Methodology verified 22 June 2026DfE SEMSPSDS Phase 4Ofgem Q3 2026 cap (gas 7.33p / elec 26.11p)
reference / methodology

Methodology: assumption register

Author: Oliver Wakefield-Smith, Founder Digital Signet. Reviewed against DfE Sustainability Strategy (Mar 2025) and Education Estates Strategy (Feb 2026). Verified 22 June 2026.

The methodology behind schoolnetzero.co.uk is published as an assumption register, modelled on a Salix bid appendix rather than as a prose narrative. Each row (A1 to A24) carries the assumption, the value, the primary source URL and the last-verified date. Auditors can cross-check the worksheet output line by line.

Assumption register A1 to A24

IDAssumptionValueSourceVerified
A1ASHP installed capital£2,000 / kW (range 1,500 to 2,500)Salix PSDS application data2026-06-22
A2Heat-loss coefficient (W/m2) by age band95 / 85 / 75 / 55 / 40CIBSE practitioner / DfE CDC22026-06-22
A3Regional capital multiplier0.94 to 1.18 (Barbour ABI 2025)Barbour ABI regional indices2026-06-22
A4Fabric upgrade £ /m2 by age band380 / 320 / 240 / 160 / 90Salix PSDS application data + Passivhaus Trust2026-06-22
A5Solar PV rooftop schools anchor£1,100 / kWp installedSolar for Schools published data2026-06-22
A6LED relamp anchor£25 / m2 (range 18 to 32)Salix PSDS + CIBSE LG52026-06-22
A7BMS flat capital by phasePrimary £38k / Secondary £70kCIBSE TM39 + Salix PSDS2026-06-22
A8DNO connection upgrade contingency£90k (range 30k to 150k)Energy Networks Association 20262026-06-22
A9PSDS Phase 4 per-project cap proxy£5,000,000Salix PSDS2026-06-22
A10Salix CIF per-school per-year cap proxy£4,000,000DfE CIF guidance2026-06-22
A11Solar PPA fraction of CAPEX replaced100 per cent (PPA carries CAPEX)Solar for Schools2026-06-22
A12Ofgem Q3 2026 default cap gas7.33 p/kWhOfgem default cap2026-06-22
A13Ofgem Q3 2026 default cap electricity26.11 p/kWhOfgem default cap2026-06-22
A14Gas boiler seasonal efficiency0.88CIBSE Guide H2026-06-22
A15ASHP seasonal coefficient of performance2.9 (design-day)CIBSE Guide F2026-06-22
A16UK grid carbon intensity 20260.165 kgCO2e/kWhBEIS / DESNZ Greenhouse Gas Conversion Factors 20262026-06-22
A17Natural gas carbon intensity0.184 kgCO2e/kWhBEIS conversion factors2026-06-22
A18Solar PV annual yield per kWp (UK schools roof)880 kWh / kWp / yrSolar for Schools / MCS reference2026-06-22
A19School heating intensity (kWh/m2/yr)90 (gas boiler baseline)CIBSE TM46 benchmark2026-06-22
A20School lighting saving (kWh/m2/yr) from LED relamp8 kWh/m2/yrCIBSE LG5 + Salix PSDS application data2026-06-22
A21ASHP utilisation factor0.85CIBSE design-day practitioner factor2026-06-22
A22Primary typical GIA1,800 m2DfE Area Guidelines BB1032026-06-22
A23Secondary typical GIA7,500 m2DfE Area Guidelines BB1042026-06-22
A24Payback horizon for Salix gateway15 yearsSalix PSDS Phase 4 payback gateway2026-06-22

Grant offset application

The grant offset rule depends on the selected grant stack: PSDS only applies the PSDS Phase 4 cap (A9); PSDS plus Salix sums the PSDS cap (A9) and the CIF cap (A10) per school per year; Blended PPA applies the PSDS cap plus the solar element multiplied by the PPA fraction (A11). The output is always net of grant and ex-VAT.

Regional multiplier source

The regional multiplier (A3) draws from the 2025 Barbour ABI regional construction cost indices, which is the benchmark commonly applied in Salix PSDS bids and DfE Construction Framework call-offs. The multiplier ranges from 0.94 in Northern Ireland to 1.18 in London.

Related
Sources cited on this page
  1. [S2]Salix PSDS
  2. [S21]Treasury Green Book
  3. [S3]Ofgem default price cap Q3 2026
Prepared by schoolnetzero.co.uk, 22 June 2026.Page: methodologyAuthor: Oliver Wakefield-Smith, Founder Digital Signet.