Residential customers often face a specific challenge when adding a garden studio or annexe: the structure is complete, the space is in use, but the energy costs are a separate problem that was never quite resolved. When Jaime Davis worked with Stuart Randall Construction to build a flat roof garden annexe, the building was well designed and well built. What it needed next was a solar PV system that could generate meaningful savings and feed any surplus back to the main property.
ProStar Energy were brought in to design and install the system once the construction and plumbing works were complete.
System Capacity: 6.5 kWp
Total Project Cost: £8,586
First-Year Savings: £1,020
Estimated Payback Period: 7 years 9 months (11.88% ROI)
Annual Carbon Offset: 1 tonne CO₂
Challenges and Solutions
Coordinating Around an Active Construction Programme
Solar installations on new builds and extensions require careful sequencing. By the time ProStar Energy were on site, the main construction works had been completed, which meant the installation team needed to work efficiently without creating disruption to the newly finished structure or the adjacent main property.
The flat roof configuration was well suited to a solar array. ProStar's engineers assessed the available surface area, confirmed the structural load requirements, and designed a 6.5 kWp system to make effective use of the space without overcomplicating the installation.
Maximising Value Across Two Properties
One of the most practical considerations on this project was ensuring that any generation surplus from the annexe could be directed back to the main house rather than exported to the grid at a lower rate. A backup battery was included in the specification to store excess generation, with the system configured so that any overflow beyond the battery's capacity flows through to the main property's supply.
This approach means the household benefits from the solar system across both buildings, improving the overall return on the investment.
Minimal Disruption, Clean Handover
Residential customers reasonably expect the installation process to be straightforward and unintrusive. Prostar's team completed the work with minimal disruption, keeping the site tidy throughout and handling the full commissioning and handover process on completion.
Key Results
The system delivered £1,020 in savings in its first year of operation, with an estimated payback period of 7 years and 9 months. The backup battery ensures surplus generation is captured rather than exported at a lower rate, with any overflow routed directly to the main house. The installation also offsets 1 tonne of CO₂ annually, contributing to the property's environmental performance from day one.
All manufacturer warranties are in place and tracked through ProStar Energy's CRM, alongside a two-year workmanship warranty backed by available servicing packages. The system is monitored and supported beyond the point of handover, which is how we approach every installation regardless of scale.
A Practical Investment for the Long Term
A 6.5 kWp system on a garden annexe is a considered installation rather than a large-scale one, but the principles are exactly the same as any commercial or residential project ProStar Energy takes on. The detail done upfront, including the site assessment, the battery specification, and the configuration to serve both buildings, is what determines how well the system performs over time.
With first-year savings of £1,020 and a payback period of under eight years, the project delivers a clear and measurable return. Modern solar panels are typically warranted to perform for 25 years or more, which means the financial case strengthens significantly beyond the payback point.
If you are considering solar PV for a residential property, the first step is a free site survey. Get in touch with the ProStar Energy team via the link below or call us on 03316 302 225 to arrange an assessment.
Testimonial
Glad we had ProStar Energy to fit solar panels with a backup battery to an annexe in our garden. All the workers were polite and we hardly knew they were there! The array is already working well with any excess from the annexe going back to the main house, which is a win for everyone. After commission service has been brill too, with a very comprehensive handover and any questions I've had have been answered quickly. Would highly recommend.
