£1000? Jesus.... no wonder he wants solar panels. Easiest way would be to check his meter reading from one day to the next. Should give a pretty accurate figure.
Thought you might like a quick update. As I thought it's impossible to do a sap rating on a barn/cowshed and there's no need to either. Lead assessor at NICEIC has said that the usual pvgis/pv sol estimates are fine - it just doesn't say that in the MCS guidance.
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