MCS PV Output Estimation and Shading

[The Solar King]

As Gavin A has pointed out, the ECA/MCS Guide says very little about design. The clue is in the Title: 'Guide to the Installation Photovoltaic Systems'. It is not a guide to design, so we get bonkers installations like the one shown. Sadly, this installation may well be fully compliant with the guide......

 

[whinmoor]

Yes, totally.

And it could be that the homeowner was made well aware of effect of the shading and he still thought it was a decent investment. Even if he's only getting 1500kWh - 2000kWh per year, he may still be a satisfied, if not particularly happy bunny.

It may be "bonkers" to a solar purist, but it's on the back of the guy's house so he probably wouldn't give it a moment's thought most days and will benefit from (marginally) lower bills and FiT. The key thing really is whether he was given accurate predictions and we'll never know that.

 

[SolarCity]

Is that a recent picture? And any idea what time of the day it was?

If it's as bad as it looks, I'd be astonished if he gets anywhere near 1500kWh a year.

 

[whinmoor]

I took it last week. About 3pm. It's near Bradford.

According to SAP 2009/2005, 4kWp system on a heavily shaded (80%) south facing 30 degree pitch will get:

SAP 2009 - 1717 kWh
SAP 2005 - 1667 kWh

So, assuming the installation was before the recent rule changes, that's what the customer should have been told. I would probably also have quoted and left it up to the customer to decide whether the figures added up.

 

[SolarCity]

Looks like it will get shade for the majority of the day then. I don't care what SAP comes out with, if that install generates 1,000kWh a year I'll eat my chin.

 

[Gavin A]

I'm going to hazard a guess that the customer was talked into it on the basis that the trees could be cut back, but they then discovered they had tree preservation orders / were in some way protected and didn't get permission to cut them back.

 

[SolarCity]

I have my MCS inspection next week. One major NC we will have is our refusal to use the MCS method of calculation. We do 3D modelling on all our work and armed with the statement below, which is part of the MCS guide, we intend on digging our heels in, if only to illustrate our dissatisfaction.

This shade assessment procedure has been designed to provide a simplified and standardised
approach for MCS installers to use when estimating the impact of shade on system performance.
It is not intended to be as accurate as more sophisticated methods such as, for example, those
included in proprietary software packages. It is estimated that this shade assessment method will
yield results within 10% of the actual annual energy yield for most systems. Unusual systems or
environments may produce different results

 

[SolarCity]

We also have data from every single one of our installs since we started installing which shows how accurate our predictions have been.

 

[Gavin A]

We also have data from every single one of our installs since we started installing which shows how accurate our predictions have been.

any chance of putting that into an annonymised table and sending it across to me?

I'm going to be doing something similar, I just haven't got round to it yet.

In my view, as long as you can demonstrate that your methodology is more accurate than the MCS method, they'd not have a leg to stand on legally in trying to force you to use a less accurate method - it would fall under the unfair restraint of free trade type legislation I reckon.

MCS steering group view that differently, but then none of them are lawyers, and tbh they as a group appear to have a fairly tenuous grasp of the law - eg not appreciating that they collectively could be held jointly liable for the health and safety implications of their guidance if someone did come a cropper as a result of trying to follow their methods.

 

[SolarCity]

I certainly will send it over. I'll post it up on the forum too after I've made a few adjustments.

It is currently in Excel format and I'm trying to tidy it up a bit and removed specific addresses. There is a fair bit of information on it with different panels and inverter combinations, locations, orientations etc.

Most handy is that I have my SAP calculations AND PV*Sol calculations and a percentage comparison of them both. PV*Sol is clearly more accurate, as you would expect.

 

[Gavin A]

we've got a spreadsheet like that, I just don't have the real world data to input yet - intended to request it from customers and download it from those we've got remote monitoring for.