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In the UK’s fledgling PV industry there are some unexpected issues emerging.
One of those is related to the grid or ‘mains’ voltage. This should be, as
in the rest of Europe, fixed at 230V. However it can vary, and in reality
the legal limit is 230V plus or minus 10 percent, generally being higher
voltage close to the nearest transformer, and getting lower voltage as you
move away.

This is important for PV systems because the inverter must convert direct
current into mains compatible AC with an acceptable voltage level. Inverters
are also designed to shut down if there is a problem with the grid for
safety reasons.

In Germany, the voltage level is very precise (perhaps as you might expect)
and since Germany is the world’s largest solar market by far, most inverters
in Europe have their settings with Germany in mind. This means that when the
grid strays slightly away from 230V, the inverter temporarily shuts down. In
the UK, the grid is much more likely to deviate from 230V, meaning that with
German settings, an inverter could well spend more time off than on.

Luckily the problem is generally easily fixed by changing the inverter to
new settings which make it tolerant to a wider voltage range. The key point
to remember is that with the inverter shortage, products are being sold
which are completely unchanged from their German settings. This means you
need to be extra vigilent when buying an inverter to ensure compatibility.

A related topic, that I will soon cover, is to do with how lots of solar
energy connected to the grid can actually affect the grid voltage and
frequency – but that’s another issue.

If an inverter is used in the UK without any change in settings then chances
are, with our fluctuating grid, you will have peaks or dips in the voltage
that shut off the inverter from time to time. The solution is simply to
change the settings via the firmware to allow the inverter to carry on
working in a wider range of voltages.

Understanding how to design a PV system is not rocket science, but it is more complex than many people consider. Here’s a very quick overview of the important points.

Solar panels produce direct current (DC). This means you need an inverter to turn that electricity into mains frequency alternating current (AC).  Inverters come in a range of power ratings. The more solar panels you have, the more power the inverter has to deal with, so the size and cost increases. It’s very important to match the size of the inverter to the number of solar panels.

If the inverter is too small, you will lose out on some of the energy that your system produces. If it is too large, the inverter may not perform at its optimum efficiency, and you will have paid for more than is necessary. In the UK, the optimum situation is to have an inverter that is rated at 80% of the power rating of your PV system, since it is rare you will be producing at 100% power.

More critically than getting the power right, you need to ensure the voltage and current of your solar panel system remains within the input range of the chosen inverter. To re-cap, solar panels on your roof are generally connected together in series, in a ‘string’. This increases the system voltage, but does not increase the current. Once a certain number of solar panels have been connected in series, the voltage will become too high and the system needs to be arranged in two strings, each of the same number of panels, connected in parallel. This generally occurs after a string exceeds 8 – 11 solar panels. When strings are connected in parallel, the currents add-up, but the voltage remains constant.

By adding more and more strings in parallel, the current and voltage can be controlled to remain in the inverter limits. For large solar installations, inverters can used that that have a very high power capacity, or alternatively it is possible to use many small inverters connected in parallel.

It is important to remember certain constraints. Inverters come in several sizes, but there may be some numbers of solar panels for which no inverter is ideal. For instance, because it is necessary for all stings to be equal in size, you can only use an even number of solar panels when using multiple strings. In addition, all solar panels must receive the same amount of sunlight when connected to the same inverter. It is no good to have some solar panels facing different directions on different parts of the roof. New technologies, soon to become widely avaialable that will make this process much easier. Namely micro-inverters, which convert DC to AC at every solar panel, will mean that solar panels can face different directions, however these are not yet widely available.

If you have a sales visit from a solar company, make sure the salesman understands these points as he’s designing your system.