Most Cost Effective Technologies?

Just musing on DECC's announcement the other day that the FITs review will include a focus on the most cost effective technologies. Such thinking is very likely to influence the design of RHI tariffs, too, I would have thought. That raises quite a conundrum: It is acknowledged that a lot can be done by engagement at household or small community level (Jim Watson's post on the visit of Charles Hendry to SPRU gives some useful pointers) but at this level one does not necessarily achieve optimum paybacks. For instance, in early work on the subsidy costs required to make RHI work, solar thermal was shown to requires some £172/MWh(heat) - more than double the next technology cost. But solar thermal is one of the choices that householders will (relatively) readily install. So, how does HMG incentivise and encourage small-scale engagement and yet avoid a very expensive outcome? (Answers on a postcard, please).