So what do you think about performance? Thanks Andy.

Right, if I've pieced things together correctly, I think you have 8 blocks with 8.5kWp on each so a total of 34kWp.

By way of a random comparison, my home 2kW system had produced something like 26,000kWh over about 14 years - so lets say that's about 900kWh/kWp/annum. That's for a system facing almost due south, on a roof at about 35 degrees, with almost no shading (just a few mature trees to the east that shade it slightly only in the very early morning), but on the misty side of the Pennines, and a fair way 'up north'.

On that basis, one might expect your 34kWp systems to produce a total generation in the region of 30600kWh per year or 244800 over 8 years - which seems to be in the same ball park as your 31k kWh 'expected' figure. I suspect therefore the estimate you have didn't take account of the shallow panel angles or shading. You might be further south or east than me though, so might do better on sunshine hours and intensity, so maybe that would bring it up a bit. It's hard to be sure at this distance.

 I suspect your best way forward would be to get a competent PV installer to come and have a look, do the calculation on the basis of the conditions you have there and then see if there are any faults that need fixing.

   - Andy.