As part of Innovate UK’s Net Zero Living programme, Energy Systems Catapult partnered with Belfast City Council, exploring new ways to accelerate rooftop solar deployment while improving the viability of a low‑carbon city centre heat network.

Building on earlier Net Zero Living programme work and the Belfast Local Area Energy Plan (LAEP), the project identified where rooftop solar could be installed – and even oversized – to supply surplus aggregated power to major energy users through a Portfolio Power Purchase Agreement (PPA).

Our engagement with stakeholders and innovators shows how LAEP-identified opportunities for scaling clean energy innovation can be turned into deliverable projects.

Key Facts

• Up to 95 MW of untapped rooftop solar capacity discovered
• Around 68 GWh of annual surplus electricity identified
• Indicative PPA price of 13.8-18.7p/kWh

The challenge

Belfast has significant untapped rooftop solar potential, yet deployment remains low. Most non‑domestic systems are sized only to meet on‑site demand, as export tariffs for surplus electricity offer limited incentives. At the same time, the proposed low‑carbon heat network faces high electricity costs, creating affordability and connection risks.

Previous Net Zero Living programme research led by Energy Systems Catapult identified the Portfolio PPA as a promising solution, but further evidence was needed to test whether it could work in practice and support delivery at scale.

The solution

Energy Systems Catapult brought together advanced energy system modelling, business model innovation expertise and market‑led innovation to move the Portfolio PPA from concept toward delivery.

We combined our unique local energy modelling capability with deep commercial insight to understand not just whether the Portfolio PPA could work, but how it would need to operate in practice.

Using half‑hourly demand and generation profiles drawn from previous Belfast LAEP analysis, we quantified surplus rooftop solar potential across the city and tested how aggregated supply could align with demand from large off‑takers including a low‑carbon heat network.

These technical insights were then handed directly to our business model innovation specialists, who mapped the Portfolio PPA end‑to‑end. This included defining roles across generators, portfolio managers, suppliers and off‑takers – identifying commercial assumptions and stress‑testing paybacks, pricing and value sharing.

By combining energy systems modelling and business model innovation, we pinpointed where real‑world barriers could undermine viability.

A critical challenge we identified was the availability and granularity of data, particularly the lack of widespread half‑hourly smart metering across behind‑the‑meter (BtM) solar assets. Rather than treating this as a blocker, we used it as a design challenge.

Drawing on the Catapult’s cluster‑style approach to innovation, we actively pulled in energy innovators to tackle these hard‑to‑treat elements of the transition.

Through an innovator hackathon, we invited market‑ready organisations to propose practical solutions to issues including behind-the-meter data access, portfolio coordination, system optimisation and cost reduction.

This mirrored our wider cluster work creating the conditions for innovators to engage with real system problems and accelerate solutions that can unlock scale.

Alongside this, we engaged generators, aggregators, suppliers and potential off‑takers to make sure the model is grounded in what it takes to get things done, ensuring regulatory, technical and commercial constraints in Northern Ireland were fully understood.

This project shows how LAEPs provide a trusted evidence base to underpin the scale up of clean energy innovation, helping to attract innovators to investment opportunities that maximise local value and resilience.

By combining system insight, commercial rigour and innovation pull – we supported Belfast City Council in progressing its clean energy ambitions from strategy to pilot delivery.

The impact

The assessment highlighted and prioritised which of Belfast’s non‑domestic buildings could host surplus solar. We discovered that oversizing rooftop solar systems can create large generation volumes that are financially attractive. The Portfolio PPA model demonstrated clear potential to:

• Unlock larger rooftop solar installations: from the top 100 buildings in Belfast with the greatest solar generation capacity, there is the potential for 95 MW of total installed capacity.
• Generate surplus energy for local use: of those 95 MW, 55 MW could deliver excess generation, leading to 68 GWh of surplus energy each year.
• Provide competitively priced renewable electricity: the excess energy could power a local heat work through a competitive PPA price of 13.8–18.7p/kWh.
• Strengthen investor confidence: through a scalable, place‑based model.

The project concluded with a clear recommendation to progress to a proof‑of‑concept pilot.

Debbie Caldwell, Climate Commissioner for Belfast City Council, said: “This work has helped turn an innovative idea into a credible pathway for scaling the uptake of rooftop solar in Belfast whilst also providing an opportunity for a local business to avail of 100% renewable electricity at a reduced rate.”

Reace Edwards, Senior Consultant at Energy Systems Catapult, said: “By combining energy and financial modelling with real‑world commercial design and hands‑on innovator engagement, we’ve shown how places can de‑risk emerging business models and turn LAEP‑identified opportunities into deliverable projects. This work doesn’t just unlock clean power for the city, it offers a practical model any area can use to accelerate its energy transition and attract Local Power Plan investment.”

What next?

The Catapult helps local authorities and innovators accelerate clean energy innovation, turning opportunities identified in local area energy plans into scalable projects. To start a conversation about how we can support you, email info@es.catapult.org.uk