OK, I see we are talking a different type of solar to what I was thinking on the roof.
I was thinking of these types which, would be to much weight aloft.
Seeing as we are talking about just the panel on the roof with remote storage tank, am I right that you were thinking I could use the existing insulated copper tank from the existing HWS for storage?
The first hurdle I see there is that tank is down near the engine room and the roof where the panels would go is near 30ft above it so a fair bit of pump would be required to circulate that water.
A quick google would suggest that the pump alone would be well on the way to the cost of a 45 litre Force10 HWS, which, as it would replace the existing large domestic HWS would be a relatively simple install (water circuit and power is already there) and would free up storage space.
The second is, based on my household solar HWS experience where I turned off the electric heating side of it, what happens when its raining for more than a few days?
Without an alternative heat source it was back to cold showers.
FAQ on the Heliatos site says "What happens at Dark?
First you use the water that was heated by the sun during the day. Then if you need more hot water
then your gas or electric will kick in."
So I still need electric or gas in addition to the solar for the times when its raining which has been several days where I am now and cold as well.
Based on that I feel I am better off going electric and an extra $300 for 500 watts of solar panels in the space that the solar hot water panels would have gone.