Calculations

The concern about the heat pump is the running costs. Even though the efficiency is 1 to 4 (1kWh in gives 4kWh out), the economics & CO2 savings are somewhat different.

My last bill including VAT

• 1 unit of electricity = 14.58p
• 1 unity of Gas = 5.17p (see note about split costs below)
• No. kWh of Gas used = 2389
• Gas bill was £124.40

As a rough approximation, if we needed the same number of kWh (i.e. 2389) and using a Coefficient of Performance (COP) of 1 to 4 for the heat pump, then I would only need 597.91 kWh of electricity to generate 2389 kWh.

But, hold on guys, lets work out the electricity costs :-

• 597.91 kWh will cost 597.91*14.58 = £87.08
• My Gas bill was £124.40
• Saving is £36.32
• Difference = 29.4% saving on £££

Hmm, and what is the CO2 savings (units from EST) ?

• 1 unit of gas = 0.206 KgCO2
• 1 unity of electricity= 0.537 KgCO2
• CO2 generated from GAS = 2389*0.206 = 492.1 KgCO2
• CO2 generated from ELECTRICITY = 597.91*0.537 = 320.7 KgCO2
• Saving is 171KgCO2
• Difference = 34.8% saving on CO2

Conclusions

This is only looking at less than 1 quarter’s bills (24th Dec 08 to 2nd Feb). Since I have split unit prices on my Gas (i.e. the first 1145 units costs 7.465p and the rest at 2.577p), then the more gas I use in a quarter, the lower the unit price as compared to the calculation above. This would imply that my real unit cost of 14.58p per unit will drop and therefore the economics will work out to be less favourable.

The other point was that I was hoping to be able to offset my electricity usage with PV, but since the amount I would get would be less than what I am currently using, this is a non-starter. So getting to zero via this route meets a hard stop if you have to use electricity. You could offset your CO2 from electricity usage but the savings aren’t all that favourable.

I would probably have to have 2 bore holes drilled (2 x 60m) and at £37/m, thats £4440 capital costs.

So my thinking is to sink my capital into significant insulation (hence reducing the need for energy) and this would also keep the inside of the house cool in summer. If we’re going to have summers like Morocco or Southern Spain, then this will help.