I have a 7 year old Solartwin panel on my roof which I believe is performing poorly and is not metered. The company say that the harder water is likely to be too much for the panel and that it needs an indirect panel. Thus I feel I need to replace it. It is a landscape shape high on my roof, now surrounded by PV panels, so its replacement needs to fit the same space. Given the space constraint, what is the best panel/system to install? I cannot easily find outputs per square metre gross.

To maximise the solar gain from a limited area you are probably best using an evacuated tube collector as they have a higher yield per square metre than a flat plate collector.

This is due to the increased insulation provided by the vacuum. By containing the absorber within a vacuum, heat loss from the absorber is almost completely avoided. This also increases the yield in low radiation and cold conditions as the absorber is not susceptible to the ambient temperature.

An evacuated tube collector will have an output of approximately 600-650 kWh per m2 per year compared to approximately 500 kWh per m2 per year for a flat plate panel. But as panels come in all different shapes and sizes it could be that a flat plate panel will fill the available space better than a evacuated tube panel.

There are various institutes that publish test data from various collectors. Most comprehensive is the SPF Swiss test facility. It has information on collector outputs as well as collector sizes.

If the roof space is too restrictive solar thermal panels can sometimes be mounted in different locations than the roof. Different types of panels have various mounting options. Most can be mounted on A-frames, some can be mounted on flat walls, and we have mounted panels on pergolas in the garden which can make a nice feature.