Whole life costing: Solar hot water systems

Peter Mayer

Will the new Part L see mass up-take of
solar hot water systems?
Peter Mayer of Building LifePlans looks at
the specification options and whole life costs.


On the face of it solar hot water systems are an attractive proposition: they can provide up to 60% - 70% of the energy for hot water in the average home. In the spectrum of energy conservation measures installation of solar hot water systems are a long term option.

The real issue for solar hot water systems in the UK is that the period they are needed most — the winter months of November to February — is the time when there is the least solar energy available. Options to overcome this include increased hot water storage or increased area of solar collectors.

Nevertheless, the legislative and economic environment seems to be swinging the balance in favour of solar energy systems. Fuel prices are likely to increase. Part L, implemented in 2006, encourages the use of low and zero carbon energy supply systems such as solar hot water systems as they can make a substantial contribution to meeting the Target carbon dioxide Emission Rates.


Specification options

Solar hot water systems are commonly categorised by solar collector type. The two most common are flat plate and evacuated tube solar collectors. BS EN 12976 is the British Standard which specifies durability, reliability and safety requirements for factory made solar systems. Assurance is given for resistance to freezing, internal corrosion, thermal performance and pressure. Ancillary components such as heat exchanger, pipework, circulation pumps, hot water storage, controls, and valves are defined by reference to other European standards. The standard for solar collectors is BS EN 12976.


Flat plate solar collectors

These are flat panels or planks which can be integrated into or be a substitute for traditional roof coverings.

Panels are assemblies 0.7 – 8mœ, generally square or rectangular set in a box or frame with insulation, comprising: an absorber plate usually of aluminium and copper combined or copper. The panel is covered by glazing, typically toughened glass, single or double glazed with or without enhanced surface coatings such as low reflection glass. Polycarbonate plastic is an alternative.

Service lives between 10 and 25 years can be expected. Glazing system may need to be replaced during the service life. Panels cost in the range of £200 – 600/m˙ for metal and glass assemblies, plastics panels are generally cheaper.

Planks can be manufactured in lengths to span the roof and are typically of powder coated aluminium hollow sections connected with hoses.

Aluminium planks should give in excess of 60 years performance provided the system is designed to avoid internal corrosion. The powder coating will need replacing at 20 – 30 years depending on the exposure and rate of colour fading to maintain absorption. See cost table for whole life costs.


Evacuated tube solar collectors

Evacuated tube solar collectors are more efficient than flat plate collectors but more expensive and are often installed on existing roofs as they are lighter.
Evacuated tube solar collectors comprise banks of glass tubes connected into a manifold. Tubes can be replaced in the event of the vacuum being lost.
Service lives of between 20 -35 years are quoted. Costs are in the range of £500 – 1200/m˙.


Design and operational issues

Early integration with the overall building design particularly in terms of building orientation, aspect, insulation and backup heating system, is essential to maximise the benefits solar hot water systems.

Confirm that the panels or planks are designed to withstand roof wind and dead loads.

Where used for potable water ensure that the requirements of the water byelaws are met.

Ensure regular checking of dosage where protective inhibitors are used.

All systems will require inspection, maintenance and replacement of ancillary components during their service life.

Use underfloor heating system in conjunction with solar hot water systems.

Solar hot water systems are ideal for outdoor swimming pools as they require large quantities of low temperature heat during summer when most solar radiation is available.

The capital costs of plank or panel type of solar collector can be offset against the cost of traditional roof covering.

Designing solar hot water systems as mini-district heating systems can reduce unit costs.

Enhanced capital allowances or Clear Skies renewable energy grants may be available.

A control panel with diagnostic systems will warn of system problems.



Specification options


  Capital cost
Net present value for 60 years £/unit Expected service life
Aluminium interlocking plank solar hot water system 34,000 40,100 60
Aluminium interlocking plank solar hot water system with 40% enhanced capital allowance grant 20,400 26,500 60
Aluminium interlocking plank solar hot water system as a mini–district heating system of 6 dwellings 19,200 25,300 60
Flat plate solar collector. Toughened glass to BS 6206. Corrosion resistant collector components: stainless steel, aluminium or copper alloys. EPDM seals. System to BS EN 12976 4,000 8,520 25
Evacuated tube solar collector. Glass evaculated tubes. Corrosion resistant collector components: stainless steel, aluminium or copper alloys. System to BS EN 12976 5,000 9,850 25
Conventional heating system with condensing boiler and concrete tile roof 13,200 25,800 components
vary from
10 - 60 years



Table notes

• Net present values include allowances for inspection, maintenance and replacement of components for the whole system as well as energy costs. The complete system is included in the analysis including extra-over components to enable a fair comparison.

• Panel systems comprise pumps, pipework, roof fixings, controls and valves as well as the solar collector which are included in the whole life costing. Other related components such as the condensing boiler, radiators, thermal store or insulation are not included nor are energy costs. Panel collectors are sized for supplying energy to a 100m˙ house.

• A discount rate of 3.5% is used to calculate net present values.

• A cost analysis based on project specific information comparing complete systems, including assessment of each component, is essential for a realistic best value appraisal.

First published in Building 2006




Further information

BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450.