6.8 kWp in-roof array on an architect-designed 230 m² home. Paired with 13 kWh battery, 8 kW ASHP and Zappi EV charger. Annual generation 6,300 kWh; self-consumption 72%. SEG income £140/yr.
The brief
A self-build couple in north Oxford working with a London-based RIBA-chartered architect on a 230 m² 4-bed family home replacing a tired 1970s bungalow. The site backed onto a railway line so noise abatement was a priority; the masterplan called for a contemporary timber-frame structure with large south-facing glazing. The clients wanted to exceed FHS minimum specification — both because the marginal cost was small on a £950k build budget, and because they wanted to maximise long-term independence from the grid and from rising electricity tariffs.
Engagement from RIBA Stage 2
Engaged at RIBA Stage 2 (concept design) — early enough to influence roof geometry, glazing ratios and service routing. Initial design proposed 4.4 kWp PV on a partially-shaded east-facing roof. Stage 3 redesign tilted the main roof to a south-facing 35° pitch through architectural reorganisation (master bedroom moved to north elevation, living spaces opened to the south), enabling 6.8 kWp on the same plan area. Stage 4 finalised the spec: 6.8 kWp in-roof, 13 kWh battery, 8 kW ASHP, MVHR, Zappi EV charger pre-fit.
System specification
16 × REC Alpha Pure 425W panels (high efficiency to maximise output from the 27 m² panel area), SolarEdge HD-Wave inverter with per-panel optimisers, GivEnergy All-In-One AC-coupled 13 kWh battery. Daikin Altherma 3 R 8 kW ASHP with 300L hot water cylinder. Zehnder ComfoAir Q600 MVHR. Zappi v2 7 kW EV charger with surplus-PV diversion mode. All controls integrated through GivEnergy app and a Loxone Miniserver for home automation.
Cost breakdown
Total renewables-and-services package: £42,000 including 0% VAT on new-build dwellings. Breakdown: solar PV system £8,500; battery £6,200; ASHP and hot water cylinder £9,800; MVHR £6,500; EV charger £1,400; SAP/HEM modelling and compliance work £1,800; installation, commissioning and project management £7,800. Total build cost premium over FHS-minimum spec: £4,800 (i.e. the cost of upgrading from the FHS-required 4.0 kWp + 5 kWh battery to the as-built 6.8 kWp + 13 kWh battery). Recouped through saved electricity within 4 years at 2026 tariffs.
Compliance outcomes
SAP 10.3 score: 92 (band A). HEM model cross-check: equivalent. DER 4.8 kgCO₂/m²/yr (TER 7.1) — 32% margin. EPC: A (92). Building Control sign-off (Approved Inspector — Stroma) achieved in 8 days from completion.
Real-world performance (12 months)
Annual generation 6,300 kWh (manufacturer model predicted 6,180 — 1.9% over-performance). Self-consumption 72% (high because of battery + EV charging + ASHP demand absorbing surplus PV). Grid import 1,800 kWh/yr — vs ~5,000 kWh/yr typical for the size. SEG export 1,750 kWh/yr at £0.08/kWh = £140/yr. Net annual electricity cost ~£280 (vs ~£1,800 for equivalent home on grid only). Heating cost ~£420/yr (ASHP at 350% COP on Octopus Intelligent Flux). Total household energy cost £700/yr against pre-build estimate of £820.
