To determine the viability of a solar PV array, you need to estimate the expected output from your solar panels. This is determined by three main factors, the amount of direct solar radiation the panels will receive, the angle of elevation of the panels and the amount of shading at the site.
Measuring the amount of solar irradiation
Solar irradiation is split into 2 categories, direct (where a surface receives direct sunlight) and diffuse (where sunlight comes through cloud cover). In the UK, the majority of the solar irradiation we receive is of the diffuse variety (you probably won’t be surprised to hear!). Solar PV panels operate best under direct sunlight, although they will still produce electricity in diffuse light conditions. Levels of solar irradiation vary considerably with the seasons, with the vast majority of direct irradiation received during summer in the UK.
Measuring solar radiation directly at a site requires a piece of equipment called a pyranometer, and you are taking your own readings, it will be necessary to collect data for at least 12 months at the site to ensure you have a good understanding of seasonal variations. However it is quite possible to get a very good estimate of the amount solar irradiation at your site from published solar maps and local Met Office data, both of which are widely available. Because the amount of solar irradiation tends to be less variable at the local level than wind speed would be, it is more acceptable to use these published figures as the basis for viability calculations. Only if you have specific reason to believe that site factors (such as excessive shading) will be significantly affecting solar irradiation at your site should you need your own pryranometer.
Angle of elevation
If you are building your solar installation on an existing building, you will be restricted in the angle and orientation of your panels. However, if you are building a free-standing PV array you have the chance to set your panels at the optimum angle to ensure they receive maximum levels of solar irradiation across the year. In the UK, the optimum orientation of a PV array is between south-west and south-east, with a panel tilt of around 40 degrees.
You can get an estimate of how panel tilt and orientation affect the output of solar PV systems using a variety of online calculators.
If you are building a free-standing PV installation, you can also consider a tracker system, which tracks the level of the sun throughout the year and adjusts the panels to achieve optimum output. These installations can improve energy outputs by up to 20%, but also require additional energy and maintenance once in operation.
Assessing the shading at the site
Having determined the size and position of your potential solar PV array, it is important to ensure that there will be no significant shading affecting your site. The simplest way to do this is to mark out your proposed panel area and observe the site for a period of time to establish that no significant shadows are cast over the panel area.
A more detailed method of assessment involves using sun charts to plot the potential shading obstacles for your site. Your consultant will be able to assist in this more detailed process.