At a glance: What the updated MCS PV guide means for installers

Last week saw the official unveiling of the long-awaited third revision to the Guide to the Installation of Photovoltaic Systems. After a three-month transitional period, use of the new guide's requirements will become mandatory for the MCS scheme. 

The updated guide introduces a raft of changes which can be viewed in full here. Below is a brief summary of the most significant changes being introduced in the updated guide:

New performance estimates

The old method for calculating the performance of a solar array was based on SAP. The calculations produced were based on a single geographic in Sheffield and didn’t accurately take into account the pitch and orientation of roofs.

The new system combines the current geographical zones introduces in the new SAP combined with the PVGIS European database of irradiance and performance. PVGIS is widely acknowledged as a more accurate database and the new system should provide a much more accurate prediction of system yield up and down the UK.

Shade calculation

Under the previous guide, shade calculation was incredibly crude and difficult to audit. The new guide introduces a revised shade calculation that MCS hopes will be far more accurate than the previous method. The key point of the new method is that the calculation is auditable. Although the new calculations can be done easily with specialist instrumentation they can also be produced with the most basic of equipment – a handheld compass and something to measure the inclination. Martin Cotterell talks through the various methods available to installers in his latest blog.

Selection of residual current devices (RCDs)

The updated guide includes a new chart for the selection of RCDs. Previously there had been a lot of confusion amongs installers over the selection of suitable RCDs; the new guide provides a much clearer description of how installers should approach the selection process.


The updated MCS guide now mirrors the International Electrotechnical Comission (IEC) standards. Under the new guide, if the inverter has a built in isolator you do not need to fit a second one providing it meets some basic criteria. The updated guide is designed to stop the fact that a number of certification bodies were asking for a second isolator even if inverters had one built in.

String fuses

The third edition of the MCS PV guide will introduce a different way of determining the fuse requirements of PV systems. As all modules now show the maximum series fuse value on data sheets, fitting fuses can be judged by the real ratings of the modules. The new method is also aligned with PV-specific fuses (Type G) to weed out systemic failures of PV systems as a result of inappropriate fuses being fitted.

Wind loading

The calculations used to determine wind loading has been simplified under the new guide, MCS hopes that the new assessment process, with specific geographical areas will prove more accurate.

Slate roof mounting

The fitting of solar arrays to slate roofs has been a contentious issue for many solar installers. The new requirements lean heavily against any drilling into slate, an issue that has caused a number of issues for consumers and installers alike.

The full raft of changes can be viewed online here. Hard copies of the new guide will be distributed to all attendees at the ‘Restarting the Market’ series of solar roadshows for free. Martin Cotterell, a key member of the MCS PV working group, will also be taking to the stage at the Solar Power UK roadshows up and down the country answering any queries surrounding changes outlined in the guide. Tickets and dates for the roadshows can be viewed here.