Frequently Asked Questions

All projects need a detailed design to determine the best position for the PV array, the optimum system configuration and the predicted output from the system. However, there are some general rules that will allow you to get an idea of what the system can do to get you started.
  
Note: this page uses common terminology for the components of the system and the operating parameters. If you are unfamiliar with PV systems, you might want to read the Introduction to PV Technology page first.

Where should the PV array be located?
How big should the system be?
How is the system size expressed?
What area will the array be?
How much electricity will the PV system generate?

Where should the PV array be located?

Ideally, the array should be in a place where it gets the maximum amount of sunlight and is not shaded. In most locations, a south facing system will collect the most sunlight, but there is only a small loss for choosing a direction between south east and south west. So, don't worry if your roof or façade is not directly south facing.

The amount of sunlight also varies with the angle of the array, but typical roof pitches of between 30 and 45 degrees are quite acceptable and only result in a small difference from the ideal condition. Vertical systems lose a bit more - in the UK, their output is typically about 15% lower than for a module tilted at around 45°.

The array can be shaded by other nearby buildings, trees, telegraph poles and other parts of the building it is on (e.g. chimneys, window casements, aerials and decorative features) so you need to choose a location where the shading is not severe. Remember that the position of the sun, and therefore the potential shading sources, will vary with time of day and season. Arrays can be designed to cope with small amounts of shading, but the output can be reduced substantially if care is not taken.

The inverter system is usually located inside the building as close as possible to the array but also convenient for the connection to the grid. In domestic properties, it is often in the loft, but there is a range of possible locations within a commercial building.

How big should the system be?

The PV system will be contributing to the electricity loads of the building and the size of the system depends on how much of that load you want to meet. The maximum system size is determined by how much roof and/or façade area is available, but there are several other factors that then affect the choice of system size.

• Do you want to export electricity from the building?
• Are there specific loads you want to meet?
• Is the array designed to be multifunctional and how does the size affect the other functions?
• What visual aspect is desired – may require smaller or larger array to provide an attractive look?
• What is the budget?

These factors vary from building to building and depending on the wishes of the owner. A system designer will help you to decide on the best size for your system.

For information on the physical size of the array for different module types, please go to: What area will the array be?

How is the system size expressed?

The electrical output of a PV module varies with the sunlight and temperature conditions, so it is usually expressed at particular values of these parameters known as Standard Test Conditions (or STC). The output information you find on the product data sheet is measured under these conditions.

Because STC represents high sunlight levels, the power output at STC is also sometimes referred to as the peak rating or expressed in Watts peak (Wp). Normally, the module will give a lower output because the sunlight levels will be lower than STC, but the Wp value can be used by the system designer to provide an estimate of the system output in your location (see How much electricity will the PV system generate? ).

The system size is usually expressed as a power rating, in Wp or more usually in kWp, and this is found by totalling the power rating of all the PV modules in the array. The rating of the inverter system is usually a little below the rating of the array, since this gives the most efficient operation of the system.

What area will the array be?

The area of the roof or façade required for the array depends on the modules you choose, but there are some general rules of thumb that can be used to make the first estimate.

Modules based on single crystal silicon are the most efficient on the market at the moment - around 13-14% is a common figure. This would require an area of about 8m² to install an array rated at 1 kWp.

1 kWp of polycrystalline silicon modules, which are slightly lower in efficiency, would require an area of about 10m², whereas amorphous silicon modules would need an area of 16-20m² (depending on the module chosen).

If semi-transparent, architectural modules are chosen, then the area required for a given array rating will depend on the transparency of the modules, but will generally be higher than the numbers for standard modules given above.

How much electricity will the PV system generate?

The amount of electricity generated by the PV system depends on the weather conditions, the orientation of the array and other factors such as shading, choice of system components and the way it is integrated into the building. Your system designer will provide an output prediction for your system once these factors have been decided.

However, you can get a rough idea of the output of the system by using the following information.

For an unshaded system in the UK, we can expect 750-800 kWh of electricity to be produced each year for every 1 kWp of array installed.