We installed a GSHP about 18 months ago and have monitored performance and costs.
Our house dates back to 16thC. We did upgrade insulation where possible but that was limited because of clay lump walls. All radiators had to be replaced, and we also opted to put in some under floor heating whilst carrying out other works in some of the ground floor areas.
The first full year of operation cost us an additional £490 in electricity compared to our electricity consumption the previous year. Consumption over the past last six months is very much in line with the first six months so I would expect our 2nd full year additional electricity cost to be in-line with year 1.
Previously, we were using an oil fired boiler and spending around £2000pa on heating oil.
The past two winters have been relatively mild so accurate comparisons with earlier years are almost difficult to achieve.
A further big plus is that the house is much warmer. The old oil boiler used to be on about 8 hours a day so we had significant parts of the day without heating, whereas the GSHP system runs 24/7 and the house is cosy all of the time
So: a warmer house, a 4:1 energy efficiency resulting in a 75% reduction in energy spending
What did it cost ?: around £30,000, excluding the under floor heating which was part of a larger project
What about the RHI payments ?: We get a quarterly RHI payment which, over the 7 year period that it will be paid, will amount to around £30,000. So we get our money back (without interest) and will also save around £1500 a year on oil
Conclusion: It's a big outlay. £30K would buy a lot of oil. Without the RHI it would never stack up. It works for us, and we are very happy with the performance