[webjunkie]

A word of caution: It's worth factoring in battery depreciation. That 7p→15p arbitrage isn’t "free" profit: you pay round-trip losses and you burn cycle life. If you assume ~£X installed, ~Y usable kWh, ~Z cycles to 70–80% capacity, the wear cost alone is often several pence per kWh throughput, which can wipe out most of the spread.