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 | £ 89 | £ 99 | £ 109 | £ 118 |
| Indicative installed cost of PV system | £ 5500 | £ 5500 | £ 5500 | £ 5500 |
| Payback period (years) | 62 | 55.8 | 50.7 | 46.5 |
| Payback period if energy prices double (years) | 31 | 27.9 | 25.3 | 23.2 |
| Payback period with the Feed-in Tariff (using the Energy Saving Trust calculator and the data above). This is no longer valid see below, the FITS rules have changed. | 24 years | 21 years | 19 years | 18 years |
| *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 £ 5500. If electrical mains supplied 800kWh of electrical energy it currently costs on average £ 100. So irrespective of maintenance issues, cost of financing a project or loss of interest on otherwise invested funds; the unit would payback in about 50 years.
Well the part of this article below (in the orange block) on FITS is now incorrect, because the Feed in Tariff rules have changed. This Article is once again based on the real economy of Solar PV and not a FITS subsidy. There are still subsidies and a less generous Feed in Tariff with new strings attached, as I read it insulate first! This kind of policy change is the main reason this site tries to avoid calculation based on government incentives and tries to stick to the real economics.
But then there is FITS! The main calculations in this article do not allow for the Feed-In Tariff (FITs) incentive schemes for renewable energy systems in the UK. They show raw calculations based on the true cost of grid electricity which is being replaced by the renewable energy system. With the FIT's scheme you are essentially paid by the government via electricity companies much more than the electricity is worth, even if you are using what you have generated. With the Feed-In Tariff scheme solar photovoltaics looks a lot better, use this website, Energy Savings Trust Cashback Calculator, to calculate the payback time and use this article to compare how uncompetative solar photovoltaics is without it in the UK (not much sun!). Also use this website Energy Saving Trust FITs, to determine the current rules regulations and eligability.
As example using their calculator have a look at the table above where the FIT's payback time is included at the bottom for the same examples. The payback times are still high, so you need to aim for a low install cost, perhaps less than £ 6000, to get a payback time of 12 years.We just need to hope FIT's is here to stay.
Have a look at this link
http://www.navitron.org.uk/forum/index.php?topic=16066.0
Most seem to be quoting about 3 kWh per day on average for a 3ish kWp system. This is entirely consistent with the article above
Your daily average power gen is so much better than most of these. We find that the efficiency difference of solar cell with temperature across the UK is very small sub one percent, and at the very most light conversion efficiencies are 14% for a reasonably priced system.
Sincerely do you have some tips for others??
Also we cannot find an install cost as cheap as you suggest, nearest we can find is £16,000 for 3.5 kWp. But we agree they have come down in prices and we have added a new price to the article £5,500 for 1 kWp.
We keep being told the payback quoted here is too long, but no one provides any evidence. We like facts.
Can you send me some hard facts and a full study on this and I will be happy to do a full case study on this using this data, provided that it is verified and factual. How efficient do you say the cells are?
my data suggests a 3.5kWp system will provide 2268 kWh per year. so if the true number is 3100, based on more efficient cells and with the install cost is £10,000, then yes the article needs to be revised. I look forward to the data.
All the best
Richard
Other discrepancies
1. Solar PV panels are more efficient when colder (20 degree optimum), so although there is less light they are more efficient in the north
2. The EST calculator takes no account of RPI (guaranteed for FITs) or energy price increases over 25 years. A bank would not be allowed to give such misleading figures.
please quantify what you are saying, are your comments based on solar technology or on the feed in tarriff scheme in the United Kingdom. The Feed in tarriff scheme does indeed provide a much better payback time and it probably is about 8-10 years as you say. But it is a market distortion and is likely to be tempory, it pays 5 times (about) the market rate for the green electricity. The article above is based only on the physics of solar cells, invertor efficiency, the sun, your geography installation cost and the standard electricity tarriff. In this context the only thing that can make the above out of date is much more efficient solar cells, from 12% efficient to over 50%. This has not happened.
all the best
Richrad