To see the rates we are using click here.

The estimate – which we are obliged to calculate under the terms of our MCS accreditation – is a "SAP 2009 calculation".

This is the government's official calculation methodology – SAP stands for 'standard assessment procedure'. Whilst this takes account of roof orientation, slope and shade, it doesn't take account of latitude – thus Manchester is considered to be on a par with Truro in sunshine terms, which everybody knows is not a true representation of reality! See below for a more in depth explanation of its limitations.

The second estimate – which we have found to be much more accurate is made by the Photovoltaic Geographical Information System (PVGIS).

To run your own estimate click here, enter your postcode, change the estimated system losses to 10%, and select Free Standing for the system. The slope will be that of your roof, or 15° for a system mounted on a flat roof. The Azimuth is the orientation of your roof, so 0° is due South, -90° is East etc.

1. System Size (kWp):
2. Tilt (degrees): Horizontal 30° 45° 60° Vertical
3. Orientation: North North East North West East South East West South West East West South East South West South South East South South West South
4. Level of Overshadowing: Heavy Significant Modest None or very little
5. Will Panels be Fitted to:
   an Existing Building
   a New Building
   Not applicable to systems over 4 kWp.
6. Your Electricity Tariff:
   10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 14.0 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 15.0 Pence
   If in doubt select 13p.
7. Are you at home during the day:
   Yes No