The main ingredient in most photovoltaic cells is silicon – the same element that makes computer chips possible. Silicon is the second most abundant element in the Earth’s crust, but unfortunately it is normally found in the form of silica (the chemical symbol for silica is SiO2) – you might know it as sand.
Various methods exist to extract the pure silicon, but the most common is carbothermic reduction, where the silica is heated to 1700°C in the presence of carbon. As the silicon cools it forms crystals.
The speed at which the silicon cools is one of the critical factors that determine the crystal size: the slower the silicon cools, the larger the crystals. With care the silicon can be extracted as one large crystal. As you might imagine, that’s more difficult, which means it’s more expensive.
The difference between monocrystalline vs polycrystalline solar cells is simply that one is produced from a single crystal of silicon and the other is produced from a piece of silicon consisting of many crystals.
So what is the impact on cell performance?
Since polycrystalline cells contain many crystals, they have a less perfect surface than monocrystalline cells. This means that they absorb slightly less solar energy and produce slightly less electricity per square metre. On the plus side, the process of creating the silicon for a polycrystalline cell is much simpler, so these cells are generally cheaper per square metre.
On balance, the cost of monocrystalline vs polycrystalline based panels per Watt of power output works out about the same, but the polycrystalline panels will be slightly larger than equivalent monocrystalline panels. This is generally not a problem unless you have a very limited area available for the installation, in which case you will want to maximise the power output per square metre.
Monocrystalline and polycrystalline can also look different. Monocrystalline cells will usually have a perfectly uniform appearance, but polycrystalline cells will appear “grainy” – think of how a granite worktop looks and you’ll get the idea. From a distance this will not be noticeable, so if they are going on your roof this is unlikely to worry you.
So which should I choose?
At the end of the day, unless you are very space constrained, your choice of panel will probably be dictated by factors other than whether they are made up of mono or polycrystalline cells.
The price per Watt is an important factor, and that is largely unaffected by the choice of monocrystalline versus polycrystalline cells. In some circumstances, the area available for the installation may be a factor that pushes you to go for monocrystalline cells.
But the most important thing is to make sure that you choose a reputable installer and manufacturer. Your panels will most likely give you many years of trouble free operation, but for your own peace of mind you will probably want to choose a manufacturer that is likely to be around for long enough to honour the terms of the guarantee – which may be up to 25 years!