Hopefully this is because of:
1. the more efficient Sunbox design and the addition of Tubes, and
2. partially that it was cold enough to keep the GSHP heating for longer, which always helps to draw more energy from the Sunbox.
3. I haven't lifted the manhole cover to the manifold for a while, but the good temperature also hints that more energy may be going down if the manifold stays dry.
From 10 May to 1 July 2010: 601 kWh (SB). [PV 891 kWh], Must have been a good summer!
From 10 May to 1 July 2012: 488 kWh (SB) and 145 kWh (Tubes) = 633 kWh. [PV 733 kWh]
The graph above shows how strange the spring temperatures have been.