A Shady Situation: Your Solar PV Inverter

Solar PV systems are a great way to generate electricity where it will be used and have become very popular in the last few years.  The technology is reliable and well proven, and can be a good investment for homeowners.

At times solar panels will be shaded (by other buildings, chimneys etc.) and this leads to reduced performance.  Of course the best thing to do is to install them in such a way that that there is very little shading, but this is not always possible.  There is technology that can reduce the effect of shading and that’s what we’re going to look at this week.

Solar: What Are Inverters?

Solar PV panels produce DC (direct current).  The function of an inverter is to convert the DC that the solar panels produce into AC (alternating current) that can be used to power appliances in the home.

The majority of solar PV (photovoltaic) systems installed in the UK employ string inverters.  Alternatives such as micro inverters and DC optimisers exist and these can reduce the effect of shading on the output of a PV system.

String Inverters

In a solar PV system with a string inverter the solar panels are connected in series (a string) to the inverter.  In a typical domestic system there may be one or two strings.  This is a well proven arrangement and the lowest cost to install.  The effect of partial shading of solar panels using a string inverter can be significant.  A single panel in shade will affect the output of the whole system, not just the shaded panel.

Alternative Options

The alternatives to string inverters can reduce the effects of shading.  Micro-inverters (including AC panels which have an integrated micro-inverter) and DC optimisers (including “smart” solar panels with integrated DC optimisers) are two alternatives to string inverters.

Micro-Inverters

The clue is in the name, these are small inverters.  A micro-inverter is mounted behind each panel.  This means each panel will operate in the optimum state for the conditions it is in as each micro-inverter has its own mppt (maximum power point tracker).  Any panels that are shaded won’t be reducing the output of the other panels.  Where a string inverter is used there are usually only one or two mppt’s so the shaded panels will nearly always be reducing the output of the unshaded panels.

DC Optimisers

DC optimisers combine some elements of a string inverter system and some elements of a micro-inverter system.  An optimiser is mounted behind each panel.  It has an mppt so each panel operates in its optimum state as with a micro inverter.  The DC to AC conversion however takes place in a separate unit usually inside the property.
Manufacturers claim that these systems can increase outputs by up to 25% in some installations.  There are other advantages such as module level monitoring and automatic module shutdown for safety purposes.  A disadvantage is that more bits of equipment are installed in hard to access locations potentially leading to higher maintenance costs.  Installation costs are also higher.

What’s right for you?

Despite the extra costs, in some situations an installation using micro-inverters or DC optimisers will provide the best solution both in terms of maximising output and financial benefits.  This is just one of many factors to consider when designing a solar PV system and every installation needs to be assessed on its own merits to find the optimum inverter specification.

If you would like to talk to a renewable energy consultant about your project, whether solar or involving other renewable technologies, pick up the phone and call 08445 003383

Doug Johnson

Doug is a chartered mechanical engineer and has had the pleasure of being involved in the UK renewable sector since 2008. With wide-ranging operational and design experience in both the domestic and commercial sectors across a wide range of recognised technologies he hopes to be able to share his experience to help enlighten and save you hassle along the way.