Commercial solar PV design in the UK now runs on tighter rules than a domestic roof job, and the biggest single change lands mid-2026: MCS made structural and wind loading calculations mandatory for any mounting system not already certified under MCS 012, effective from 18 June 2026 (MCS, 2026). UK solar capacity reached 22.3 GW by June 2026, with cost per kW still falling (pv magazine, 2026), and a growing share of that pipeline sits on commercial roofs rather than houses.
This guide walks through commercial pv system design the way an installer actually needs it: the site survey and structural checks first, then string sizing, grid connection, layout, financial drivers and fire safety. The angle is what changes once you move from a 4 kW residential array to a 50 kW warehouse roof or a 250 kW distribution centre, because the standards, the paperwork and the failure modes are different.
Key Takeaways
- MIS 3002 clause 5.5.5 now requires a structural engineer's loading calculation for any mounting system without MCS 012 certification, mandatory from 18 June 2026.
- Systems above 50 kW need a full G99 application to the DNO, not the simpler G98 notification used for small domestic jobs.
- Flat roof ballast can add 50 to 80 kg per square metre versus 15 to 25 kg per square metre for rail-mounted systems, so the structural survey has to come before the layout, not after.
- The Annual Investment Allowance still gives 100% tax relief up to £1 million a year even though solar sits in HMRC's special rate pool.
- Commercial systems above 100 kW typically cost £700 to £1,100 per kWp installed and pay back in 3 to 5 years, against 5 to 7 years for smaller sub-50 kW systems.
What does commercial pv system design involve?
Commercial pv system design covers everything between a confirmed site survey and a signed-off, commissioned array: structural assessment, electrical string design, DNO connection, shading and layout, and the compliance paperwork that MCS and the DNO both expect to see. It differs from residential design mainly in scale and process, because a 30 kW-plus system triggers structural sign-off, a formal grid application and often a three-phase electrical design that a 4 kW house roof never needs.
The core sequence rarely changes: confirm roof condition and load capacity, size the strings to the inverter's voltage window, model shading losses, submit the DNO application in parallel with detailed design, then finalise layout once the connection offer and structural report are both in hand. Skipping the order, particularly quoting a layout before the structural survey, is the single most common reason commercial jobs get re-designed mid-project.
The deliverables also look different from a residential job. Expect to produce a shading and yield estimate, a single line diagram, a structural report reference, a G99 or G98 application pack, and a commissioning certificate that ties back to the MCS Installations Database. Larger sites often ask for all of this before signing a contract, not after, so building the documentation trail into the design workflow from day one saves time later.
Site survey and structural assessment come first
A qualified structural engineer must now assess the imposed load from the array and any ballast on the roof structure for flat-roof ballasted systems, following MIS 3002 V6.0 clause 5.9.13(h) (MCS, 2026). Rail-mounted systems typically add 15 to 25 kg per square metre to a roof, while ballasted flat-roof systems can reach 50 to 80 kg per square metre depending on wind zone and building height (nu.energy, 2026).
The engineer works from aerial imagery, planning records and available building data to produce Eurocode-verified calculations against BS EN 1991, then issues a pass, conditional or fail result. A conditional result usually means redistributing panels, adding roof reinforcement, or switching from ballast to a mechanically fixed system, all of which change the layout you eventually quote. Getting this report before committing to a panel count avoids re-quoting the whole job later.
String sizing and inverter selection
Once the structural picture is confirmed, string sizing follows the same electrical logic as any PV system design: keep the string's open-circuit voltage under the inverter's maximum DC input across the coldest expected temperature, and keep the operating voltage inside the maximum power point tracking window across the full temperature range the roof will see. Commercial roofs often run hotter than domestic ones, particularly dark membrane flat roofs in summer, which pushes string lengths shorter than a simple room-temperature calculation would suggest.
Three-phase inverters become the default above roughly 30 to 50 kW, both to balance load across phases and because many DNOs require it for larger connections. Mounting hardware itself has its own certification requirement: from August 2025, flat-roof mounting systems generally need MCS 012 certification, and MIS 3002 sets out the fallback evidence installers must supply when a certified system genuinely is not available for the site (Westech Solar, 2025).
DC cable runs and combiner boxes also need more thought at commercial scale, since a multi-inverter array often means several combiner points feeding a shared AC distribution board rather than one string running straight into a single domestic inverter. Voltage drop across longer commercial cable runs can quietly erode yield if conductor sizing is left as an afterthought, so calculate it alongside the string design rather than after the layout is fixed.
How do you connect a commercial system to the grid?
Any generation above the G98 notification threshold needs a G99 application to the local Distribution Network Operator, and above 50 kW that means the full technical study rather than the simplified process available to smaller systems (Energy Networks Association, 2025). The DNO assesses local network capacity and can impose an export limit below the system's nameplate capacity, which materially changes the economics if self-consumption on site is low.
Timelines and costs vary sharply by connection type and network headroom. The table below gives working figures for planning a commercial quote; always confirm current fees and timelines with the specific DNO before committing to a delivery date.
- G98 notification Typical capacity: Up to G98 threshold (per phase); First DNO response: Automatic / rapid; Connection offer: N/A; Typical application cost: Low or no fee
- G99 Type A Typical capacity: Up to 50 kW; First DNO response: Around 12 weeks; Connection offer: Included in response; Typical application cost: £500 to £8,000+
- G99 Type B-D Typical capacity: Above 50 kW, full study; First DNO response: 12 to 16 weeks; Connection offer: 16 to 24 weeks; Typical application cost: £1,500 to £35,000+
Source: Energy Networks Association, 2025; Grid Guru, 2026.
Submit the G99 application as early as possible and run it in parallel with detailed design, because the connection offer, not the panel delivery, is usually the critical path on a commercial job. A site with a constrained local network can add months to the programme regardless of how fast the design and structural work move.
Shading and layout optimisation
Commercial roofs frequently have rooftop plant, parapets, adjacent taller buildings or neighbouring chimneys that residential layouts rarely deal with at scale, so a proper shading analysis using real 3D obstruction data matters more here than on a house roof. Row spacing on flat commercial roofs is a direct trade-off between capacity and self-shading losses in winter, and getting it wrong either wastes usable roof area or accepts avoidable output loss across the system's life.
Orientation flexibility is one advantage commercial layouts have over pitched residential roofs. East-west arrays on flat commercial roofs can pack more capacity into the same footprint than a south-facing-only layout, even though peak output per panel is lower, because the roof area is usually the binding constraint rather than the inverter or grid connection. Model both orientations against the site's actual electricity demand profile before settling on a layout, since a business that consumes power steadily through the working day often gets more value from a broader east-west spread than from a smaller south-facing peak.
Modelling software matters here too. MCS documentation requirements now expect a shading and yield estimate as standard evidence, so a 3D obstruction model built from an accurate roof survey, rather than a rough compass-and-tilt estimate, is worth the extra hour it takes. On a large industrial roof with rooftop plant scattered across it, the difference between a rough estimate and a proper obstruction model can be several percentage points of annual yield, which on a 250 kW system is a meaningful sum of money over a 25-year design life.
What financial factors shape commercial pv system design?
The Annual Investment Allowance remains the main route to tax relief on a commercial system: it gives 100% relief on qualifying spend up to £1 million a year in the year of purchase, even though solar panels sit in HMRC's special rate pool rather than the main pool (GOV.UK, 2026). Outside the AIA limit, special rate assets only qualify for a 6% writing-down allowance a year, so most SME-scale commercial jobs never need to worry about that lower rate.
Commercial electricity prices in 2026 typically run 22p to 35p per kWh depending on contract and load profile, and the return on a commercial system comes overwhelmingly from displaced grid import rather than export income (Business Solar Calculator, 2026). Systems above 100 kW usually cost £700 to £1,100 per kWp installed and pay back in 3 to 5 years, while smaller 10 to 50 kW systems cost more per kWp, typically £1,000 to £1,200, and pay back over 5 to 7 years (Spirit Energy, 2026). Self-consumption rate is the biggest lever in that range, which is exactly why the layout and demand-profile modelling done earlier in the design process matters financially, not just technically.
Fire safety and compliance on commercial roofs
Commercial installations carry a heavier compliance load than domestic ones because more people occupy the building and the DC cabling runs are longer. Wiring and system design still follow BS 7671, with MCS 012 and MIS 3002 setting installation-specific requirements for shading documentation, yield estimates and system performance evidence (MCS, 2026). Fire safety evidence, including how fire performance and weather-tightness are maintained after installation, is now an explicit requirement installers must be able to show under the updated clause 5.5.5 (MCS, 2026).
Cable routing on a large commercial roof also needs a documented rapid shutdown or isolation strategy, since fire services expect a clear, labelled way to de-energise the DC side. Build this into the layout early, because retrofitting isolation points after the racking is installed is far more disruptive than planning conduit runs and combiner box positions alongside the panel layout itself.
How does battery storage change a commercial pv system design?
Adding storage to a commercial PV design shifts the string and inverter sizing conversation toward the site's demand profile rather than just its roof area, because the battery's value comes from peak shaving and network-charge avoidance as much as from storing excess solar generation. A behind-the-meter battery lets a site discharge through the highest-priced demand periods, which for many commercial electricity contracts is the single clearest saving available once the PV array itself is designed and sized, and it changes the commercial battery storage cost conversation you have alongside the PV quote. Reonic's platform is one option installers use to turn the site's consumption data and PV design into a costed, DNO-ready commercial proposal without re-doing the sizing calculations by hand for every quote.
FAQ
What is the difference between residential and commercial PV system design?
Commercial design adds structural sign-off for larger, often flat, roofs, a full G99 grid application above 50 kW, and usually three-phase electrical design. Residential systems below the G98 threshold skip most of this, using a simplified notification process and lighter documentation for both the roof and the grid connection.
Do commercial solar systems need planning permission in the UK?
Most rooftop commercial systems fall under permitted development, though listed buildings, conservation areas and unusually tall arrays can trigger a planning application. Always check the local planning authority's position before finalising a layout, since a rejected application after design work is a costly redo.
What size commercial system needs a full G99 application?
Any system above 50 kW needs the full G99 technical study rather than the simplified Type A process, and the DNO's technical assessment can take 12 to 24 weeks depending on local network headroom before a connection offer is issued.
How much does structural assessment add to a commercial solar quote?
Costs vary by roof type and engineer, but a desktop structural report is typically far cheaper than a full site survey with intrusive investigation. Since MIS 3002 now mandates this evidence from 18 June 2026 for non-MCS-012 mounting, budget it into every commercial quote rather than treating it as an optional extra.
Can a commercial PV system use ballasted mounting on any flat roof?
Only if the structural engineer's report confirms the roof can carry the added ballast load, which can reach 50 to 80 kg per square metre. Weaker roof structures may need a lighter mechanically fixed system instead, which changes both the layout and the cost of the mounting hardware itself.
