What Makes Heat Pump + Solar Systems Perform Well in West Wales
Heat pump and solar PV systems can work exceptionally well together — or they can underperform significantly. The difference usually comes down to a handful of engineering decisions made at design and commissioning stage. This guide explains what those decisions are and illustrates them with lessons from our own West Wales installations.
Combining a heat pump with solar PV is increasingly common in West Wales, and the pairing makes good engineering sense: the heat pump runs largely during the day in milder weather, overlapping with solar generation, while the solar array offsets a meaningful share of the electricity the pump consumes.
That said, a poorly designed or commissioned system can produce a SCOP well below 3.0 regardless of how many kilowatts of panels are on the roof. The guidance below draws on MCS best practice, our own project records, and publicly available PVGIS data for West Wales irradiance figures.
The Single Biggest Lever: Flow Temperature
Flow temperature — the temperature at which the heat pump supplies water to the heating circuit — has more influence on seasonal efficiency (SCOP) than almost any other variable. Every 1°C reduction in design flow temperature typically improves COP by around 2–3%, so the difference between a 55°C and a 35°C flow temperature is substantial over a heating season.
Why Flow Temperature Matters So Much
Underfloor Heating
UFH typically runs at 35–45°C flow temperature. This is the ideal range for an air-source heat pump: the refrigerant cycle works efficiently, and weather compensation can modulate supply temperature smoothly in response to outdoor conditions.
- • Design flow temp ≤ 40°C is achievable in well-insulated homes
- • Weather compensation is straightforward to configure
- • Thermal mass of the slab can act as a buffer — useful for solar divert
- • Lower cycling risk than a small radiator circuit
Radiator Systems
Existing radiator systems were often sized for a gas boiler running at 70–80°C. Before installing a heat pump, the key question is: how large are the radiators, and how much heat do they emit at 45–50°C? In many cases, upsizing radiators in the key rooms (living areas, bedrooms) allows the flow temperature to drop enough for efficient heat pump operation.
- • Target ≤ 50°C design flow for good efficiency
- • Oversized radiators compensate for lower flow temp
- • Poor radiator sizing is the most common cause of low SCOP
- • A proper heat loss calculation (MCS/EN 12831) is non-negotiable
Weather Compensation
Weather compensation automatically reduces the flow temperature as outdoor temperature rises. Instead of running at full design temperature on a mild 8°C day in October, the heat pump supplies cooler water — improving efficiency significantly. Most modern heat pumps support this out of the box; the critical step is commissioning it correctly, setting the right heating curve for the building's heat loss and the emitter system.
Well-Commissioned System
- • Weather compensation enabled and curve set correctly
- • Flow temperature tracks outdoor conditions continuously
- • Long, steady run times at low intensity
- • SCOP significantly above 3.0 in a West Wales climate
Poorly-Commissioned System
- • Fixed high flow temperature regardless of outdoor conditions
- • Frequent on/off cycling, especially in mild weather
- • Lower efficiency, higher running costs
- • Often misdiagnosed as a "heat pump problem" when it is a settings issue
West Wales Climate: A Genuinely Good Environment for This Technology
West Wales sits in a maritime Köppen Cfb climate — mild, wet, and rarely either very cold or very hot. This turns out to be well-suited to both technologies.
Climate Conditions (PVGIS / Met Office typical for West Wales)
Solar Resource
- Annual irradiation: typically 950–1,050 kWh/m² for West Wales coastal locations (PVGIS typical meteorological year)
- Summer peak: June–August generation can be substantial, often producing significant surplus beyond immediate household demand
- Winter generation: Low but not zero — south-facing panels produce usefully on bright winter days
- Diffuse radiation: Cloud cover means a large fraction of annual yield comes from diffuse light, which panels harvest effectively
Heat Pump Climate Fit
- Winter temperatures: West Wales rarely sees prolonged periods below −5°C, the range where air-source efficiency drops sharply
- Mild shoulder seasons: March–May and September–November are the sweet spot — reasonable solar generation and low heating demand at relatively mild outdoor temperatures
- Summer: Heat pump typically runs only for hot water; solar can supply much of that energy via an immersion divert
- Humidity: Higher humidity does increase defrost frequency, but this is manageable with modern defrost logic
Seasonal Interaction Between the Two Technologies
Where They Complement Each Other
- • Spring/autumn: good solar generation coincides with active heating demand
- • Summer days: solar surplus can heat the hot water cylinder instead of the heat pump running off-peak grid electricity
- • Heat pump's daytime operation during mild weather overlaps naturally with peak solar output
Where They Don't (and What to Do About It)
- • Deep winter: high heating demand but minimal solar — the heat pump runs largely on grid electricity
- • Battery storage or a time-of-use tariff can help shift some overnight charging to cheaper/greener periods
- • Thermal mass (UFH slab, large hot water cylinder) can store solar energy generated during the day for evening use
Correct Sizing: Neither Too Big nor Too Small
Oversizing a heat pump is one of the most common installation errors. A unit that is too large for the building's heat loss will short-cycle — turning on and off frequently rather than running in long steady periods. Short-cycling reduces efficiency and causes wear. The fix is a proper heat loss calculation before any equipment is selected.
Sizing Principles
Heat Pump
- Capacity: Size to 100–110% of the calculated design heat loss (MCS / EN 12831 methodology) — not to the property square footage or the old boiler size
- Flow temperature: Design for ≤ 45°C wherever possible; lower if the emitter system allows
- Run time: Aim for long run cycles rather than frequent short bursts
- Hot water cylinder: 200–250 L for a 3–4 person household; larger if the solar divert will be used regularly
- Buffer tank: May be needed if total system water volume is low — prevents rapid cycling
Solar PV Array
- Roof area and orientation: South-facing at 30–40° pitch is optimal; southeast or southwest loses only a few percent
- Capacity: For a heat-pump household with higher-than-average electricity use, a larger array (6–10 kWp) is often worth considering
- Self-consumption: Adding a heat pump as a load increases the proportion of generation used on-site, which improves the economics of a larger array
- Battery storage: Shifts unused daytime generation to evening — particularly valuable where the heat pump runs after dark in winter
Smart Control: Making the Two Systems Work Together
A heat pump and solar array installed independently but not integrated will still deliver savings — but integrated control can improve results further. The core idea is to shift as much heat pump operation as possible to times when solar is generating.
Control Strategies That Work
Solar Divert for Hot Water
The simplest integration: an immersion divert controller detects surplus solar export and redirects it to the hot water cylinder. On a summer day this can heat the entire cylinder for free. In shoulder seasons it supplements the heat pump's DHW run. No complex integration with the heat pump controller is required.
- • Devices such as myenergi Eddi or Solic 200 handle this automatically
- • Works independently of heat pump brand or controller
- • Good return on investment for households with a large hot water demand
Pre-Heating During Peak Solar
On days when solar generation is forecast to be high, the heat pump can be scheduled to run its main heating cycle during mid-morning — using solar electricity directly rather than drawing from the grid. The building's thermal mass (especially a UFH slab) then holds that heat into the evening.
- • Requires a heat pump controller that accepts time schedules
- • Most effective in spring and autumn when heating is still needed but solar generates well
- • Some energy management systems (e.g. Solis, Sigenergy) can automate this based on live generation data
Time-of-Use Tariff Optimisation
For households on a time-of-use electricity tariff (such as Octopus Agile or Go), the heat pump can be controlled to run during the cheapest overnight periods in deep winter when solar is not available. A battery can be charged overnight and discharged during the evening peak. This strategy requires more sophisticated control hardware but can materially reduce annual electricity costs.
What You Need
- • A smart meter (mandatory for TOU tariffs)
- • A heat pump controller with time-window scheduling
- • Optionally: a home energy management system that reads tariff prices
- • Adequate thermal mass or buffer volume to carry stored heat
Realistic Expectations
- • TOU optimisation helps most in winter when solar contribution is low
- • In summer and shoulder seasons, solar divert delivers more benefit
- • The gains are real but depend heavily on tariff price spread and household usage pattern
- • Do not expect to eliminate grid imports entirely — overnight DHW and deep-winter heating will always require some grid electricity
Lessons from Our West Wales Projects
Two installations illustrate the engineering principles above in practice.
The Grove
Pembrokeshire — ASHP + UFH + Solar PV + Battery · Sykes Holiday Cottages Best New Property Award 2025
What Made It Work
- • Full UFH throughout — flow temperature designed to 29°C at CIBSE 99.6% outdoor design condition
- • Heat loss calculation carried out before equipment selection; the heat pump was sized to the building, not to rule of thumb
- • Weather compensation commissioned and tuned during the first heating season
- • Solar divert handles summer DHW; heat pump DHW runs are minimised June–August
- • Battery shifts lunchtime surplus to evening cooking and EV charging
Monitored Performance
- SCOP 4.41 — measured over the full 12-month period Aug 2024–Jul 2025 via Open Energy Monitor
- These are the only quantitative figures cited for The Grove in this article; further performance detail is on the Grove case-study page.
Wolfsdale
Pembrokeshire — ASHP + Upgraded Radiators + Solar PV
The Challenge
- • Existing radiator circuit originally designed for a gas boiler at 70°C
- • Key rooms had undersized radiators that would have required high flow temperatures to maintain comfort
- • Solution: radiators upsized in the main living spaces before heat pump installation
- • Design flow temperature achieved: ≤ 48°C on the coldest design day
Outcome
- Radiator upgrades combined with correct heat pump sizing and weather compensation delivered the efficiency improvement expected for a well-commissioned system.
- Solar self-consumption: ~71% (with battery) / ~38% (without battery) — monitored figures from the Wolfsdale installation.
- The solar PV contributes meaningfully to running costs during the spring and autumn heating season when solar generation and heating demand overlap.
What to Measure and Monitor
A system that is monitored can be improved; one that isn't is flying blind. The key metrics to track are straightforward and most modern inverters and heat pump controllers make them available.
Heat Pump
- SCOP (seasonal COP): heat output ÷ electricity consumed over the season. Most heat pumps with an energy meter can report this directly
- Flow and return temperatures: confirm weather compensation is working as intended
- Run time vs on/off cycles: excessive cycling suggests an oversizing or controls issue
- DHW vs space heating split: understanding what the heat pump is doing and when is the starting point for any optimisation
Solar + Battery
- Specific yield (kWh/kWp): total generation divided by array capacity. Compares your system against PVGIS expectations for the site
- Self-consumption %: how much of what you generate you actually use on-site
- Export: what goes to the grid and earns SEG payments
- Battery cycles and state of charge: ensures the battery is being used effectively and isn't being over-cycled
Summary: What Makes the Difference
In our experience, the installations that perform best share a consistent set of characteristics. None of them are exotic — they are all standard engineering practice applied correctly.
What the Best Systems Have in Common
- • Heat loss calculated properly before any equipment is chosen
- • Heat pump sized to the building, not to the old boiler
- • Emitters (radiators or UFH) sized to deliver design heat at ≤ 50°C
- • Weather compensation enabled and correctly commissioned
- • Solar divert set up to handle DHW in summer without the heat pump running
- • Performance monitored so problems can be spotted early
Common Causes of Underperformance
- • Oversized heat pump causing short-cycling
- • Radiators too small — forces high flow temperature
- • Weather compensation not enabled or incorrectly set
- • Solar and heat pump not integrated — missed self-consumption opportunity
- • No monitoring — poor performance goes unnoticed for months
- • Incorrect refrigerant charge from poor commissioning