Here's an insiders' take on the topic. As a developer of solar water heating systems, with the hope of taking a lead in the industry, I have been interested in minimising energy and carbon budgets for some time. So it is interesting to see that academics are taking an real interest in this area now. In summary, as a business and product developer there are 3 mains "boundaries" that I have looked at. These are:
1/ How we operate as a business. Minimising impacts here has meant that we do not usually visit a householder to make a quotation, even though this offer a lower "quote to close" ratio. Instead we try to quote by phone and email. (Obviously there are other areas to achieve a balance like between trying to market wholly electronically versus paper based marketing such as what are called leaflet drops, which have a higher carbon impact per person contacted.)
2/ Technology design and manufacture up to the point of commissioning. Among other things, this favours solar thermal systems which can retrofit to existing hot water cylinders, where possible, since replacimng them as an enormours environmental footprint. Also It favours materials which are as sustainably sourced as possible - and the minimisation of the actual quantities of these which we use in the first place.
3/ Technology operation during its life, length of life, and end of life issues. This approach favours a long design life and a low end of life impact. Also solar hot water systems which do not need to be run on mains electricity, since plug in solar negates its carbon savings in the home, at the power station instead. This carbon clawback figure is typically around 20% according to DTI funded research. This is something which the conventional solar water heating suppliers is rather sensitive about.
Steve Allen of Bath University has calculated the energy payback of three domestic microrenewable technlogies: Micro-wind, PV and Solar Thermal (using our system, Solartwin).
His conclusions are (in summary) that the financial payback is well over a decade for these renewable energy technologies. But in contrast to this, that energy payback is about 2 years for our solar water heating system. It can be even less for micro-wind in a windy areas and it tends to be a bit longer for PV's.
There is more on our website at:http://www.solartwin.com/index.php?page=97
I think hat the cost curves that you are referring to Nick, are in Steve Allen's report. I sincerely hope my reply is technically interesting and will not not be deleted.