COP 4.8 in Scotland.
No borehole. No noise.
Nearly three years of proof.

What the numbers actually say

The data comes directly from HeatpumpMonitor.org — an open-source community platform run by OpenEnergyMonitor where heat pump owners share independently metered performance data. Barry Sharp's NIBE S1155 PVT installation in Broxburn, West Lothian is currently the only system of its type on the platform. The monitoring runs from June 2023 through to May 2026 — 954 days — with near-perfect data coverage throughout.

954 days is approximately two years and seven months. This is not a snapshot performance figure taken on a perfect spring morning — it is a sustained, whole-system average across two full heating seasons, including Scottish winters, including defrost events (of which there are none on a PVT system — more on that shortly), including the full annual cycle from cold January brine temperatures to warm summer ambient collection. A COP of 4.8 held over that period is one of the strongest sustained performances on the HeatpumpMonitor.org platform for any system type, anywhere in the UK.

How does 4.8 compare — and why does it matter?

The difference between 4.8 and the Electrification of Heat trial average of 2.81 is not academic. It represents a 71% difference in the amount of electricity consumed to deliver the same amount of heat. Put another way: a poorly installed heat pump achieving SPF 2.81 would have consumed 21,261 kWh of electricity to deliver the heat that this system delivered for 12,542 kWh. That is 8,719 kWh more electricity — at 24p/kWh, an additional £2,093 over the monitoring period. Over a typical heat pump lifespan of 20 years, the compounded difference approaches £16,000 in electricity cost for the same heating output.

What PV-T actually does — and why the roof becomes the ground loop

To understand why this system performs so well, it helps to understand what makes it different from both a conventional ground source heat pump and a conventional air source heat pump. The NIBE S1155 PVT is a ground source heat pump — but one that has no ground. Its heat source is a roof-mounted array of photovoltaic-thermal panels. Each panel simultaneously generates electricity from sunlight and extracts low-grade thermal energy from the ambient air and solar radiation via a heat exchanger on the panel's rear face.

The NIBE product manual summarises the key principle: the heat pump regulates the temperature of incoming brine in the summer and closes the shunt valve in the winter when the temperature reaches the set value. The system manages both seasonal extremes — in summer it harvests abundant solar thermal energy, in winter it extracts aerothermal heat from the ambient air even when the panels are cold.

The four reasons PVT outperforms ASHP — especially in Scotland

No defrost cycles. An air source heat pump in Scottish winter conditions must periodically reverse its cycle to melt frost that accumulates on the outdoor unit. Each defrost event consumes energy and temporarily stops heating the building. A PVT system has no outdoor fan unit. The brine circuit extracts heat from the panel passively. There is nothing to frost, nothing to defrost, no energy wasted on the reversal.

More stable source temperature. An ASHP's COP fluctuates sharply with outdoor air temperature — it performs well at 10°C and poorly at -5°C. The PVT panel's brine temperature, averaged over a 24-hour period, is more stable because it integrates solar gain, ambient temperature, and the panel's thermal mass. The brine arriving at the heat pump has already been partially warmed by the panel before the compressor does any work.

Cooling the PV panel improves electrical output. Standard PV panels lose approximately 0.4–0.5% of electrical efficiency for every 1°C rise above 25°C. On a warm summer day, an unventilated roof-mounted panel might reach 60–70°C, losing 14–18% of its rated output. The brine circulating through the PVT panel keeps it cooler, improving electrical yield by a documented 11% in optimal conditions. This means the system generates more electricity from the same roof area than standard PV would — partially offsetting the heat pump's electricity consumption.

No mechanical noise. ASHP units require an outdoor fan that creates noise — the source of many planning disputes and neighbour complaints. A PVT system operates in complete silence outdoors. The only moving parts are internal circulation pumps and the compressor inside the building. This is a material advantage in Scottish conservation areas, urban settings, and any home where outdoor noise is a concern.

What the numbers mean in pounds — and tonnes of carbon

The headline comparison is striking: at standard electricity rates, the heat pump running cost (£1,153/year) is already below the equivalent gas bill (£1,780/year) despite electricity costing roughly 3.4× more per unit than gas. That is the COP doing its work — 4.8 units of heat for every unit of electricity purchased more than compensates for the price difference between electricity and gas.

On an off-peak tariff, the case strengthens further. At Octopus Cosy (approximately 15p/kWh for six hours of cheaper-rate heating), the annual electricity cost falls to £721 — less than half the equivalent gas bill. At Agile prices on a windy Scottish day (when curtailed wind pushes wholesale prices near zero and Agile reflects this), the marginal cost of a unit of heating from this system approaches pennies.

The borehole you didn't need to drill — and why that matters in Scotland

A conventional ground source heat pump requires either a borehole (typically 70–100 metres deep, costing £8,000–15,000 per borehole for a domestic installation) or a horizontal ground loop (requiring approximately 500–700 square metres of garden at 1.2 metres depth). In Scotland's urban and suburban West Lothian, neither option is readily available to most homeowners. Broxburn is a town of mixed housing types — semis, terraces, new builds — where drilling through West Lothian's geology is expensive and garden space is limited.

The PVT array replaces the ground loop entirely. NIBE's own documentation indicates that the required collector area is approximately three times the heat pump output in square metres — so a 12 kW system needs approximately 36 m² of PVT panel area. That is a typical south-facing roof on a 1960s–1990s Scottish detached or semi-detached home. No drilling, no groundworks, no planning application for a borehole, no garden disruption.

What one installation in West Lothian says about Scottish heating

Scotland's Heat in Buildings Bill sets out a requirement that all homes move off fossil fuel heating by 2045. The Scottish Government has targeted one million low-carbon heating installations by 2030. Current heat pump installations in Scotland number approximately 278,000. The gap is large. The question of which technology pathway closes it is consequential.

The conventional wisdom has been that air source heat pumps are the default mass-market solution — lower upfront cost, simpler installation, no drilling required. Ground source heat pumps have been positioned as premium niche technology for new builds and high-budget retrofits. Barry Sharp's Broxburn installation challenges that framing in two ways.

First, it demonstrates that PVT can deliver genuinely exceptional performance — COP 4.8 — in Scottish conditions, without drilling, on a standard residential roof. The efficiency advantage over a typical ASHP installation is large enough to materially reduce running costs and carbon emissions over the system's lifetime.

Second, the data quality itself is a contribution. HeatpumpMonitor.org's near-perfect monitoring record — 100% data coverage on flow temperature, return temperature, electricity and heat output for most months — provides the kind of independently verified performance record that the heat pump industry desperately needs. Too many claims about heat pump performance are based on manufacturer test conditions or cherry-picked weather periods. This is 954 days of real West Lothian weather, independently metered, publicly accessible.