Solar PV Design Guide

What a solar PV design actually needs to decide

A solar PV design is not only a drawing with panels placed on a roof or site. It is a sequence of technical decisions that must work together.

A useful design should answer these practical questions:

·         Where can modules safely and sensibly be installed?

·         How many modules make sense for the available area, energy demand and connection capacity?

·         What will shading do to annual generation and daily output?

·         How should modules be grouped electrically into strings?

·         Which inverter arrangement is suitable for the site and the project objective?

·         How will the modules be fixed, supported or ballasted?

·         How will the system connect to the existing electrical infrastructure?

·         What drawings and documents are needed for installation, approval, procurement and handover?

When these questions are answered clearly, the project becomes easier to price, approve, build and maintain.

Good design starts with good input data

The quality of the design depends heavily on the quality of the information provided at the start. Missing drawings, unclear electrical information or unconfirmed site constraints often lead to redesigns, delays or unrealistic expectations.

Site and roof information

Useful information includes roof plans, elevations, site plans, drone images, roof covering type, roof pitch, parapet heights, skylights, plant areas, access zones, fragile roof areas and any known structural limitations.

Electrical information

A design should be based on the real electrical supply. Useful information includes the point of connection, main incoming supply, distribution board details, existing generation, metering arrangement, earthing information, available capacity and any known export limits.

Energy use information

Annual consumption is useful, but half hourly data is far more valuable where available. It shows when electricity is used, how much can be consumed on site and whether the system should be optimised for self consumption, export, battery storage or future loads.

Project objective

The best design depends on the goal. A project designed for maximum roof coverage may look different from 1 designed for self consumption, export revenue, PPA modelling, carbon reduction, battery readiness, planning constraints or staged future expansion.

Usable area is not the same as empty area

A common mistake is to treat every empty roof space as available solar area. In reality, a solar layout must account for access, maintenance, roof zones, fire access, drainage paths, obstructions, wind exposure, warranties and safe installation practice.

What should be checked

·         Roof geometry, pitch, orientation and available module zones.

·         Obstructions such as vents, rooflights, plant, parapets, chimneys and existing equipment.

·         Shading from buildings, trees, roof features and future developments where known.

·         Access routes for installation, maintenance and emergency considerations.

·         Roof covering type, fixing method, ballast potential and waterproofing constraints.

·         Structural limits, wind exposure and any requirement for separate structural review.

A good layout does not force modules into every available corner. It balances capacity, yield, safety, maintenance and buildability.

The right system size depends on demand, capacity and purpose

The largest possible system is not always the best system. A strong design considers how much electricity the site uses, when it uses it, how much export is allowed, what the roof or land can support and what the client wants the project to achieve.

Main sizing factors

·         Available roof or land area after practical exclusions.

·         Annual electricity consumption and half hourly demand profile.

·         Import capacity, export capacity and electrical infrastructure limitations.

·         Expected self consumption and export behaviour.

·         Future loads such as EV charging, heat pumps, process electrification or building expansion.

·         Budget, payback target, PPA assumptions and carbon reduction goals.

A commercial or industrial site with strong daytime demand may justify a larger PV system than a site with low daytime load, even when both buildings have similar roof space. This is why consumption behaviour matters as much as roof area.

Yield estimates are only useful when assumptions are clear

Solar yield is affected by more than the number of modules installed. Orientation, tilt, shading, module type, inverter selection, cable losses, temperature, soiling, downtime and modelling assumptions all influence the final estimate.

What a yield review should explain

·         Annual generation estimate in kWh.

·         Specific yield where relevant, usually shown as kWh per kWp.

·         Assumptions for orientation, tilt, losses and system performance.

·         Shading impact from surrounding buildings, trees and roof features.

·         Monthly generation profile, not only annual generation.

·         Any limitations in the model caused by incomplete information.

A realistic yield estimate is more useful than an optimistic estimate. It helps clients, investors and project teams make decisions with fewer surprises later.

String layout, inverter selection and protection need to work as 1 system

The electrical design connects the module layout to the real electrical infrastructure of the site. It must consider voltage, current, inverter limits, cable routes, protection, isolation, earthing and the point of connection.

String layout

The string layout shows how modules are grouped and connected to inverter inputs. It must respect voltage limits, current limits, temperature effects, module orientation and inverter MPPT configuration.

Inverter selection

Inverter selection affects efficiency, monitoring, clipping behaviour, export control, stringing options, roof zone flexibility and maintenance. The inverter arrangement should suit the system size, site layout and electrical connection strategy.

Protection and cable design

Cable sizing, voltage drop, isolation points, protective devices, AC connection, DC routing and earthing all need to be considered. These details are critical for safe installation and clear handover.

The single line diagram should show how the system connects electrically, what protection is included and how the main components relate to each other.

A solar layout is only useful if it can be built safely

Mounting design affects module spacing, load distribution, roof warranties, drainage, fixing points, ballast, wind resistance and maintenance access. It should be considered early, not added after the panel layout is already fixed.

Roof mounted systems

For flat roofs, the design may need ballast zones, wind calculations, parapet effects, row spacing and maintenance walkways. For pitched roofs, it may need rail positions, fixing points, roof covering details and safe cable routes.

Ground mounted systems

For ground mount projects, the design should consider row spacing, tilt, terrain, access tracks, fencing, cable routes, inverter locations, soil conditions and the chosen mounting structure.

The mounting approach can change the final module count. A technically strong design treats structure, electrical layout and site access as connected decisions.

Design around the point of connection from the beginning

Grid connection requirements can influence system size, inverter selection, export control, protection settings and project timescales. This should be considered early, especially on commercial, industrial and larger residential projects.

What needs attention

·         Existing supply capacity and connection arrangement.

·         Proposed inverter capacity and export expectation.

·         DNO application requirements where applicable.

·         Export limitation strategy where required.

·         Protection settings, metering and monitoring requirements.

·         Clear single line diagrams and supporting technical documents.

A design that ignores the grid connection can look attractive on paper but become difficult to approve or operate. The connection strategy should be part of the design from the start.