If you have the older, two tank system,ie one tank for cold water one for the heated water which operates by gravity relying on the height of the tank above the shower to give pressure,you could change to the newer single tank system which operates by mains water pressure,when we changed ours
the pressure was so good it blew the shower head off!
All of the solar sales shops sell the newer system,scout around,ours was fitted by Engin,is he still around?