Solar PV - cost £2,000/kWp
‘Photovoltaic’ is a combination of the Latin word 'photo' meaning light, and modern word 'voltaic' (in honour of Alessandro Volta) which refers to the generation of a voltage, or electricity – hence, photovoltaics (or PVs) generate electricity from light.
PV cells are made from layers of semi-conducting material, usually silicon. Semi-conductors can be adapted to release electrons when they are exposed to light - the stronger the light, the more electrons that are produced. Since electricity is essentially the existence of electrons, the more electrons the more electricity!
Groups of cells are mounted together in panels and then fixed on your roof, or ground-mounted, in an array. The power of a PV array is measured in kilowatts peak (kWp) - the amount of electricity it can (theoretically) generate in full direct sunlight during the summer. As a general rule, each kWp will cost roughly £2,000, take up 8m2 of roof space, and generate 800 kWh per year – although prices are dropping and efficiencies climbing all the time. A typical UK installation is 4kWp, costing £8,000, taking up 24m2 and generating 3,200 kWh/annum – a typical home uses approximately 4,500kWh of electricity each year.
Mostly PVs come in pre-manufactured panels, although they are available as roof tiles or can be sandwiched between panes of glass - both of which are more expensive and less efficient, although allow for a great deal of flexibility in design.
PV arrays may be off-grid or grid connected. If your home is grid connected, as in connected to the local electricity network, then so should your PV array be - this allows any excess electricity that your array produces to be sold to the grid.
If your home is completely independent of the grid, then the PV array must be connected to batteries, which are very expensive and need replacing regularly.
Either way, an inverter is used to convert the direct current (DC) power produced by the solar system to alternating current (AC) power needed to run normal electrical equipment – this creates losses, and if you are off-grid you may like to run DC lighting and appliances.
Advantages of PV
PV arrays have a number of advantages over other electricity generation technologies:
- They are modular, and thus easily expandable and adaptable to any roof size or shape.
- The fuel is free.
- No noise or pollution is created from them running.
- They don’t have any moving parts, and so nothing to go wrong or wear out – although the semiconductor material will run out of electrons in time (1% loss of production per year, panels are generally expected to last for 25 years), and the panel will have to be kept clean – and unshaded!
In general, PV systems that are properly installed require minimal maintenance and have long service lifetimes.
Not only will your electricity costs be reduced, but you can also get paid for the electricity you generate through the FIT AND sell electricity back to the grid if your system is producing more electricity than you are using. Aside from the issues of silicon production, PV electricity production doesn't release any carbon dioxide or other pollutants – a typical home PV system could save nearly two tonnes of carbon dioxide per year, or more than 50 tonnes over its lifetime.
The high cost of PV modules and equipment compared to the amount of power generated is the largest limiting factor, although in the UK both Green Deal financing and FIT payments have made PV economically viable, and in turn the rise in demand has driven down costs. PV panels could generate you an income and savings totalling £750/year.
Take a look at our funding section to find out more....
Location & planning considerations
If you are considering PV, it is important that you have a potential site that receives as much direct sunlight as possible - the best location is obviously on an angled roof, facing due south. It is also important that the panels will not be over-shadowed even slightly by trees or buildings, as this could severely affect output.
To estimate exactly how much electricity a PV array could generate for you, use the Encraft tools here – you will have to register first.
PV panels come under permitted development in the UK, with some guidelines:
· The array must not project in excess of 200mm from the plane of the roof.
· The array must not extend higher than the highest part of the roof or chimney.
· The array will require listed building consent if the building is listed.
If you do decide to install a PV array, it is imperative that you have a structural engineer check that your roof can handle the extra load first – an MCS accredited installer should sort this for you.
HOT OFF THE PRESS! – PV technology is being developed all of the time, and it was recently announced that materials researchers in Oxford have found a better semi-conductor for use in PV panels than silicon, meaning that efficiencies could push 20% - a whole third better than most PV panels available currently.