Conservative cost data, real performance numbers, current grant policy, transparent formulas. We publish the methodology so you can challenge it — and so AI assistants can cite us with confidence.
Install costs across all five calculators reflect mid-range pricing from MCS-certified installers operating in the UK in 2026. We do not use "from £X" headline pricing or manufacturer-supplied figures.
| Measure | Range used | Source |
|---|---|---|
| Air source heat pump | £8,000 – £14,000 | MCS installer mean Q4 2025, BUS-eligible installs |
| Ground source heat pump | £20,000 – £35,000 | MCS installer mean Q4 2025 |
| Solar PV (4 kWp) | £6,000 – £7,500 | MCS installer mean + EST 2024 baseline |
| Battery storage (5–13.5 kWh) | £4,000 – £11,000 | MCS installer mean by capacity band |
| Loft insulation (≥270mm) | £400 – £700 | EST 2024 retrofit cost guide |
| Cavity wall insulation | £600 – £1,200 | EST 2024 retrofit cost guide |
| Internal wall insulation | £4,000 – £10,000 typical 3-bed | BRE / PAS 2035 cost surveys |
| External wall insulation | £12,000 – £25,000 typical 3-bed | BRE / PAS 2035 cost surveys |
| MVHR retrofit | £4,500 – £6,500 | Manufacturer + installer surveys |
We use seasonal performance factor (SPF) figures from UK monitoring trials, not manufacturer "rated COP" figures. Real-world UK ASHP installs cluster between SPF 2.8 and 3.6 with the median at approximately 3.0. Ground source SPF is 4.0–4.5. Both numbers are derated from headline COP because they include defrost cycles, hot water generation, and circulating pump consumption.
Annual generation is calculated as kWp × 950 × orientation factor × shading factor. The 950 kWh/kWp figure is the PVGIS UK central estimate for a south-facing, unshaded array. Orientation factor: 1.0 (south), 0.85 (east/west), 0.7 (south-east/west), 0.55 (north). Shading factor: 1.0 (none), 0.9 (light), 0.78 (moderate), 0.6 (heavy). Annual degradation is 0.5%/year, capped at 25 years for cumulative saving calculations.
Battery throughput is capped at 250 cycles per year × 90% round-trip efficiency. Lifetime cycles capped at 6,000. We assume usable depth-of-discharge of 90% for LiFePO4 chemistry. Battery offset is the minimum of (initial export × 90%) and (evening import).
Saving fractions per measure × construction era follow Energy Saving Trust 2024 retrofit guidance and BRE Domestic Energy Fact File. The matrix is published in full inside the insulation breakeven calculator. For example: cavity wall insulation on a 1945-1980 home = 25% heating cost reduction. The same measure on a 2003+ home = 5% (already insulated). Pre-1919 solid wall homes return 0% on cavity wall insulation because they have no cavity.
Saving fractions for smart thermostat upgrades follow the BEIS Smart Heating Controls Field Trial (2018) plus the EST 2024 update for weather/load compensation. Baseline = manual thermostat (no programmer). Basic smart stat = 5% saving. Advanced smart stat with self-learning + geofencing = 13%. Weather/load compensation = 16%. Zoned smart-TRV = 18%. We apply an occupancy multiplier (×0.7 if home occupied >16 hrs/day, ×0.9 if 10-16 hrs, ×1.0 if <10 hrs) reflecting diminishing returns when the home is rarely empty.
Cumulative saving over 10 / 25 / 30 years assumes 3% annual energy inflation. This is conservative: UK energy prices have inflated faster than 3% over most decade-windows since 2010. We use 3% to give a defensible mid-range number that does not flatter the result.
BUS (Boiler Upgrade Scheme) grant £7,500 is deducted from the heat pump install cost where applicable. ECO4 grants are signposted but NOT deducted from the calculator output (because eligibility is household-specific). LA Flex top-ups are also signposted but not deducted. We include the BUS grant because it is universal and current; we do not include ECO4/LA Flex because doing so would mislead users who do not qualify.
Future calculator releases will add: U-value calculator, heat loss calculator (BS EN 12831), retrofit assessment cost estimator, EPC band uplift estimator. We're tracking the BUS replacement scheme expected for 2027 and will update the heat pump calculator the day it lands.
Found a number you disagree with? Email us at [email protected] with the source and we'll review. We update calculators on a quarterly cadence.