140 kW Hybrid Install on a Grade II Listed Specialist Designated Institution
- System size
- 140 kW + 100 kWh battery
- Annual saving
- £30,000
- Payback
- 6.8 years
- Location
- South of England
Scenario
A Specialist Designated Institution in the South of England — Grade II Victorian listed main building, modern 2005 accommodation block, and a 1960s teaching annex — operates a residential adult education programme for 220 students per year across the autumn-to-spring cycle. Annual electricity bill in 2025: £104,000.
The corporation board commissioned a Climate Action Plan in autumn 2025 with the new sustainability committee chair (a former civil servant and current trustee). The chair’s request was simple: a high-visibility climate action that respected the heritage estate.
This case study is an illustrative composite — institutional details are anonymised, but the design challenges represented (Listed Building Consent on a Victorian main, conservation-officer engagement, hybrid solution across heritage and modern buildings) reflect real SDI engagements.
What we delivered
Three-building hybrid design totalling 140 kW + 100 kWh battery:
- Victorian main building (Grade II listed): 28 kW — rear-elevation slate slope only, no visible panels from the public approach. Conservation officer approved after a pre-application meeting and heritage statement. 12° pitch, west aspect (acceptable yield loss in exchange for compliance with LBC).
- 2005 accommodation block: 75 kW — standing seam metal deck, south aspect, 16° pitch, ideal substrate. Rail-mounted system on standard mounting kit.
- 1960s teaching annex: 37 kW — flat membrane roof with rail-mounted ballast-free fixing. Roof was due for membrane refurbishment in 5 years; PV fixings designed to be lifted and re-laid during that refurb.
- Battery storage: 100 kWh in main plant room (LiFePO4 commercial chemistry).
The hybrid approach was essential: a pure-design 200 kW system on the main building would have failed Listed Building Consent. The distributed approach delivers 70% of the pure-design capacity at full LBC compliance.
Funding
100% Salix Decarbonisation Loan:
- Capital cost: £132,000 at £945/kW (mid-scale, with some heritage overhead on the main building)
- Loan term: 8 years
- Annual repayment: £16,500
- Modelled annual energy savings: £30,000 (the 24/7 residential occupancy drives strong self-consumption — well above sector average)
- Net cash-flow positive year one: £13,500/year
Salix approved the application in 10 weeks. The heritage element was disclosed in the application narrative but didn’t affect eligibility — Salix funds the energy savings calculation, not the heritage works premium.
Listed Building Consent process
The single biggest scheduling factor:
- Week 1: Pre-application meeting with local authority conservation officer
- Week 2-4: Heritage statement prepared (photographic record, design rationale, conservation impact assessment)
- Week 5: LBC application submitted
- Week 6-11: Statutory consultation period (no objections raised by Historic England or local heritage society)
- Week 12-13: Conditional consent issued
- Week 14 onwards: Build programme
The 11-week LBC process ran in parallel with G99 DNO application and Salix loan approval — no net delay to the project. Conservation officer was supportive throughout; the rear-elevation-only design and the wider Climate Action Plan context were both cited in the approval.
Compliance
- Listed Building Consent on main building (Grade II) — granted
- Asbestos R&D survey on all three buildings — clear status confirmed on the roof areas selected
- KCSIE 2025 not required (adult learners only; safeguarding still expected — installers DBS-cleared as a matter of course)
- ESFA Post-16 Audit Code applies (SDIs sit inside the central government sector since the November 2022 ONS reclassification)
- G99 DNO application for the combined 140 kW system; G99 connection accepted in 14 weeks
- Charity Commission compliance — the SDI operates as a charitable foundation; trustee board approval at the equivalent of a corporation board meeting
Outcomes
- Year one actual generation: 138,400 kWh versus modelled 132,000 kWh (+4.8%)
- Year one actual energy savings: £31,800 versus modelled £30,000 (+6.0%) — high self-consumption rate (68%) from 24/7 residential occupancy
- Featured as exemplar heritage-estate solar in the AoC Climate Action Roadmap launch event the following year
- Conservation officer commendation in the local authority annual heritage report
- Adult learner cohort designed an interpretive panel for the main entrance explaining the solar story and the heritage-sensitive design — installed as part of the centre’s visitor literature
What the corporation Director said
“Heritage-estate sustainability is often presented as either-or. Either you preserve the building and don’t do solar, or you do solar and damage the building. The hybrid approach gave us neither. The Victorian main carries a respectful 28 kW; the 2005 accommodation block carries the bulk of the generation; the 1960s annex picks up the rest. The conservation officer was actively supportive — they want to see heritage buildings being used and adapted, not embalmed. The build was sympathetic, well-detailed, and now part of the story we tell to incoming students.”