Well.... winter has been and gone - officially at least.
Here's how the experiment went. The home-made solar panel worked of sorts, it just wasn't big enough at first and my workmanship was not sufficient to make it completely air-tight.
If the joints suck in air then the panel empties overnight, then the little solar powered pump couldn't get the water up and over the height of the roof in the morning. I got over this by re-routing the canister filter flow to help it along and that worked well. The panel was too small however and in June I had to install a 200W heater to provide a 'floor' temp of 20C. Not bad considering we're talking a 770L tank outside in the winter - and it was still thermostat 'off' much more than it was on.
Then mid-winter all of a sudden it couldn't keep up on the cold nights. I realised eventually that the ground temperature had dropped below 20C as well (2.5m down in the ground) and I had to divert the flow from the ground coils, then that was fine too. At the same time I rebuilt the panel (again), this time out of all PVC painted black and then that worked better too.
Now it's early spring and the electric heater seems not to run at all anymore, the tank is maintaining about 21.5C.
Have I saved money? No, not yet. Has it cost me a lot to set up? No, just a a fair bit of time and effort to set up and learn from my mistakes. Next winter, touch wood, I’ll just be to just plonk it back on the roof and away it will go. Maybe start a bit earlier in the season so that the ground might start from a warmer state.
The next project might be a tank rack in the shed, in which case there will definitely be a similar larger set-up on the roof. It will be easier in fact, because the water won’t have to rise so high.
Cheers,
L