on October 12th 2005, was that the house was using an average of 20 units (kWhs) of high cost plus 90 units of low cost a day. This represented an annual running cost of £902 for our total electricity use.
Since then the average for the ten months with the system running is 57 high plus 24 low. This is an annual running cost of £2,520
So in crude terms the heat pump is costing us £1,600 a year.
This is confirmed by the observation that the main heat pump has run for 4,013 hours in this period. Since it uses 2.8 kW, this represents a rough annual cost of £1,400. This is only rough because we do not know how many of these hours were at the lower rate, and this does not allow for the 650 hours in which the supplementary cassettes (in effect immersion heaters) were also running. Overall, however, I think the figure of £1,600 a year will stand for the first year of operation.
Some points on this:
- this includes some months with non-optimal settings, so next year’s total should be a little less
- this means that the main heat pump has been on for an average of 13 hours a day, which is close to the 12 hours we had as our target. Since the last 8 weeks of the year are summer-time, this average may well come close to 12 hours a day by October.
- the oil boiler used to cost around £1,200 a year in fuel and maintenance, though oil costs have risen since then.
- increased insulation should also help next year.
So my interim conclusion is that I’m delighted we did it in terms of reducing our carbon footprint, but the cost position is at best neutral and we haven’t for some reason yet seen the kind of savings often promised for this technology.
Next steps include further fine tuning of the heat pump (especially trying to run it more at night), and installation this Autumn of our first small turbine to generate some of this electrical load.