We installed 258 panels in five weeks. In its first full calendar year on the meter, the system has generated 72,664 kWh and saved The Cambridge around £12,700 on electricity bills, with a further £444.84 earned from exporting the surplus to the grid. The original install was announced by The Cambridge in late 2024; this is what happened next.
The Cambridge Building Society is an independent mutual. It’s owned by its members rather than outside shareholders, and that ownership structure shapes how it makes decisions, sustainability included. Members are also customers, and their long-term interests sit behind most of the bigger calls the organisation makes. They wanted to a solar installation for its long standing business
The brief, when The Cambridge first came to us in 2024, was straightforward: improve the carbon footprint of the Newmarket Road head office, reduce reliance on the grid, and help them to action some of their sustainability goals.
Renewable energy sources such as solar offer a really effective way to make that positive change and our Head Office becoming solar powered is an important part of our sustainability journey.
As Facilities Manager, Matt understood about renewables and the benefits they can bring so by the time we sat down to quote, the question wasn’t if. It was how.
Our challenge: 116 kWp on a slate roof in central Cambridge
Solar arrays usually go on metal, tile, or composite roofs. The Cambridge had us working with slate, which is heavier, more brittle, and more expensive to repair if something goes wrong during install. On a substantial building in central Cambridge, with planning sensitivities to match, the margin for error narrows.
It was the biggest slate roof project we had taken on. Looking around the city centre, it’s also one of the biggest rooftop solar arrays in central Cambridge full stop.
The Cambridge head office project is the largest slate roof solar panel installation we have completed to date and is one of the largest rooftop solar panel examples in central Cambridge. It highlights perfectly what can be achieved in a commercial setting with relatively little disruption
The system in numbers:
- 258 panels across a complex slate roof
- 116.1 kWp installed capacity
- Two inverters
- Five weeks start to finish on site
- Zero operational shutdowns
The install
Five weeks on site, working around a live head office. Staff stayed at their desks, branches stayed open, members carried on being served. None of that paused.
The professionalism of the entire Spartek team really stood out to me - from start to finish of the project.
The Cambridge had spoken to two other installers besides us. We were recommended into the conversation, the proposal made the case clearly, and Matt and the team made their call.
Year one: 72,664 kWh on the meter
The system went live in September 2024 and generated 10,025 kWh across the four remaining months of that year. The first full calendar year, 2025, is now in the books.
Total generation in 2025: 72,664 kWh.
Roughly the same as powering 25 average UK homes for a year. The Cambridge used 80% of that generation on site, avoiding around £12,700 on grid electricity bills in year one alone. The remaining 20% was exported to the grid and earned a further £444.84. Total year one financial benefit: roughly £13,145.
As well as driving our sustainability ambitions forward, we have seen lower bills along with the ability to sell back to the grid unused produced power.
What a UK solar year actually looks like
Look at the chart. The best month (August) produced more than five times the lowest month (December). March and September both delivered surprisingly strong numbers thanks to clear, cool weather. Panels actually run more efficiently when the air is cold and the sky is bright, which is why a sharp March can occasionally outpace a cloudy June.
That seasonality is just the shape of UK solar. A 116 kWp system in Cambridge does its heavy lifting between March and September, still produces through winter at a lower base, and a commercial design needs to be sized and modelled with that profile in mind from the start.
What surprised Matt
He’d been championing the project internally for some time, so he came into the install without the usual concerns about disruption, payback, or whether the system would do what it said on the tin. What did get him, once the system was running, was the day-to-day reality of watching it work.
” Annual generation as a figure on a spreadsheet is one thing. Opening the monitoring portal on a clear August morning and watching kilowatt-hours stack up in real time is something else. That’s the part that has actually landed – how much the panels produce on good days of sunshine.”
Go for it.
For other commercial buildings considering solar.
A few takeaways from a year of running this system.
Slate, listed buildings, conservation areas and city-centre locations all add planning and design complexity. They rarely make a project impossible if you work with an installer who has done them before. They do mean the design conversation has to start earlier.
Year one performance is informative but not the whole story. A commercial solar system has a 25 to 30 year working life and pays itself back across that arc, not in 12 months. Year-to-year variance of 10 to 15% with the weather is normal.
The most valuable kilowatt-hour is the one you use on site, not the one you export. The Cambridge’s first-year split makes the point in pounds: 80% of generation used in the building delivered 97% of the financial benefit (£12,700). The 20% exported delivered just 3% (£444.84). Designing a commercial system around when the building actually draws power matters far more than maxing out the headline kWp figure.
Common questions
No. Five weeks on site, no operational shutdowns. The Cambridge stayed open throughout.
Yes. Slate is heavier, more brittle, and more expensive to repair than the more common commercial roof types. It needs more careful fixing and panel layout.. What does a 116 kWp commercial system actually generate?
Yes, particularly for buildings with daytime occupancy. Offices, headquarters and branches use most of their electricity at exactly the hours panels generate it, which means a high proportion of self-consumption and a stronger financial case.
Probably. We cover Norfolk, Suffolk, Cambridgeshire and the wider east of England for commercial solar projects. Get in touch and we’ll come and look.
