Yeah sorry Timmary - was responding to Rays comments (should have quoted him). Your friends sound like the ideal setup for a decent solar array, with all the hot water to heat up and use as a thermal store. 

I also have solar H2O evacuated tubes on my roof and they do a fantastic job of heating my HWC in summer (not so effective in winter down here in ChCh). When my solar PV was installed however the guys told me they recommend just installing extra PV panels rather than evacuated tubes these days, more efficient in terms of cost etc. Plus you don't have pumps to worry about and water running thru your walls!

I have a home brew PV diversion system (based on the stuff from www.openenergymonitor.org) which is a bit smarter than normal. I monitor the amount of PV I have exported each billing month, and only start diverting to my HWC once I have hit the magic 150kWh (which is when my export rates drop from 25c to 10c/kWh). This ensures I get the maximum credit each month and anything excess is used to warm my HWC. I also monitor the HWC temp and shutoff diversion once the cylinder gets above 65 degrees, so that I still get some credit for any excess. Therefore in summer when my evacuated tubes keep my HWC pretty hot, I actually have very little PV diversion to the cylinder...which of course is when I am generating the most PV...so I am trying to think of clever ways to use/store that excess (without spending 000's on batteries).

The thing is, it is a very interesting field, and there are some clever ideas out there for how to maximise the efficiency of these systems. This will only improve over time no doubt!