| The whole idea of this feed-in-tariff business is | | | | defined what is known as Standard Test |
| that you earn money by selling units of energy | | | | Conditions (STC), which corresponds to an |
| produced by your solar panels. So much so that | | | | irradiation of 1000 W/m2 and a cell temperature |
| after 25 years of operation you've made your | | | | of 25oC, when the light has a specific spectrum |
| money back and have even turned a tidy profit. | | | | (or colour) known as Air Mass Index 1.5. So the |
| This means that in order to know whether putting | | | | power of your solar panel comes from its |
| up some solar panels makes any sense, you need | | | | performance under exactly these conditions. In |
| to know exactly how much energy they're going | | | | general this is measured using special type of |
| to produce over the 25-year guarantee period. | | | | lamp called a 'solar simulator' that tries to |
| Easy, you might say - the calculation is pretty | | | | reproduce the AM1.5 spectrum as closely as |
| straightforward. You find out the average annual | | | | possible. Calibrating these lamps precisely is |
| irradiation (sunnyness level) from your local | | | | notoriously difficult and there are very few |
| weather station, and multiply by the efficiency of | | | | testing centers around the world that are truly |
| your solar panels and the number of square | | | | trusted. The National Renewable Energy |
| metres you have. This will give you a nice number | | | | Laboratory (NREL) in Colorado, USA uses at least |
| and away we go. The only problem is you might | | | | two different lamps and one outdoor |
| be more than 50% wrong because we've missed | | | | measurement to record STC performance, after |
| out a couple of variables. Variables such as | | | | a long period of calibration. |
| temperature coefficient, tilt angle, diffuse-light | | | | Because measuring the STC performance is so |
| fraction, solar cell type, shading losses, inverter | | | | tricky, the power rating you get has a plus or |
| losses, cable losses, degradation, module de-rate | | | | minus 5 percent error margin. This is hard to |
| factor, mismatch losses, anti-reflective coatings, | | | | include in your simulation. In addition, |
| snow and lightning strikes, to name a few. | | | | manufacturers will often deliberately under-rate |
| Of course there are an infinite number of effects | | | | the power of their solar panels to be sure they |
| that can influence the output of your photovoltaic | | | | don't fall below the warranty. This means you |
| system (solar eclipse, anyone?). The question is | | | | may well get considerably more power than you |
| whether you have considered the important ones | | | | expect. |
| or not. | | | | Another factor that adds to uncertainty is the |
| Knowledgeable installers use one of a number | | | | degradation factor. When you buy solar panels |
| computer programs designed specifically to take | | | | they are normally guaranteed for 20 years, but |
| these factors into account. You type in what type | | | | only to 80% of the initial power output. This |
| of solar panel you're using, how many, where | | | | means the manufacturer expects them to |
| they are, what angle they're tilted at, what | | | | degrade 1% per year on average. When |
| direction they're facing and then press 'go'. It then | | | | calculating performance in the models, people also |
| calculates the amount of energy you'll produce | | | | tend to use a 1% degradation rate per year. This |
| each month and even the return on investment if | | | | is only a rough estimate however. During the |
| you want it to. Behind these models is actually | | | | certification process, solar panels are given all |
| some physics that describes the behaviour of | | | | sports of nasty treatment to test their reliability |
| solar cells under different light intensities and | | | | to breaking point. This doesn't tell you much about |
| correctly. | | | | the rate of degradation when the solar panels are |
| The most commonly used model in Europe is | | | | outside under normal operation though. The only |
| called PVSyst, developed at the University of | | | | reliable way to test degradation over 20 years is |
| Geneva. This software package contains | | | | to wait 20 years, but this is complicated by the |
| information on a large number of different solar | | | | fact that technology improves reliability much |
| panel types and is capable of taking into account | | | | faster than that. So the degradation of solar |
| many of the above listed factors. Installers across | | | | panels made in 2008 has only been tested since, |
| Europe use this software package to predict the | | | | well, 2008. |
| energy yield of residential solar systems, as do | | | | What these issues highlight is that understanding |
| many banks pondering whether to provide | | | | the energy yield output of your solar panels is not |
| multi-million euro loans to super-large PV power | | | | as straightforward as it may at first seem. When |
| projects. Even with this advanced software | | | | having your system designed, make sure |
| package however, some of these factors are | | | | who-ever you're dealing with has some |
| very complex, and improving these models is an | | | | experience, and if possible, get a second opinion. |
| active area of research. | | | | The other critical piece of information for |
| Here, I'll deal with a couple of these complications | | | | understanding the financial viability of a solar |
| as examples. When you buy a solar panel, it | | | | installation is how much you will get paid per kWh |
| invariably comes with a power rating. Full size | | | | under the feed-in-tariff. Unfortunately, the UK |
| modules are generally around 200W. What does | | | | government has not released the final figures yet, |
| this mean though? In principle, the power rating | | | | which means no-one in the UK can make a reliable |
| indicates what you get when the panel is | | | | financial plan for getting solar panels, even when |
| illuminated by full-sunlight. 'Full sunlight' is not very | | | | the launch date for the feed-in-tariff is just 4 |
| specific, so the international community has | | | | months away. |