The terms solar energy and solar power are often used to describe technologies which collect the energy of the sun and redistribute it for our use. These terms are however a little ambiguous. For example, solar energy effectively drives our whole planet, wind, wave and fossil fuels could ultimately be regarded as solar energy. For the purposes of this site our only concern is for solar energy that can be collected directly by our dwelling and used to, for example, heat water or to drive our electric devices. For this article only Solar Photovoltaic power generation is considered. Please follow the link to take a look at Solar hot water.
| If you are interested, all light is made up of photons, which might be best described as tiny little packets of energy. There is a whole range of different energies or “packet sizes” which if you put them all together from small to large would constitute the spectrum of light. |
| Solar Hot Water... |
To collect solar energy a method is required to collect photons of light and to convert the energy in this light into another form of energy for storage or use.
In photovoltaic’s this energy is directly transferred into electrical energy using solar cells, the energy may then be stored in a battery, used immediately in your house or sold back to the grid.
| In solar cells the light photons are absorbed by transferring electrons in a semiconductor to higher energy levels. They are transferred to what is called the conduction band of the semiconductor, where they can flow through a circuit and provide electrical energy to your property. Semiconductor physics is a bit much for this site; there is a good wiki on the subject. http://en.wikipedia.org/wiki/Semiconductor. |
Is it worth it?
On the face of it, it should be quite straight forward to calculate how sensible an investment in photovoltaic power generation is likely to be. The number of variables is however extraordinary, here are the more important
- How sunny is it where you live.
- The orientation of solar panel, (North, South, East, West).
- The angle of the panel.
- The size of the intended panel.
- The efficiency of the solar cells.
- Efficiency of the electrical systems, (invertors and power storage)
- The cost of the installation.
- The cost of mains electricity.
The first term that you generally come across when looking into solar modules is the term Kilowatt peak (kWp) or peak power. This value specifies the output power achieved by a Solar module under solar radiation under set standard test conditions. Standard test conditions use a solar radiation level of 1,000 watts per square meter (W/m2).
As an example, an 8m2, 1kW peak system (1 kWp) is often specified as an entry level system. This means that under test radiation conditions of 1000 W/m2 this 8m2 panel is capable of producing a 1kW power output. A general consensus of suppliers indicates that such a unit should produce 750 kWh of electricity per year in the UK, if correctly oriented.
The table below give an indication of how these values might be derived and considers different radiation level across the UK, but it is very rough. A payback is also presented to show what happens if the electricity price doubles. If you are interested there is also a solar radiation map of Europe at the bottom of the article.
| Light energy avaiable to you see graphic kWh/m2 per year* | 900 | 1000 | 1100 | 1200 |
| light energy incident on a 8m2 panel * kWh per year | 7200 | 8000 | 8800 | 9600 |
| Light capture efficiency, or Gross efficiency over full area of solar collector | 12% | 12% | 12% | 12% |
| Energy converted to electricity 8m2 collector kWh per year | 864 | 960 | 1056 | 1152 |
| Electrical efficiencies, storage and invertor | 75% | 75% | 75% | 75% |
| Useable electrical energy kWh per year | 648 | 720 | 792 | 864 |
| Equivalent standard rate mains electric price of this energy per year | £ 76 | £ 85 | £ 93 | £ 102 |
| Indicative installed cost of PV system | £ 8000 | £ 8000 | £ 8000 | £ 8000 |
| grant | £ 2500 | £ 2500 | £ 2500 | 3 2500 |
| Payback period (years) | 71.9 | 64.7 | 58.9 | 53.9 |
| Payback period if energy prices double (years) | 36 | 32.4 | 29.4 | 27 |
| *The value for the light energy availiable assumes that the panal is optimally inclined at about 38 degrees and is south oriented. | ||||
To determine if a system is financially viable in the UK it is instructive to use the installers own figures, however the installers do not generally state which part of the UK their figures are applicable to, probably Cornwall! Generally a 1kWp system costs about £ 8000, and there are grants available to reduce this to £ 5500. If mains supplied, 750kWh of electrical energy currently cost on average £ 88.50. So irrespective of maintenance issues, cost of financing a project or loss of interest on otherwise invested funds; the unit would payback in about 62 years.
At the other end of the scale a large 40m2, 5kWp system would cost about £ 24000 to install and produce 4280kWh per year, which if mains supplied would cost £ 505. This unit would then take 47.5 years to payback.
I guess you should make your own mind up.
