Domestic solar hot water (Thermomax) installed 1993
Solar PV (Sanyo 1.7kWp) installed 2009

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Ensure your solar water heating system has a control panel or extension of it that allows you to see the tank temperature without needing to expore the dark recesses of an airing cupboard etc.. Then on marginal days, you can decide a) when to take a shower, b) if it's worthwhile or necessary to top up heat from gas.
I also built an interlock arrangement into my SWH so that if the solar pump is running, the gas boiler is inhibited from firing up. (with a manual override)
Finally, I put the SWH control unit and pump power supply on a simple clockwork timer so that it switched off the pump at 10pm. Otherwise in the summer I can find the pump is running most of the night if not much hot water has been drawn off the tank