We have substituted a mixture of mains gas and LPG heating, which in total consumed 90k KWhr. One surprise has been that the heat pumps are required to deliver only 70k KWhr, possibly reflecting our additional insulation efforts (or that we are not keeping as warm as before?)
We measure the electricity taken by the heat pumps and this is 20k KWhr, reflecting an efficiency ratio of 3.5x. As we have not balanced our electricity usage to match our output, across all 3 phases, we still end up importing and paying at full rate. This somewhat offsets the otherwise rather generous assumption on 'deemed usage' that we use only 25% of our output which would otherwise result in zero additional cost for using part of the output for heating. On top we have an annual maintenance contract - a condition of the RHI scheme but probably a good idea anyway
The in river heat exchangers ended up being quite expensive as we ended up specifying stainless steel food-grade kit for security over lifetime and avoiding corrosion risks. A future scheme could probably halve these costs. Even so, the payback is set to be respectably under 10 years. Were we to have reduced the heat exchanger costs and balanced our electricity usage to avoid importing, the payback could in theory have been an impressive 6.2 years - reflecting the benefit of having very low/ zero opportunity cost for the electricity required to drive the heat pumps.
Looking at the combined hydro/ electric and heat pump system, if we exclude the special costs of the new river gate and the fish pass, then the combined payback should be 8.2 years, and should be slightly better as we have not made allowance for rpi increases in the FIT and RHI payments. (On the other hand we have not adjusted for changes in future gas, lpg and electricity prices which may be going down before they go up). Even if we include the costs of the new river gate and fish pass the combined payback is still 11.4 years, slightly outside our target but not bad.