Does your roof have multiple orientations? Read this before going solar

Published: 4 December 2018

Can you install solar panels if you have a roof with multiple orientations? Let’s find out.

Installing solar panels can be more complicated when all sides of the roof don’t face the same way – we call this multiple roof orientation. And since houses come in different shapes and sizes, solar panel installation has to as well.

Installing a PV system

To design a PV system, a solar installer must take into account several factors like solar irradiance, power size, temperature ranges, soiling, wiring, selection of components, shading and more.

One of the most important tasks is to size the system in order to fulfil the energy demands of the homeowner, but also to ensure that the PV system fits within the available roof space for solar panels.

Utility or commercial-scale solar power plants generally have extensive land to install their solar panels pointing to south or north whichever is more convenient. 

However, as the solar market keeps growing, PV systems become desirable to more householders whose homes might not have the large spaces required for solar panels.

Or, while often there may be enough roof space for solar panels, it doesn’t always face in a single direction or the most desirable direction. In such cases, two or more orientations – multiple orientations – are needed for the same PV system.

Enphase - Multiple Roof Orientations solar panel installation

Why Maximum Power Point (MPP) is Important

As we know there are several components in a PV system.

There are solar panels, wires, inverters, meters, batteries and more. 

The heart or brain of the system is the inverter and this equipment is able to track the maximum power point (MPP) of the array in order to ensure maximum energy yields. 

The inverter is able to track this point of operation by using a maximum power point tracker (MPPT) which generally has one or two inputs.

In the figure below we can see an image that describes an I/V curve and where we can see the MPP point. 

Check this link if you want to know more about MPP: Solar Basics: What is Maximum Power Point (MPP)

I/V curve and where we can see the MPP point

The Dilemma of Multiple Orientations

Now, installing a PV system with a single orientation towards a single MPPT input assures that all the I/V curves of the solar panels will have the same shape. 

However, when some solar panels need to be placed facing north while others face east (for example) due to space constraints, then the I/V curves are different. 

The reason is that an I/V curve changes according to irradiance and temperature values. 

Irradiance is associated with current values and temperature is associated with voltage.

Since the solar panels that face east are positioned at a different angle towards the sun, they receive less solar irradiance than those located towards north – at least, in Australia. 

Therefore, the I/V curve of the solar panels located towards the east will be different from the ones facing north.

If all the solar panels are wired together towards a single inverter with a single MPPT input, then mismatch losses appear in the scenario.

Mismatch losses are attributed to any disparity in the PV system that could lead to reductions in energy yields due to tracking errors. 

The general conception in the industry is that connecting solar panels with different orientations towards a single MPPT input is impossible. 

The reason argued is that mismatch losses will be so big that your PV system will reduce its efficiency radically and you will not have the energy yields that were promised.

Therefore, if two orientations are needed, the common solution is to install two inverters (one for each orientation). Since inverters have specific power ratings, this could lead to an oversizing of the system, leading to higher costs as well.

The other solution is to install an inverter with two MPPT inputs. 

This would be the ideal solution truly, since these two inputs are entirely independent. However, it will probably mean purchasing a more expensive product. 

In any case, is it worth it to purchase two inverters? 

The Fronius and SMA – Multiple Roof Orientation Solution

Given that this topic is like quicksand for many people, remarkable brands of solar inverters like Fronius and SMA (BROKEN LINK) dedicated a research to investigate the true impact of mismatch losses in a PV system due to multiple orientation configurations towards a single MPPT input. 

They developed a test where they installed a single east-west solar array towards a single inverter (from each brand) with a single MPPT input. 

On the other hand, they installed the same array but separated with two inverters (one for the string facing east and another for the string towards the west).

Surprisingly, they both discovered that the energy yields were almost the same in both cases and that mismatch losses were below 1%!

The reason is that despite the fact that irradiance values are different between the two orientations, temperature values are not different from each other since they are located on the same roof. In other words, the current values between the two orientations are different, but the voltage values are similar.

I/V curve and where we can see the MPP point

Therefore, the I/V curve only changes in the y-axis and the MPPT is able to track appropriately because the reference for the input is usually voltage and not current.

I/V curve and where we can see the MPP point

Why Should I Care?

This is quite amazing when you consider that householders are now able to avoid the installation costs of another inverter which also leads to more wiring and O&M costs.

Choosing a single inverter with a higher power rating (or nominal power output) is always more cost-effective than installing two inverters with lower power ratings to cover the same power size.

Actually, when selecting a single inverter, it’s possible to reduce the power rating of the inverter by 5%, which also reduces capital costs.

Conclusion

The best solution with if you have multiple roof orientation is always microinverters or DC optimizers. 

However, if you are considering the installation of a grid-tied PV system with a central inverter and it is necessary to install several panels facing different orientations, then you might want to discuss this with your solar installer. 

It is important to notice that these results were mainly carried out for east-west and NE – NW orientations with SMA and Fronius inverters; there are no studies concerning other configurations. 

Finally, another important conclusion from Fronius was that a single inverter with a single MPPT input has no disadvantages when compared to an inverter with two MPP-Trackers. If you want to see more details from the Fronius experiment, you can check the photo below.

Multiple orientation roof and solar panels, energy yield
Photo credit: Your Energy Solutions, Energy Matters, SMA, Fronius

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