Posted By: borpinFirstly just to check, evac tubes have a smaller collector area than physical area - right? If so then per m2 of roof they are less efficient?

Yes, and sometimes. But at higher working temperatures (i.e. DHW rather than swimming pools) and especially when the sun isn't so bright and it's cold outside, their efficiency is better than most alternatives and that compensates. And earlier and later in the day, they're more efficient at collecting the oblique light (IAM). And of course mirrors in the gaps between the tubes help.

Don't forget that you can use wall space as well as roof space for thermal panels. Cuts down the summer overheating in relation to the winter output, and doesn't compete with PV for roof space.