Most commercial rooftop solar installations take around one to four weeks on site, depending on the size and complexity of the system.
That is the physical installation. It is not the project.
Our largest commercial installation to date, 228.8kWp at Wenzel’s the Bakers in Watford, took two weeks on the roof. The DNO approval for that same job took fourteen.
If you are planning a commercial solar project, that ratio is the single most useful thing on this page.
Key Takeaways
- Most standard commercial rooftop installations take one to four weeks on site.
- The full project, survey through to switch-on, is typically three to four months, and the DNO approval is usually the longest single stage.
- On ground-mounted systems and sites in protected areas, planning can take longer still.
- Most businesses can keep operating throughout the installation.
- Weather affects rooftop work in both directions, wind and ice in winter, extreme heat in summer.
- A proper physical survey before installation is the best protection against avoidable delay.
A Real Commercial Timeline: Wenzel’s the Bakers, Watford
228.8kWp. 558 Canadian Solar 410W panels. Two Solis 100kW inverters. Around 188,000 kWh a year.
Here is how the programme actually ran.
| Stage | Duration |
|---|---|
| Site survey | 1 day |
| Design and back-of-house work | 2 days |
| Structural engineer’s report | 4 weeks |
| DNO application and approval | 14 weeks |
| Scaffold | 3 days |
| Installation on the roof | 2 weeks |
| Commissioning and testing | 1 day |
| Paperwork, handover and client demonstration | 2 days |
Two weeks of that programme was panels going onto a roof. The structural report and the DNO approval ran alongside each other, but between them they accounted for the overwhelming majority of the elapsed time.
Nobody plans a commercial solar project around the fortnight the installers are on site. They plan it around the fourteen weeks nobody warned them about.
Which Stage Takes the Longest?
On commercial rooftop projects, it is almost always the DNO.
A 50kW system on our patch currently takes around ten weeks for approval through UK Power Networks. Wenzel’s, at more than four times that size, took fourteen. The timescale is not entirely predictable and depends on the system size, the existing supply to the building, and the state of the local network.
The exception is ground-mounted systems, and anything in an area of outstanding natural beauty or a similarly protected setting. Those need planning permission, and planning can run to six months. On those projects planning is the longest stage of all.
The sequencing mistake that costs a full quarter
We see this regularly. A business submits its planning application and waits. Planning is granted after several months. Only then does the DNO application go in, and that adds another ten to fourteen weeks on top.
Those two processes can run in parallel. We start the DNO alongside planning as standard, and we will do it before a job has been formally awarded.
What we tell clients – We are happy to prepare and submit the DNO application for you. It does not tie you to using us, and it does not commit you to installing. What it does is start the clock, and on a commercial project the clock is the thing that costs you.
What Decides How Long the Installation Itself Takes?
Two businesses could both be installing a 200kWp system and have completely different programmes.
One might have a single accessible roof with straightforward cable routes and space for lifting equipment. The other might have several roof areas, busy loading bays, and the electrical intake at the opposite end of the building.
Same size system. Different job.
| Factor | What it means for the programme |
|---|---|
| System size | More panels and mounting equipment generally means more time on the roof. |
| Roof construction | Flat, standing seam and trapezoidal roofs all need different mounting methods. |
| Roof access | Scaffold, MEWPs or lifting equipment have to be planned in. |
| Number of roof areas | Several arrays usually means more installation stages and more cable routes. |
| Electrical work | Switchgear changes, long cable runs or export limitation can add significant time. |
| Site operations | Work may need to fit around production, deliveries, staff or customers. |
| Weather | Rooftop work stops when conditions are not safe — in either direction. |
This is why we would rather see the building before telling anyone exactly how long their installation will take. An estimate based on system size alone tells you part of the story.
Does the Business Have to Close?
Usually not. Most of the work happens on the roof, so a commercial solar installation can normally proceed while the business carries on.
Wenzel’s is a good example, because a bakery does not stop.
We stayed out of everyone’s way and kept the installation deliberately lean. We were in regular contact with the management team throughout and worked out when the quietest shifts were, then scheduled the internal cabling work for those hours. For the AC connection, we agreed a specific time for the power shutdown well in advance.
It went to plan, and the disruption to production was minimal.
That is the point. There will be some disruption on any commercial installation, panels have to arrive, installers need roof access, lifting equipment needs space, electricians need access to the building’s electrical infrastructure. What matters is that it is planned rather than discovered.
Before installation starts you should know where contractors will be working, which areas need to stay clear, when deliveries are happening, and whether there will be any interruption to the electricity supply.
Can Weather Delay a Commercial Solar Installation?
Yes, and in both directions. Most people think about winter. Summer has caught us out more recently.
Wind is the winter problem, not rain
Commercial panels are large, around two metres by one metre. They catch wind easily, and handling one at height in a gust is a genuine hazard. We will work in a degree of rain. We will not put people on a roof handling large panels in high wind.
HSE data shows falls from height accounted for 8% of non-fatal employee injuries reported under RIDDOR in 2024/25, and work at height has to be properly planned, supervised and carried out safely. A realistic programme has to allow for weather days.
Heat is the one nobody plans for
We recently completed a commercial installation close to our Hemel Hempstead office during the UK heatwave. Roof temperature hit 36°C. The panels were too hot to handle comfortably even through gloves.
We are not going to have people working full days in that. We started early, built in frequent hydration breaks, and finished earlier each day. It slowed the job down.
That was outside our control and we would make the same call again. The health of the team on the roof is not something we trade against a programme date.
Can Electrical Work Extend the Programme?
Yes, and this is the part most often underestimated.
A commercial solar installation is not panels on a roof. The system needs inverters, isolators, cabling, protection equipment and a connection into the building’s existing electrical infrastructure. On a straightforward site that work is relatively simple. On another, the cable route might be long, switchgear may need changing, or the work may have to happen outside operating hours.
Export limitation is the one that catches people out. If the DNO requires it, and on constrained parts of the network that can mean zero export, a commercial system usually needs a separate export power manager, current transformers at the origin, an enclosure and a hardwired data run back to the inverters. On a retrofit, that cable route can be one of the more expensive parts of the job.
It looks like a setting. It is an electrical installation.
Because Synergy Group Solutions is an established commercial electrical contractor with NICEIC-approved electricians, that work stays in-house rather than becoming a second contractor to coordinate.
What Should Be Sorted Before the Team Arrives?
| Before the team arrives | Why it matters |
|---|---|
| DNO approval received | We will not install without it. Neither should anyone else. |
| Structural engineer’s report passed | Confirms the roof can carry the array. Mandatory on every commercial job we do. |
| Roof construction evidenced | Established from underneath, not inferred from the top surface. |
| Access agreed | Scaffold, MEWPs and lifting equipment organised properly. |
| Delivery areas confirmed | Stops equipment deliveries clashing with normal site activity. |
| Cable routes and inverter locations agreed | Prevents redesign once electrical work begins. |
| Electrical requirements understood | Any upgrades, export limitation or shutdowns can be planned. |
| Business operations considered | Installation can work around production, staff and customers. |
Most avoidable delays are not caused by fitting panels. They are caused by something on that list not being sorted beforehand.
What Happens After the Panels Are On?
Panels on the roof does not mean the job is finished. The electrical installation has to be completed, the system tested and the protection verified before commissioning. Monitoring needs to be working so the business can see what the system is generating.
Then there is the part that runs for the next twenty-five years.
On Wenzel’s, we are still on site regularly. We visit monthly for basic checks and a visual inspection. The array is cleaned quarterly by our specialist partner, because the site attracts a significant build-up from birds and surrounding vegetation. We carry out a deeper inspection and hardwire testing annually.
Remote monitoring runs continuously and we review it at least monthly across every commercial system we have installed. If something drops off, we would rather find it than have the client find it.
How Long Should You Allow?
Allow one to four weeks for the physical installation on a typical commercial rooftop.
Allow three to four months for the project.
A straightforward job can be quicker and a large or complicated site will take longer, but the honest answer is that the installation is the short part. Roof access, structural approval, DNO consent, cable routes, electrical work and business continuity all need working through before anyone climbs a ladder.
That is how we approach commercial solar. We physically survey every site before finalising the design, because what looks straightforward on a drawing or a satellite image is not always straightforward when you are standing underneath it.
Synergy PV Renewables installs commercial solar across Hertfordshire, Bedfordshire, Buckinghamshire and North London. For a realistic programme based on your actual building, call 01442 921055 or email hello@synergypvrenewables.co.uk.
Frequently Asked Questions
How long does a commercial solar project take from start to finish?
Typically three to four months. On our 228.8kWp installation at Wenzel’s the Bakers in Watford, the physical roof installation was two weeks, but the DNO approval alone took fourteen weeks, with a four-week structural report running alongside it.
How long does a 100kW commercial solar installation take?
There is no reliable figure based on capacity alone. A 100kW system on a straightforward accessible roof can go up quickly, while the same capacity spread across several difficult roof areas takes considerably longer. Access, roof type and electrical work all matter more than the headline kW.
What is the longest part of a commercial solar project?
Usually the DNO approval. A 50kW system currently takes around ten weeks through UK Power Networks. On ground-mounted systems or sites in protected areas, planning permission takes longer still and can run to six months.
Can my business operate normally during installation?
Usually yes. At Wenzel’s we worked around the bakery’s quietest shifts for internal cabling and agreed the AC shutdown time well in advance. Most rooftop work can proceed while the business carries on.
Will my electricity need to be switched off?
Potentially, for a short period when the system is connected. If an isolation is required it should be identified and agreed before installation begins so it can be scheduled for the least disruptive time.
Can commercial solar be installed at weekends?
Yes, where it makes sense, particularly for electrical isolation work or areas that are difficult to access while the business is operating.
Does bad weather delay installation?
It can, in both directions. High wind is the main constraint in winter, commercial panels are roughly two metres by one and catch the wind badly. Extreme heat is an increasing issue in summer; on a recent Hemel Hempstead installation at 36°C we shortened working days and built in hydration breaks.
