Case studies

Scenario 1: Flats, £1,950 per unit
Scenario 1 is a twenty-six dwelling development in the West Country with a target to meet Code level three. The block was already half built when the developer decided to offer the properties to a Social Housing provider.

Having reviewed many renewable energy options including solar water and air source heat pumps, the developer chose photovoltaics to provide the final 10% reduction in emissions to meet the Dwelling Emission Rate (DER). A simple PV module on a frame system installed in one south-facing array on top of the block provides the total amount of solar PV required for all dwellings.

Photovoltaics can easily be installed, even late on in the development as they require just an inverter and some wiring between the roof and the main fuse board. Typically, a small installation can take as little as one day.

At an installed cost of just £1,950 per dwelling, PV was the cheapest option of all renewable technologies. The landlord will also benefit from Renewable Obligation Certificates (ROCs) for exported energy, which is expected to provide an additional revenue stream of £1,200 per annum from 2009 onwards.

Scenario 2: Flats, £1,450 per unit
Scenario 2 is an eighty-dwelling block of flats in London. This project had come to Solarcentury with a remit to use a solar water system to make up the final 10% of the required 25% reductions to DER.

Whilst the actual panels were cheap and the solar thermal route at first seemed like the most cost-effective solution, the additional plumbing required to pipe heat to the basement, back-up boiler and other infrastructure raised the cost of installation considerably.

By switching to photovoltaics, the developer reduced the cost to just £1,450 per dwelling by installing a module array on the flat single-ply membrane roof area. This avoided the added complications to the build process that would have resulted from using a solar thermal solution. The 'buildability' of photovoltaics and simple installation process radically reduced the overall cost of installation per unit.

Scenario 3: Houses, £2,400 per unit
Scenario 3 comprises twenty houses in Gloucestershire. Solarcentury was asked to provide costs for an integrated roof tile product providing solar PV systems to twenty houses. The houses were between 76m² and 80m² with an external wall U value of 0.28 and an air tightness of 5m at q50.

The cost and practical comparison was made against using solar water or air source heat pumps. Aesthetics were considered important because whilst the development was not in a conservation area, the planners wanted to maintain an attractive road scene.

In this scenario, due to the fabric upgrades, the PV system sizes were small and no dwelling required a roof area of greater than 8m².