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Solar PV across different facing roofs

Posted by Chris Rudge on 3 September 2010 at 12:59 pm

Q: I'm thinking of installing solar PV panels on two roof aspects, one south facing the other east. Can you please explain in layman's language what is meant by 'series' and 'parallel'. Different installers are recommending one or the other or both! 

A: Unlike battery charging PV systems, all grid connected systems need to have the solar modules connected in series which will provide a high voltage DC input to the inverter. This voltage is typically in the range of 150 to 500 volts.

For larger systems (over 3kWp), these series circuits (strings), will need to be divided into two or more as the total voltage will be too high for an inverter input. These separate strings will be connected in parallel into an inverter.

Another issue that is sometimes misunderstood, even by installers, is the problem of installing on roofs with variable or different aspects. As an example, if you have a double aspect roof, with one area facing south and the other east, both can have PV installed, but will need either two separate inverters, or a single special inverter with dual maximum power point tracking (MPP) inputs.

If both the South and East arrays were connected together to the same input, you would find the array with the higher output being pulled down to match the lower output array. It would emulate the whole system as never having any direct sun on it.

This can be likened to connecting a good car battery onto a poor battery, the poor battery will eventually make the good battery useless.

Photo by computerhotline

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one

Holly James

Holly JamesComment left on: 24 October 2011 at 5:05 pm

I found your post about pv across two different angle and orientations very useful. I have recently started installing solar Pv and am finding it a bit difficult to find all the answers as the course I went on was a bit vague.

The question I have about the above post is. How do you calculate the outputs if you have two roofs. Do you do the SAP on each individual array and then add them together? Even if you are using a dual tracking inverter.

Many thanks


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Explore Solar

Explore SolarComment left on: 16 November 2010 at 6:47 pm

Chris you have raised a good point, I am often finding that installers are recommending clients to install on different aspects on the roof, to make it worthwhile doing the installation, but quoting for 1 inverter only. They are not taking into account using two separate inverters, which will be required. If it’s a large system you can get away with using the SMA 4000TL with dual MPP tracking system, which can be used to install two separate arrays at different angles. You are able to install panels arrays at different pitches i.e. 4 panels at 30 degrees  and 4 panels at 5 degrees, so long as you take into account the net voltage from both arrays and make sure that sufficient voltage will be generated to make the inverter kick in at lower light levels or you will find that you panel array work inefficiently.

I am glad you have raised this point, since both installers and the general public are not aware of this. One way of find out if your installer is any good is to ask this question?

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superkinradeComment left on: 15 September 2010 at 3:49 pm

Thank you Chris Rudge and Paul Hutchens for the various comments posted regarding solar PV.

Being new to the business of posting comments I'm not certain how to kick off an 'original' blog.  I would, however, like comments from anyone who has contacts in house sales on the effect that solar PV (or thermal) is having on resale values - i.e. does it improve sales prospects or hinder them?

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