What does a commercial solar feasibility study actually involve?

“Feasibility study” gets used loosely in commercial solar. Sometimes it means a proper independent assessment. Sometimes it means the free site visit an installer runs before they quote. The two are not the same thing.

An installer’s survey exists to produce a sale. It checks enough to design a system and price it, then stops there.

By contrast, a genuine feasibility study has no such incentive. It exists to answer one question honestly: is this site worth developing, and on what terms? Sometimes the honest answer is no.

This is what actually happens between instructing a feasibility study and receiving the report. It covers the desktop review, the structural survey, and the grid application. It also covers the standards everything gets checked against. Finally, it details what a defensible go/no-go recommendation looks like when it lands on your desk.

The sequence, step by step

Every credible commercial feasibility study follows roughly the same order. Each stage exists to catch the failure that would make the next stage pointless.

1. Desktop review. Aerial imagery, existing EPC data, any structural drawings on file, and — where available — 12 months of your half-hourly electricity data. This is cheap to do and quickly rules out sites with an obvious problem (heavy shading, a roof clearly too small, no usable land).
2. On-site structural survey. A physical inspection of purlin and truss condition, roof membrane age, and the dead-load capacity available for panels, ballast or penetrative fixings. This step usually confirms or rules out a promising-looking site.
3. Shading and orientation study. Using the site’s actual pitch, aspect and nearby obstructions to model realistic annual generation — not a generic per-kWp assumption pulled from a brochure.
4. Grid-connection review. An initial DNO enquiry (and, where warranted, a formal application) to establish whether the local network has capacity, and what a connection would cost and take.
5. Indicative capex and payback. A cost range and payback estimate built on the site-specific yield and grid position established above, not a generic system price.
6. The go/no-go report. A written recommendation stating the viability conclusion, the assumptions behind it, and the risks that could still change the answer.

The structural survey usually comes first for a reason. A corroded steel frame or a tired membrane is the most common reason a promising-looking roof turns out unviable. It’s far cheaper to catch this early, before you commission a shading study or open a grid enquiry, than after.

The structural survey: what’s actually being checked

Roof structural loading is a genuine go/no-go gate, not a formality. Engineers design every commercial roof to carry a specific combination of loads. Dead load covers its own weight plus fixed services. Live load covers wind, snow, and maintenance access.

Solar hardware — panels, mounting rails, and either ballast blocks or penetrative fixings — adds permanent dead load. That load has to fit inside the margin the original design allowed. If it doesn’t, the structure needs strengthening before anything goes on it.

A surveyor checks:

• Purlin and truss condition, and any visible corrosion, deflection or previous repair
• The roof covering’s age and remaining service life — a membrane due for replacement in three years changes the economics of fixing solar to it now
• Whether the original structural design has any spare load margin, and how much
• Fixing method — penetrative bolted fixings versus non-penetrative ballasted systems, which load the structure differently
• Access and maintenance routes, and any rooflight or plant obstructions

Where original structural drawings exist, this step is faster and cheaper. Where they don’t — common on older industrial buildings — a structural engineer assesses load capacity from first principles. That adds time, but it isn’t optional. Guessing at load capacity on an ageing roof is exactly the shortcut a feasibility study exists to avoid.

The G99 grid application: process and realistic timeline

Anything larger than the smallest domestic-scale array falls under Engineering Recommendation G99. This is the Energy Networks Association’s standard for connecting generation to the local electricity distribution network. It’s the natural next gate after structural viability. A site can pass the roof survey but still fail if it can’t get a usable grid connection.

The process runs in stages:

• Initial enquiry. A preliminary check with the local Distribution Network Operator (DNO) — the regional company that owns the wires and substations the site would connect into — on whether headroom looks available in principle.
• Formal application. A completed application on the DNO’s standard G99 form, with the technical detail of the proposed system.
• Technical assessment. For larger commercial systems, the DNO studies local network capacity, fault levels and any reinforcement the connection would require. Only then can it issue a firm offer.
• Connection offer. A formal offer stating the connection terms and any reinforcement cost. Often the part that surprises buyers most is the connection date itself — it can land well after both sides agree the technical work, if the site needs reinforcement.

DNOs work to a statutory response window for that technical assessment. But the full path runs from first enquiry to a usable, accepted connection offer. That commonly takes a few months to the best part of a year. It depends on the DNO, local network headroom, and system size.

A well-connected site near a substation with spare capacity moves quickly. A site further from the network can take much longer. The same is true where local generation already uses up available headroom. That can mean a material reinforcement cost too.

Ofgem’s ongoing grid-connections reform package aims to speed this up over time. For now, though, the practical advice hasn’t changed: get the enquiry in early, as part of feasibility. Don’t wait until after a tender has gone out on the assumption that grid capacity isn’t a problem.

The standard everything gets specified against: MCS MIS 3002

Any UK-accredited installer designing, installing and commissioning solar PV work does so against MCS’s MIS 3002 standard. This is the technical benchmark covering system design, structural fixing, electrical safety and handover documentation. It underpins MCS accreditation and most funders’ and insurers’ requirements.

One detail is worth getting right, because it’s easy to misstate. MIS 3002 formally applies to installations up to a maximum DC output of 50kWp. Most commercial rooftop systems worth a proper feasibility study sit well above that threshold.

In practice, MIS 3002 still functions as the reference point for competent design and installation practice. That covers structural sign-off, electrical safety, and documentation. But a system above 50kWp isn’t a routine MCS job scaled up.

It needs specifying, commissioning and warrantying as a genuine bespoke commercial project. Referencing the relevant standard precisely in the specification is what lets you compare installer tenders like-for-like later, at procurement stage.

How long it actually takes

A single-site feasibility study, from instruction to a written go/no-go report, typically takes two to four weeks. That covers the desktop review, structural survey, shading study and an initial grid enquiry. It doesn’t include a full formal G99 application, which runs separately and longer.

It stretches beyond four weeks in three cases. First, a structural engineer may need to assess load capacity from first principles when no drawings exist. Second, a more detailed grid enquiry adds time when headroom looks tight. Third, assessing several sites together — as with a portfolio review — takes longer too.

Sometimes the study opens a formal G99 application, not just an initial enquiry. When that happens, its connection-offer stage runs on its own multi-month timeline in parallel. It doesn’t hold up the rest of the report.

What a real go/no-go recommendation looks like

This is what separates a feasibility study from a sales survey. A defensible go/no-go report states:

• The conclusion — proceed, don’t proceed, or proceed with a named condition (e.g. “viable if roof reinforcement is included in scope”)
• The site-specific numbers behind it — usable roof or land area, modelled annual generation, structural loading margin, grid position
• Indicative capex and payback range, stated as a range with the assumptions attached, not a single misleadingly precise figure
• The risks and assumptions that could change the answer — an unresolved grid reinforcement cost, a roof condition finding still pending specialist sign-off, a planning constraint
• What the specification needs to cover at procurement stage, so bids can be compared like-for-like

A report that only ever says “yes, proceed,” regardless of what the survey finds, isn’t independent. It’s a sales document with extra steps. Paying for a study rather than accepting a free survey has one real value: “no” is a genuinely available answer. And when the answer is “yes,” numbers back it, built for board or investment-committee scrutiny, not a supplier’s optimistic defaults.

Where this fits

A feasibility study is the first, cheapest gate in a commercial solar decision. It exists to stop weak projects before you commit to design fees, tenders or site visits from installers.

If you’re weighing whether a specific site is worth taking further, our feasibility studies service covers exactly this process. It includes the structural survey, grid review, indicative numbers, and a written recommendation for your board or investment committee. And we run it independently of any installer with a system to sell you.