PV System Design
and Consenting

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PV system design of commercial-scale systems is complicated, with many different variables to consider which often cause an iterative step in the process. The diagram below illustrates the whole process from start to finish. If you want more detail on a particular step, click on it and more details will be shown.

Outline Design

Outline specification

The starting point for PV System Design – would normally come from the Solar PV Feasibility Study that we completed, or could come from other sources provided they were competent at solar PV specification and followed the IET Code of Practice for Grid-connected Solar Photovoltaic Systems, 2nd Edition.

Hardware specification

This would cover the main hardware elements, so solar PV panels, inverters, optimisers (if used) and mounting system. These need to be specified early in the design process because all of the physical dimensions, weight and performance etc. are required for the following design stages. Note that as a principle, we only specify solar PV panels with a minimum efficiency of 21% from one of the Silicon Module Super League (SMSL) suppliers, prefer to user optimisers with the inverters due to the increased performance and module-level monitoring this provides, and use high efficiency inverters (typically SolarEdge due to their high efficiency, reputation and compatibility with optimisers). For mounting systems, our preferred supplier is Schletter due to their long track record of supplying quality mounting systems that are backed up with thorough design guidance and rigorous long-term testing.

We also consider the logistics of supply (lead times etc.), because this can be a significant issue for solar PV projects depending on global demand at that moment, which can fluctuate a lot due to changes in energy policy in the larger economies.

Initial Designers Risk Assessment

Regardless of whether the project falls under the CDM Regulations 2015, as designer we have a duty to identify risks and try to design these out, or at least minimise them, and then communicate any residual risks to the wider project team. We meet this requirement by completing an ‘initial’ Designers Risk Assessment which aims to identify all risks and suggest mitigations which can feed into the solar PV system design. Then once the design is completed, we will update the risk assessment and include this in the Construction Design Package so it can be feed into the installation-stage planning.

Solar PV panel layout

Based on the selected panels, we will take into account the roof / ground topography, roof furniture and structure, roof membrane type, surrounding physical objects and trees that could cast shadows, the target power output from the system and aesthetics to determine the optimum layout of the solar PV panels.

Structural Assessment

Weight, wind and snow loading

Using the proposed panel layout, we will calculate the additional weight on the roof structure from the solar PV system, then calculate the wind loading using the BRE DG 489 methodology and work out any additional loads from snow loading.

Structural assessment

There are two elements to this – the structural integrity of the roof covering outer membrane, and the ability of the underlying roof structure to withstand the additional loads imposed from the solar PV system. The checks on the outer roof covering are normally a visual inspection to check:

• The general condition.
• The number and position of fixings to the underlying structure.
• What underlying elements are present (i.e. is the roof over-clad, or is it insulated and if so what type etc.).
• Any evidence of asbestos.
• For trapezoidal and standing-seam roofs, establish the key dimensions.

Then the underlying roof structure would be checked to ensure it could take the additional loads from the solar PV system, and if necessary what strengthening would be required, or changes to the panel layout to reduce the loading.

Grid Connection Consent

All renewable energy projects that generate more than 16 Amps per phase (approx. 11 kW three-phase) require permission to grid connect the system from the Distribution Network Operator (DNO) for your area. This permission is needed even for sites that could consume all of the generated energy onsite. It is important to submit this application as early as possible in the design process, because if there are capacity issues that could limit the amount of electricity that could be exported, this could impact on the design of the system. Including how many Solar PV panels to install. Worth noting that in situations where the DNO will not allow any export of power to the grid, it is still possible to get permission to grid connect it provided you can demonstrate that the system has a control system that can limit the export to the agreed amount, including zero. This is useful where you still need to connect in parallel with the grid so that you can supply onsite loads using grid and Solar PV supplied electricity.

On projects where the export of excess electricity is limited it is worth considering integrating energy storage into the system. This will store excess electricity on site for periods when demand exceeds generation, such as overnight. To find out more please head to our Energy Storage pages or tick the “Storage” box when submitting your enquiry. The process for applying is described in detail here https://connections.nationalgrid.co.uk/get-connected/solar-and-wind/generation-g99, but in summary requires the G99 application form to be completed and submitted to the DNO. The forms are extremely complicated, so normally Renewables First would complete these and manage the application process through to completion. Once submitted, the DNO would check the local distribution network for capacity and the local infrastructure for compatibility, and then get in touch to discuss the various options before replying with an offer letter which often includes a quote for any upgrades required to facilitate the connection. Some DNOs charge for making grid consent applications – we will determine this at an early stage for your particular project.

Mechanical Design

Mounting system design

The mounting system is what holds the solar PV panels in place, either onto a roof or a ground-mounted framework. We generally specify Schletter mounting systems due to their long track record in the industry and because they can provide comprehensive testing and loading data for all of their mounting systems which adds credibility and can be useful if insurance companies ask in the future to see such documentation.

There are mounting systems available for almost every roof and ground-mounted scenario, and we would select the most appropriate for your application. A key part of mounting system design is the fixing / interface with the roof or ground, which would also be considered in detail to ensure it could take the extreme wind and snow loading scenarios calculated, including a safety factor.

Cable penetration(s) design

Getting the DC cables from the strings of solar PV panels on the outside of the building onto cable trays inside the building in a safe, neat and tidy way that doesn’t leak at any point in the future is not as straightforward as you might first think. The issue is that the size and shape, design and construction methods and materials used in buildings is extremely varied, so there is no single optimum solution. We apply some common-sense principles to our cable penetration designs:

• Design the cable layout to minimise the number of penetration points, and if possible, have just one.
• Try to position them in sheltered locations that are free-draining, have no standing water and are subject to minimal direct rainfall and sunlight.
• Use materials that will not degrade for the lifetime of the solar PV system.
• Follow good practice for roof flashings and do not rely on silicone sealants for the primary waterproofing.
• Make vermin-proof and do not compromise fire barriers.

There are a number of options available (hockey stick, bonded and bolted junction box with glands, various special flashings and tiles, MCT blocks for sidewall entry etc.), but rest assured we will specify the most appropriate for your site.

Visual Imagery to Support Planning

Planning Consent

Planning consent – PA56

A lot of roof-mounted Solar PV systems up to 1 MW in size (about 2,000 panels) are ‘permitted development’ so do not need to obtain full planning consent. You do however still need to get PA56 (56-day Prior Approval), which is basically where you notify the local authority at least 56 working-days before you intend to start works on site, and if they have any concerns, they will get in touch within the 56-day period, or consent is deemed to have been given. In practice, we would always advise clients to get written confirmation that the Local Authority is happy for the project to proceed under the Permitted Development rules for peace of mind, but this is still significantly more straightforward than having to make a full planning application. The basic criteria that must be met to come under the permitted development rules are:

• Solar panels installed on a wall or a pitched roof should project no more than 200 mm from the wall surface or roof slope.
• Where panels are installed on a flat roof the highest part of the equipment should not be more than one metre above the highest part of the roof (excluding the chimney).
• Equipment mounted on a roof must not be within one metre of the external edge of that roof.
• Equipment mounted on a wall must not be within one metre of a junction of that wall with another wall or with the roof of the building.
• The panels must not be installed on a listed building or on a building that is within the grounds of a listed building, or on a site designated as a scheduled monument.
• If the building is on Article 2(3) designated land the equipment must not be installed on a wall or a roof slope which fronts a highway (note this related to conservation areas, AONBs, National Parks etc.).
• If the equipment is on the roof of the building the capacity for generation of electricity across the whole of the site cannot exceed 1 megawatt.

Full planning consent

If your project is larger than 1 MW, you do not meet the permitted development requirements above or is ground-mounted (other than tiny household-sized systems), you will need planning consent. As planning applications go, Solar PV ones are ‘relatively’ straightforward, but as systems get larger the scope of the planning application gets more onerous. We won’t describe what is required for the planning application here because this information is readily available on the Planning Portal or your Local Authority’s website, but needless to say, Renewables First are adept of preparing planning applications and managing them through the process to conclusion.

Electrical Design

Stringing arrangement

This is working out how the panels are connected together in series and parallel strings. There are some basic electrical voltage and current limits to adhere to, but also some performance optimisation considerations to string together solar PV panels that might be susceptible to shading and keep them separate from strings that will never be impacted by shading. Also DC cable routing and building penetration points for cabling are taken into account to minimise cable lengths where possible.

External earth bonding / surge arrestors / Lightning Protection System integration

This is a complicated area with many variables to consider. Not many buildings have Lightning Protection Systems (LPS), but if yours did have one we would ensure that the solar PV system integrated properly with that. If it didn’t have an LPS we would assess the risk of a lightning strike and specify appropriate measures to mitigate the risk, which would often be appropriate Surge Protection Devices (SPDs) or in extreme cases a new LPS.

Depending on the materials of the roof structure, it may be necessary to provide equipotential bonding, array frame bonding or functional earthing (the latter depending on manufacturers recommendations for particular hardware). We would determine the requirements and specify an appropriate system.

Internal cable routing / containment

This mainly relates to the DC-cabling between the solar PV panels on the roof and the inverter. The internal AC wiring between the inverter(s) and distribution board is standard industrial wiring that would be familiar to any commercial electrician.

The DC-cabling normally operates at close to 1,000 VDC, which if incorrectly handled could pose a significant safety and fire risk. This risk is mitigated by using special double-insulated Class 2 DC cables with special ‘multicontact’ MC4 connectors at the ends. Cables are separated based on polarity, and also separated from other cables, generally by using a dedicated containment system with special labelling to ensure people are aware that they are live during daylight hours, even during power cuts.

Obviously, the cable sizing needs to be appropriate for the maximum current, and also achieve a maximum voltage drop of 3%. Note that the inverter normally incorporates DC-cable insulation resistance and residual current monitoring to further reduce the risk of DC-cable faults. If the inverter cannot do this (unusual nowadays), there are external systems that can be fitted.

The routing of DC cabling is determined by the layout of the building structure, but generally speaking, the DC cables are routed to minimise their length and keep them out of harms way.

Inverter positioning

This generally comes down to the layout of the building and ensuring good access for any future maintenance, but as a general comment the inverters are generally located close to the distribution board that they connect to on the AC-side, but they can be located further away if that is more convenient.

Inverters come in all shapes and sizes, wall and floor mounted. Generally speaking a number of smaller modular units are installed rather than one enormous inverter, though even if a single large inverter is fitted, this will have a modular design so individual, smaller, power modules can be easily removed and replaced in the event of failure.

Site electrical system integration

For most sites this is all about connecting the AC-output from the solar PV inverter(s) into your onsite low-voltage power distribution system so you can consume as much of the generated electricity onsite as possible. Generally, this just means connecting into the nearest 400 VAC three-phase distribution board and the electricity will then just find the nearest load by itself, and only if more was being generated than could be consumed on site would it flow out through an export meter and be sold to the ‘grid’.

Assuming there is space in the existingexiting distribution board(s), it is simply a matter of specifying an appropriate circuit breaker and AC cabling. If the existing distribution board is full, we can work out the best way of extending it.

For larger systems it might be necessary to connect the system at 11 kV, and for these we can either work with your preferred 11 kV contractor or we have a number of contacts in the HV industry who we have worked with before who can assist.

Communications / remote monitoring

All good quality inverters come with sophisticated remote control and monitoring systems which provide constant performance data and various maintenance alerts. If your system uses optimisers, it can normally provide module-level performance data and maintenance alerts, which can be invaluable and save significant time in the event that a fault occurs.

At the design stage we just need to make sure appropriate data connection points are accessible at the inverter location. Once installed, setting up these systems can be quite complicated, but we can set it up and configure it to meet your requirements, and if preferred we can provide a monitoring service so we receive any alerts and act on them right away to avoid any unnecessary downtime.

Technical Drawings and Planning

General Arrangement drawings

This is the set of engineering drawings showing the whole for your PV System Design in detail. The CAD model that these are derived from also forms the basis for the setting-out drawings below and any visual images of the system needed to support an planning consent applications.

Setting-out drawing

These use the GA drawings to provide the key reference points for your  and dimensions used while installing the system, to avoid the need for installation engineers to have to work out everything from scratch on-site.

Bill of Materials

A long list of all of the materials needed to construct the system, from the solar PV panels and inverters, and all the way down to the smallest bolt in the mounting system.

Schedule of rental equipment

This covers things like scaffolding, scissor lifts, mobile anchor points, telehandlers & forklifts, site welfare unit, waste skips etc.

Procurement plan / payment schedule

Solar PV systems contain a large number of small parts from a range of global suppliers, and with constantly varying levels of demand which affect lead times.

We use our industry contacts to get realistic lead times and put together a detailed plan of what needs ordering and when to ensure that the system can be installed in the most efficient manner.

The solar PV panels themselves make up the single largest item of expenditure and generally need paying for up-front, before delivery. Other items are generally available on 28-day credit terms. We will work with you to produce a payment schedule that ensures no key pieces of hardware are delayed due to payment issues to make sure the project remains on track in regard to timescales.

Proposed build sequence

This is a useful document that aims to ‘tell the story’ of the sequence of operations to actually construct the solar PV system, using many images from the 3D design model. It breaks the construction stage down into a large number of small steps, and describes them all. It is a really useful document to go through with the client, the site staff and any other stakeholders involved. It often generates useful feedback that can be incorporated into a version 2 to improve the efficiency of the project based on local knowledge, or plan around other site operations to ensure the minimum amount of disruption is caused.

We often use a version of this in the Risk Assessment and Method Statement for the construction stage, again because it is a useful way of communicating what we are doing to all stakeholders.

Designers Risk Assessment

This builds on the ‘initial’ Designers Risk Assessment carried out early during the design process and communicates all of the identified risks and shows what has been done to either eliminate or mitigate their impacts. This forms an essential Health & Safety document that feeds directly into the Construction Phase Plan.

Transport, access and lifting plan

This considers the logistics of getting all of the materials delivered to site and either stored or moved to wherever they are needed – often up on the roof or into a field. This can be quite complicated, particularly when solar PV panels are delivered direct to site from the port and each container contains around 500 panels – and the container generally needed unloading straightaway!

It’s also important to consider contingency plans – such as what to do if it is very windy or snowing, which can all significantly impact site logistics.

Construction Design Pack

This pulls all of the design information for your PV System Design into a single package ready for handover to the installer (normally Renewables First as well).