95 kW Solar + 80 kWh Battery — Cardiff Sixth Form, Welsh Government Energy Service + Salix Cymru
- System size
- 95 kW
- Annual saving
- £22,500
- Payback
- 5.4 years
- Location
- Cardiff, Wales
Scenario
A standalone sixth form college in north Cardiff, serving roughly 850 students across A-level and Welsh-medium provision, was operating from a 1990s-built main block plus a 2008 sixth-form study-centre extension. The college’s 2024-25 electricity bill came in at £210,000, and the Welsh Government’s Net Zero Wales plan plus the Curriculum for Wales sustainability strand were both putting the corporation under direct pressure to publish a credible Climate Action Plan.
In Wales the funding architecture is different from England — Salix Cymru operates alongside the Welsh Government Energy Service (WGES), which provides free technical scoping, project development support, and access to a separate interest-free loan facility specifically for the Welsh public sector. The Sustainability Lead (the Deputy Principal carrying the portfolio alongside curriculum) had engaged WGES at the scoping stage and used their free technical work to firm up the feasibility before commissioning detailed design.
The college’s load profile carried a meaningful evening tail — adult community provision and Welsh-for-Adults classes run two evenings a week, plus exam-period revision sessions through the spring term. The WGES feasibility flagged that battery storage would materially lift self-consumption and shift the payback inside the corporation’s 6-year hurdle. The corporation board approved the bundled scope in February 2026.
This case study is an illustrative composite based on representative Welsh sixth form college engagements. Specific identifying details are anonymised.
What we delivered
Total scheme: 95 kW solar + 80 kWh battery storage, sized for the constrained roof envelope and the evening-tail load profile.
- Main teaching block (south-aspect pitched tile): 65 kW (120 × 545 W panels, 22° tilt, in-roof clamp mount on the 1990s clay tile, single string inverter in the main plant room)
- Study-centre extension (south-west aspect membrane): 30 kW (55 × 545 W panels, 10° tilt, mechanically fixed, single string inverter in adjacent service cupboard)
- 80 kWh battery storage system in the main plant room — sized to capture mid-afternoon excess generation and discharge through the evening-class window (17:30-21:00 two evenings a week, plus weekend revision sessions and exam-period extended hours)
- Live-generation and battery-state dashboard in main reception, with a secondary screen in the sixth form common area showing the day’s CO2 avoided as a learner-engagement piece
DBS-cleared install crews. KCSIE 2025 safeguarding induction by the Deputy Principal at programme kick-off. Welsh-language signage on the dashboard and the reception display (bilingual default per the college’s Welsh Language Standards).
Funding stack — Welsh Government Energy Service + Salix Cymru
The blended Welsh stack covered the full capex with no corporation cash contribution:
- Welsh Government Energy Service technical development support: in-kind (free feasibility, business-case authorship, technical specification) — saved the corporation an estimated £8,000 in external consultancy fees
- Salix Cymru interest-free loan: £88,000 (full capex), 8-year term, £11,000 annual repayment
- Battery element capex: £33,000 (loaded into the £88,000 total at £412/kWh installed)
- Solar element capex: £55,000 (loaded into the £88,000 total at £846/kW — lower-than-portfolio rate reflects the in-roof clamp mount on existing membrane and no scaffold-heavy access work)
- Modelled annual energy savings: £22,500 (£18,300 solar-direct + £4,200 incremental battery time-shift)
- Year-one net cash position: £11,500 positive (savings minus Salix Cymru repayment)
- Total 25-year nominal benefit: £390,000 (real-terms model)
The WGES technical pack went in alongside the Salix Cymru application as a single bundled submission. Salix Cymru returned acceptance in eight weeks. Corporation approval was recorded in the February 2026 board minute, with the Welsh Language Standards bilingual signage commitment minuted as a condition.
Programme and compliance
- Easter 2026 mobilisation, Summer 2026 build window: scaffold up 1 July, panels live 15 August, battery commissioned 22 August
- G99 DNO application to Western Power Distribution (now National Grid Electricity Distribution south Wales region) — accepted at 12 weeks with no export-limiter required at 95 kW
- Asbestos R&D survey: the 1990s main block returned a clean survey; the 2008 study-centre extension is post-2000 and cleared on documentation alone
- Structural engineer survey on both roofs — the 1990s clay tile required a 6 mm batten upgrade on two slopes, costed into the £88,000 total
- Welsh Language Standards: bilingual signage on the dashboard, the safety signage, and the reception display — designed before install, not bolted on after
- KCSIE 2025 safeguarding induction at every contractor mobilisation
- ESFA-equivalent Welsh audit framework: Salix Cymru loan agreement lodged as an auditable workpaper; corporation board minute formally records the loan
Outcomes
- Year-one actual generation: 3.8% above design model
- Year-one actual energy savings: £23,400 versus modelled £22,500 (+4.0%)
- Battery self-consumption performance: lifted total self-consumption from 76% (PV-only) to 93% with the 80 kWh battery active; the evening-class load was almost entirely covered by stored generation through the spring and summer terms
- EAUC Scorecard: moved the college from “Bronze” to “Silver” on the Operations and Estates module
- Curriculum for Wales integration: generation and CO2-avoided data feeds were adopted into the college’s A-level Geography and Environmental Science schemes of work, with a Welsh-medium version of the dashboard CSV export commissioned in spring 2027
- Net Zero Wales reporting: the project was submitted to the Welsh Government’s annual public-sector decarbonisation return as a lead exemplar in the FE sixth form category
- Phase 2 scoping opened in autumn 2026 around adding 30 kW on a new sports-hall extension scheduled for 2028, with WGES already re-engaged at the feasibility stage
What the Deputy Principal said
“What the Welsh Government Energy Service unlocked for us was speed and confidence at the front end. We didn’t have to commission an external feasibility, we didn’t have to write the business case from cold, and we walked into the Salix Cymru application with WGES’s name on the technical pack. That’s the difference between a corporation board sceptical about an unfamiliar project and a corporation board signing off in one minute. Welsh sixth forms not engaging WGES at the scoping stage are working harder than they need to.”