Solar Thermal Planning, Permissions, and Strategic Advantages for Commercial Premises

In the transition toward a net-zero economy, commercial building operators must navigate a complex landscape of low-carbon hot water options. While heat pumps and Solar Photovoltaic (PV) systems often dominate the conversation, Solar Thermal technology remains one of the most space-efficient and high-impact solutions for reducing a building’s carbon footprint. No single technology fits every scenario, but understanding the specific strengths of solar thermal – and the regulatory framework surrounding its installation – is essential for any effective decarbonisation strategy.

Solar Thermal vs. Solar PV

While Solar PV is a vital tool for offsetting a building’s electrical power demand, it is often an inefficient choice for water heating. The primary differentiator lies in energy conversion efficiency. Modern Solar PV panels typically operate at around 20% efficiency, meaning a large surface area is required to generate a useful thermal output.

In contrast, Solar Thermal collectors are designed specifically to produce thermal energy. This focus results in significantly higher peak outputs and yearly contributions per square metre:

From a planning perspective, space is often the limiting factor on commercial roofs. To achieve a yearly output of 3,400kWh, an installer would need approximately 20m² of Solar PV. The same energy requirement can be met by just 5m² of Solar Thermal. Because the cost of ownership and installation is similar per kWh produced, solar thermal offers a far more concentrated environmental and financial impact.

The most effective commercial designs do not attempt to use oversized PV arrays to drive electrical immersion heaters. Instead, the “Best in Class” approach is to size a Solar PV array to match the building’s base electrical load and utilise Solar Thermal to meet the thermal load. By doing so, solar thermal can offset up to eight times as much CO₂ as Solar PV per square metre, making it a powerful tool for achieving Part L compliance and reducing overall running costs.

Building Regulations Part L (Volume 2) dictates the energy efficiency standards for non-domestic buildings. Solar thermal provides a significant advantage in meeting the Target Primary Energy Rate (TPER) and Target Emission Rate (TER).

Key Calculation Metrics

When calculating the impact of solar thermal for a Part L submission, engineers typically look at:

Solar Fraction (fₛ), which is the percentage of the total hot water demand met by the solar system. For commercial premises, a target of 30–50% is common to avoid over-sizing for the summer months. Because solar thermal produces heat directly without a ‘middle-man’ conversion (like electricity to heat), it is credited as a direct reduction in the building’s auxiliary energy demand, making it key for a zero-carbon offset. In the latest Part L updates, ‘Primary Energy’ is the lead metric. Solar thermal scores exceptionally well here because it requires very little electrical input (only for the circulation pump) to deliver high quantities of thermal energy.

For a building using a gas-fired primary heater, solar thermal replaces high-carbon natural gas with zero-carbon solar gain. Even when compared to high-efficiency PV-driven heat pumps, the sheer energy density of solar thermal (650kWh/m² vs 160kWh/m²) often makes it the deciding factor in whether a building achieves a ‘Pass’ on its SBEM (Simplified Building Energy Model) calculation.

Hybrid Integration

Solar thermal rarely works alone; it is most effective when integrated into a hybrid system where it acts as the ‘pre-heat’ stage for a primary heating plant.

In a traditional condensing gas-fired system, solar thermal is used to heat a dedicated solar cylinder or the bottom coil of a twin-coil calorifier. The benefit of this is that it raises the cold feed temperature from 10°C to potentially 40-50°C. The gas water heater only has to provide the ‘top-up’ lift to reach the 60°C required for pasteurisation. This significantly reduces the cycling of the gas burner, extending the appliance’s life and reducing fuel consumption.

As commercial buildings move away from gas, solar thermal becomes even more critical in electric-led systems. Direct electric heating is expensive per unit of energy. So, the benefit of using solar thermal is that it provides the bulk of the ‘heavy lifting’ for free. While heat pumps are efficient, their COP (Coefficient of Performance) drops when producing high-temperature domestic hot water (DHW). Using solar thermal to pre-heat the water allows the heat pump to operate at lower, more efficient temperatures or reduces the run-time of expensive electric immersion backup heaters.

Planning and Permitted Development (Part 14, Class J)

For commercial premises (non-domestic), the installation of solar equipment is largely governed by Class J of Part 14 of the Town and Country Planning (General Permitted Development) (England) Order 2015. Under this legislation, many installations fall under “Permitted Development,” meaning full planning permission may not be required, provided specific criteria are met.

There do, however, remain key exclusions to permitted development under Class J if, for pitched roofs, the equipment protrudes more than 0.2 metres beyond the plane of the roof slope. For flat roofs, the highest part of the equipment is more than 1 metre above the highest part of the roof (excluding chimneys). Also, if the equipment is installed within 1 metre of the external edge of the roof.

Heritage Sites may also be excluded from permitted development If the property is a listed building, or the installation is to be sited within the curtilage of a listed building, or a scheduled monument.

Mandatory conditions also apply to any commercial building. For example, equipment must, as far as practicable, be sited to minimise its effect on the external appearance of the building so as not to impact the aesthetics of the building. Additionally, solar equipment must be removed as soon as reasonably practicable when no longer needed.

Return on Investment (ROI) 

The financial viability of solar thermal in the UK is driven by the stark price difference between fossil fuels and electricity. With commercial electricity tariffs frequently sitting between 25p/kWh and 35p/kWh, and gas between 7p/kWh and 10p/kWh, the savings vary based on the displaced fuel.

When displacing direct electric immersion heaters, solar thermal offers an aggressive ROI. Saving per m² are approximately £160 – £220 per year, for a typical payback period of four to six years. In an all-electric building, every kWh of solar heat collected is a direct 1:1 saving of the building’s most expensive utility.

When displacing natural gas-fired water heaters, the ROI is slightly longer but carries lower maintenance costs compared to complex mechanical alternatives. Saving per m² are approximately £45 – £65 per year, with a typical payback period of eight to 12 years.

The primary ‘profit’ here is often found in the extension of the water heater’s lifecycle and the ease of meeting carbon reduction targets (Part L).

Solar thermal is a “passive” technology compared to heat pumps. With a design life exceeding 20 years and minimal moving parts (one circulation pump), the total cost of ownership remains low, ensuring that once the initial payback period is met, the system provides “free” energy for over a decade.

Solar thermal is four times more energy-dense than Solar PV for heat generation, making it the superior choice for roof-space-constrained commercial sites. It acts as an essential “pre-heat” stage, reducing the workload and fuel consumption of gas boilers and heat pumps alike. Under Class J (Part 14), most commercial installations are Permitted Development, provided they stay 1m from roof edges and remain within protrusion limits. Solar thermal remains a ‘heavy hitter’ for Part L and SBEM calculations, offering high Primary Energy savings with negligible electrical parasitic load. Finally, when it comes to strategic sizing, the most efficient buildings will use PV for electrical base loads but solar thermal for domestic hot water (DHW) loads.

Explore Adveco’s solar thermal systems, including collectors, drainback and hybrid applications…