The whole idea of this feed-in-tariff business is that you earn money by selling units of energy produced by your solar panels. So much so that after 25 years of operation you’ve made your money back and have even turned a tidy profit. This means that in order to know whether putting up some solar panels makes any sense, you need to know exactly how much energy they’re going to produce over the 25-year guarantee period.

Easy, you might say – the calculation is pretty straightforward. You find out the average annual irradiation (sunnyness level) from your local weather station, and multiply by the efficiency of your solar panels and the number of square metres you have. This will give you a nice number and away we go. The only problem is you might be more than 50% wrong because we’ve missed out a couple of variables. Variables such as temperature coefficient, tilt angle, diffuse-light fraction, solar cell type, shading losses, inverter losses, cable losses, degradation, module de-rate factor, mismatch losses, anti-reflective coatings, snow and lightning strikes, to name a few.

Of course there are an infinite number of effects that can influence the output of your photovoltaic system (solar eclipse, anyone?). The question is whether you have considered the important ones or not.

Knowledgeable installers use one of a number computer programs designed specifically to take these factors into account. You type in what type of solar panel you’re using, how many, where they are, what angle they’re tilted at, what direction they’re facing and then press ‘go’. It then calculates the amount of energy you’ll produce each month and even the return on investment if you want it to. Behind these models is actually some physics that describes the behaviour of solar cells under different light intensities and correctly.

The most commonly used model in Europe is called PVSyst, developed at the University of Geneva. This software package contains information on a large number of different solar panel types and is capable of taking into account many of the above listed factors. Installers across Europe use this software package to predict the energy yield of residential solar systems, as do many banks pondering whether to provide multi-million euro loans to super-large PV power projects. Even with this advanced software package however, some of these factors are very complex, and improving these models is an active area of research.

Here, I’ll deal with a couple of these complications as examples. When you buy a solar panel, it invariably comes with a power rating. Full size modules are generally around 200W. What does this mean though? In principle, the power rating indicates what you get when the panel is illuminated by full-sunlight. ‘Full sunlight’ is not very specific, so the international community has defined what is known as Standard Test Conditions (STC), which corresponds to an irradiation of 1000 W/m2 and a cell temperature of 25oC, when the light has a specific spectrum (or colour) known as Air Mass Index 1.5. So the power of your solar panel comes from its performance under exactly these conditions. In general this is measured using special type of lamp called a ‘solar simulator’ that tries to reproduce the AM1.5 spectrum as closely as possible. Calibrating these lamps precisely is notoriously difficult and there are very few testing centers around the world that are truly trusted. The National Renewable Energy Laboratory (NREL) in Colorado, USA uses at least two different lamps and one outdoor measurement to record STC performance, after a long period of calibration.

Because measuring the STC performance is so tricky, the power rating you get has a plus or minus 5 percent error margin. This is hard to include in your simulation. In addition, manufacturers will often deliberately under-rate the power of their solar panels to be sure they don’t fall below the warranty. This means you may well get considerably more power than you expect.

Another factor that adds to uncertainty is the degradation factor. When you buy solar panels they are normally guaranteed for 20 years, but only to 80% of the initial power output. This means the manufacturer expects them to degrade 1% per year on average. When calculating performance in the models, people also tend to use a 1% degradation rate per year. This is only a rough estimate however. During the certification process, solar panels are given all sports of nasty treatment to test their reliability to breaking point. This doesn’t tell you much about the rate of degradation when the solar panels are outside under normal operation though. The only reliable way to test degradation over 20 years is to wait 20 years, but this is complicated by the fact that technology improves reliability much faster than that. So the degradation of solar panels made in 2008 has only been tested since, well, 2008.

What these issues highlight is that understanding the energy yield output of your solar panels is not as straightforward as it may at first seem. When having your system designed, make sure who-ever you’re dealing with has some experience, and if possible, get a second opinion.

The other critical piece of information for understanding the financial viability of a solar installation is how much you will get paid per kWh under the feed-in-tariff. Unfortunately, the UK government has not released the final figures yet, which means no-one in the UK can make a reliable financial plan for getting solar panels, even when the launch date for the feed-in-tariff is just 4 months away.

Hopefully I will be able to update you on this in the near future. For now though, it’s better to be more conservative with your numbers than too ambitious….