Unlike arable land, where most solar farms are situated, these vast airy expanses of cladding or tile cannot be used for agriculture or wildlife restoration, and impractical for other uses such as car parks.
According to a report published in August 2022 for the UK Warehousing Association by Delta EE consultants, UK warehouse roof area alone (excluding industrial buildings) is theoretically large enough to support production of 15GW of electricity production, which equals approximately the entire current solar photovoltaic (PV) installed base in the UK (15.5GW).
Rooftop solar also benefits the building’s users. The report added: “Solar PV can reduce annual electricity costs by 40-80% and protect occupiers against future electricity price rises while preparing for increased demand from electrification of heat and transport.”
The topic was discussed in May, in the first meeting of a new public-private Solar Taskforce convened to support achieving the government’s target of 70GW of installed solar PV by 2030.
Co-chair of the taskforce, and chief executive of trade body Solar Energy UK Chris Hewett said at the time: “Installing rooftop solar power, whether at residential or commercial scale, is one of the best investments available... The benefits can also be greatly enhanced by adding a battery storage system.”
Minister for energy security and net zero Graham Stuart said: “This new dedicated solar taskforce will have a laser-like focus on cutting the costs and breaking down the barriers to harnessing the power of the sun in every way we can, all while using a small fraction of this country’s land.”
The taskforce is scheduled to publish an update in October, with the final report in February 2024.
BARRIERS TO OVERCOME
As all of this activity might suggest, installing solar panels is not always straightforward. The first hurdle is being able to afford to purchase them in the first place. The UKWA report estimates that at least 70% of solar PV costs are up front. There are currently no government subsidies; they were withdrawn in 2019.
The next hurdle is getting a connection. Solar Energy UK senior communications adviser Gareth Simkins explains: “The central problem with rooftop solar is, however much commercial premises like to put solar on rooftops, the larger the designed installation, the less likely it is to be able to go ahead immediately because of the grid access challenge.”
By that he is referring to the fact that a distribution network operator (DNO) may have to defer grid connection requests for years in some cases, because of lack of capacity.
He clarifies that a grid connection application must be made (by the installer) even if the intention of the panels is to feed on-site processes. The application must cover the power capacity of the solar array, adding to that the capacity of the battery, if fitted (“which makes no sense, because the battery will be discharging when solar isn’t,” observes Simkins).
That most of the power will be used on site cuts no ice with the DNOs, the communications adviser clarifies. “You’d think that it would be okay if the sites are not exporting power, but some grid operators are very wary of this kind of proposal, and they will not accept it.
“We do know that some installations go ahead without any formal approval, and they are very much a grey market to say the least; that is not something that we would encourage.”
He is careful to point out that despite the grid connection offering the technical possibility to sell solar power back to the grid, doing so is unlikely to generate much revenue – partly since if your site is receiving sun, so are other larger ones nearby, reducing its comparative value. “The financial benefits of installing PV accrue almost entirely from the savings that they achieve, rather than money made from exporting power to the grid,” he points out, although adding that the organisation does track power rates (see www.is.gd/hizive).
This poses one of the biggest economic problems for making a case for mounting solar panels on warehouses: they don’t tend to use much power. A related issue is that the landlord might be different to the user, so financing the investment and ongoing costs may require legal agreements. Still, there are cases where solar power arrays on one site have powered neighbours; Nissan’s Sunderland car plant is one example, drawing 20% of its power needs from the 20MW Hylton Plantation Solar Farm.
There are other ways to pay for solar panels. One alternative funding arrangement is known as a power purchase agreement, in which users of the power, rather than owners of the building, pay into a scheme that covers the installation costs. Simkins explains: “It’s more relevant in community energy projects. If a school wants to cut its bills by adding solar panels, but doesn’t have the cash, it would tap into a community energy body that makes a share offer to the wider community that pays for the installation, and then the school pays for the power.”
MORE ABOUT PANELS
How many panels are required depends on the power need. Photovoltaic panels’ output is usually rated under standard test conditions: 1,000W/m2 incident light and a panel temperature of 25°C at sea level (STC). Another is NOCT: nominal operational cell temperature.
Both are unrealistic, according to Simkin, because panels heat up in the sun, and as they do so, their energy conversion efficiency declines. Since temperature correlates with kinetic energy of the atoms making up a PV panel, the faster-moving targets are harder to hit by incoming photos, making energy transfer more difficult. More realistic therefore is NMOT, nominal mode operating temperature, which includes not only solar irradience and air mass, but also back-of-module energy levels and wind speed.
Panels should last at least a generation. However, older panels are not as productive, because they do degrade in performance over time – around 0.5% per year – due mainly to heat cycling and UV radiation.
Panels do still generate power at reduced levels on overcast and rainy days, but at reduced levels. Simpkins uses as an example Saturday 22 July 2023 – an uncommonly dull and rainy day – power output was about two-fifths of what one would normally expect.
Simpkins points out that solar installations benefit from economies of scale in terms of purchase of the panels and their installation. He adds: “It is always cheaper to do the second kW or MW than the first. That’s half of the reason solar farms exist,” referring to large sites with thousands of panels arranged in rows across a field.
Solar farms over 50MW capacity are deemed nationally significant infrastructure project, so is subject to specific requirements. But for small-scale installations, regulations are much more relaxed. In England, commercial-scale solar installations are permitted development, up to a capacity of one megawatt (which is equivalent to 2,000 500W-capacity panels). In Scotland, the number is 50kW (or one hundred 500W panels), but there is a consultation out to remove that. In Wales, no particular capacity is named.
Generally speaking, over the course of the year, the most solar radiation occurs in the south and east of the UK; that means that the same solar PV panel in the South West will generate 30% more power than in the Shetlands (see map, p21). There are online calculators that predict (www.is.gd/akoril) and record (www.is.gd/ibehok) solar incidence by location and per day.
Those interested in mounting solar panels on rooftops would need a survey to confirm structural suitability. Simkins says that flat and tilted roofs can both be suitable. South-facing roofs are preferable, although those with an eastern or western aspect will work. Access will be needed occasionally for cleaning and inspection.
BOX: MAINTENANCE
Like any piece of electrical equipment, solar panels require regular inspection, according to solar panel installer Photon Energy. “The most common problem is reduced system output, undermining your investment returns. For example, the loss of one or two strings in a large system can be difficult to spot,” the company points out. It adds that a remote monitoring system can pick up on this – larger systems should include them, but they can be retrofitted. Also, it adds, they aren’t a substitute for a periodic check.
In 2022, IET published its second edition of a code of practice for grid-connected solar PV. The £76 guide covers protection and earthing, and requirements for commissioning, monitoring and maintenance. Another relevant standard is the BS7671 wiring requirements. Both are used by some installers, such as Photon Energy, both for its own work and to check others’. In addition, a free guide specifically for rooftop units was published in 2021 by a Solar Networks work group (www.is.gd/iwequb). This is its second edition of its own operation and maintenance guidance to rooftop installations. It includes an inspection checklist based on BS 7671 IET Wiring Regulations for domestic, schools and hospitals, and commercial property (though it adds that product and manufacturer guidelines should be checked too). On that list, almost every frequency for inspection tasks for commercial systems (‘commercial, industrial, agricultural/other’) is annual. There is one exception: checks of the system inverter (which receives power from the panels and changes it into a usable form) should occur weekly, or daily if remote monitoring is available.