Solar Panel Design : North-East-West

Can I face solar panels North, East and West?

It’s a solar panel design question I pose to almost every electrician and solar salesman I interview: “Can we install solar panels without optimisers on three different orientations?” Nine out of ten times, their answer is wrong.

To explain the answer, I’ll go back to the basics with the power formula. Next, I’ll explain how we design solar panel systems both in series and in parallel, and how this affects voltage and current. Then I’ll show how, with a standard string inverter, we can install solar panels in more than two orientations. Finally, we’ll consider some things we should avoid when paralleling east and west, and more efficient options that are available in these situations.

Lesson #1: Power = volts × current.

Watts, volts, amps, power, current. You may be familiar with all these terms but might be uncertain about how they relate to one another. I’m going to explain these terms with the water pipe analogy. (The way I explain it is simplified – it’s not a perfect analogy.)

• Current is similar to the size of the water pipe. The bigger the pipe, the more water can flow.  We measure current in Amps (I).
• Volts is like the water pressure on the street or the pressure behind the water pipe. In Australia, that “pressure” for electricity is 240 volts (V).
• Power (P) is the result of the two. A fire hose has a lot of power because it’s a big hose flowing at high pressure. We measure power in Watts (W).

There are three ways we can write the formula; all mean the same thing.

You can use this basic formula with household appliances.

A 2400W kettle plugs into a 240v powerpoint. How much current does it draw?
2400W = 240v × 10A

Or use it for solar design:

A single solar panel is operating voltage is 30V and 8amps. How much power is it producing?
240W = 30V × 8A

Lesson #2: Voltage is added in series

When we design a solar panel system on a roof, we generally connect them in”strings”. A string is a group of panels (usually between 5 and 13 panels) “daisy chained” together – or more correctly connected in series.

In this diagram, we have seven solar panels strung together in series. One panel (with reasonable sunshine) produces 8amps and 30 volts. As mentioned earlier, that one solar panel will produce: 30V x 8A =240W.

When we combine the seven solar panels, we have 210v but still only 8amps.
210v x 8a = 1680 W.

Note that the voltage is added in series, the current remains the same.

Lesson #3: Current is added in parallel

When we parallel two of these strings, instead of adding the parallel voltages together, we add the current together, and end up with 210 Volts and 16 amps,

210v x 16a = 3360W

This time the current is added in parallel, the voltage remains the same.

Lesson # 4: Solar panel design on a simple roof

Now let’s design the parallel string of 7 solar panels and the single string of 7 panels on the roof. To make the job simple, we’ll first choose a big house with a solar-friendly roof.

Most string inverters have two separate inputs called Maximum Power Point Trackers or MPPT’s.  As the sun’s intensity and angle on the solar panel changes during the day, the MPPT continually “Tracks” and adjusts the voltage and current to find the “Point” where it can achieve the “Maximum Power”.

Notice how each string of solar panels is facing the same orientation. We have one string of eight panels facing North, and two strings of eight panels parallelled into the other tracker facing east.

But let’s get back to reality. Not every roof is so solar panel design friendly. Let’s look at what happens if your roof is more limited for space.

Lesson # 5: A series string of solar panels must face the same direction

This one is straight out of the Australian Standards.

Here’s where many solar electricians and solar salespeople get unstuck with a solar design. They know that all panels in a string must face the same direction, but then they assume all the panels connected to one MPPT must face the same direction.

If we apply the knowledge we have so far; it seems to make sense. We face series panels in different orientations because the panels that run at a lower current will act like a kink in the hose.

The idea then is that we also cannot design parallel panels to face different orientations, because the panels that run at a lower voltage will drag down the panels that run at a higher voltage.

But, it turns out, working within inverter input current limitations, we can!

You may know enough about you household electricity to know that runs at 240volts. This figure is relatively constant, in fact by regulation it can be should sit between 220 volts and 253 volts. So when you are using more power at home, it is not because the voltage is changing – it is the current that is changing.

Now back to solar panels. When the inverter first turns on in the morning, each solar panel operates at about 30 volts and 0.1 of an amp. As the sun comes up in increases to 2amps, then 3amps and until it reaches it’s maximum of around eight amps at full sun. The voltage, however, remains about the same: around 30 Volts.

Below is Fronius monitoring data of an east-west system we installed last year. Because we did not need to parallel this job, we have the voltage data of the east and west array running on separate MPPTs.

The Blue line shows the power increased during the morning. For simplicity, I haven’t displayed current it in this image, but it follows the blue power curve. While the two DC voltages don’t perfectly match, they remain similar – usually within 5 percent of 350 volts. If this solar panel design was with two strings to only one MPPT, then the MPT would adjust the voltage to the point that would allow the most power to be produced out of the two strings.

Note, there was a point in the morning when the purple line drops off. This is the western panels that were producing very little power. If both the east and west panels were on 1 MPPT, the voltage would have remained high, and losses on the western array would still have been minimal.

How much efficiency is lost?

In 2012, a case study by Fronius showed how installing east-west panels on 1 MPPT caused losses of less than 1 percent compared to using two inverters / 2MPPTs. This case study is admittedly outdated as the inverters that were used were less efficient than inverters available today. The actual amount of  efficiency loss will depend on several factors:

• Orientations only 90 degrees apart will cause less voltage variance and less loss.
• Solar panel design on flatter roofs will cause less loss.
• Installing one array in the shade will cause significantly more losses on the other array and should be avoided.
• The inverter’s optimal power and voltage efficiency curves.

However, in the right situation, splitting panels east and west on 1 MPPT can be a cost-effective way of installing a larger system on your roof.

Complicated and shaded roofs

If your roof line dictates it, or if increased cost is not a concern, Module Level Power Electronics may be a better option. MPLE allows every panel to run independently of each other, regardless of orientation or shade. Here are the options available:

• Read this post for an introductory explanation of Tigo. This is my choice at the time of writing, but I change that opinion regularly!
• This post explains SolarEdge optimisers.
• This post explains  Maxim optimisers, although, I’m no longer a fan of Maxim.
• And I’m currently testing Huawei, but here is my Huawei, introductory video.

Just as a ballpark, a 6.6kW optimised solution can cost you about $1000 more than a standard Fronius non-optimised system.
If you are in South East Queensland, we’d be happy to advise on a solar panel design that would best suit your home.

Conclusion

When designing solar, (without optimisation) installing a series string of panels in two different orientations will cause significant power loss. For this reason, AS5033 does not allow it. However, running two parallel strings of panels in different orientations will only affect the voltage slightly, so power loss will often be minimal. As a result, a solar panel design on 3 different orientations with a dual MPPT inverter will cause minimal power loss. We take factors such as shading, pitch, and inverter efficiency curves into account, and in some situation, we recommend using optimisers.

For further reading on basic solar design, read my post:  What’s the best direction to face solar panels.