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Do I need two inverters or one for my solar PV panels?

Posted by Cliff Willson (NEF) on 20 May 2015 at 10:30 am

Question: I have had a quote for PV with 1.75kWp on my south facing roof and 2.25kWp on my west facing roof.  I wanted to use the full 4kWp if at all possible to get the most FiT.  What I wanted to know was about the inverter.  My installer has only quoted for one inverter, but a friend who has a small system on her main roof and one on her garage has two inverters.  So, do I need two inverters or just one?

Answer:  This isn’t so easy to answer.  Basically it will depend on what kind of inverter is being offered.  Your installer could use two or he/she could use one, as long as it’s the right one.

Now the technical bit. For domestic or small-scale systems up to 4kWp solar PV two reference documents will help: the Microgeneration Certification Scheme (MCS) Microgeneration Installation Standard MIS 3002 issue 3.3 (release date 1st May 2015) and its predecessor issue 3.2. 

Solar PV Systems standard MIS 3002 specifies MCS requirements for those MCS certified contractors undertaking the supply, design, installation, set to work, commissioning and handover of solar PV microgeneration systems for permanent buildings.  In turn MIS 3002 states that solar PV microgeneration systems shall be designed and installed in accordance with the joint MCS and Electrical Contractors Association (ECA) publication  “Guide to the installation of Photovoltaic Systems” (ISBN 978-0-0974827-0-8, Hard Copy / ISBM 978-0-9574827-1-5, Electronic PDF

Section Inverter Sizing (page 48) of the guide states that the sizing of an inverter for a grid connected PV system is influenced by a number of factors, including:

  • The type of inverter or inverters available for use in the UK
    - Inverters must carry a Type Test Certificate to the requirements of Engineering Recommendation G38 [for systems up to 16A, AC output per phase] or G59 [for systems greater than 16A, AC output per phase] (as applicable)
  • Array voltage fluctuations due to operating temperature
  • The maximum permissible d.c. input voltage of the inverter
  • The desired inverter to array power ratio
  • The maximum power point tracking (MPPT) voltage range of the inverter
    - Where MPPT controllers automatically adjust the operating voltage point of the inverter so that the maximum power is harvested from the PV array as the light levels vary (as you would expect in normal operation)

Where a PV installation includes multiple strings or arrays with significantly different orientation or inclination, as in your case,  the strings or arrays should be connected to either an inverter with multiple MPPT function OR in the absence of multiple MPPT function, separate inverters should be utilised, as was most likely in the friend’s installation.  This is because the two systems will be producing differing amounts of energy at different times.

To make sure that you are investing in a PV installation that will maximise the amount of harvested energy you get, you will need to check the specification of the inverter with the installer and confirm that it has multiple MPPT function.  A good installer should be happy to explain to you why they have specified the system they have and if the salesman doesn’t know the answer, someone in their technical team will.

Also see:
Help: I'm getting conflicting advice about spilt aspect solar PV and inverters
Micro inverters vs string inverters - which works best?

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About the author:

Cliff is a senior energy specialist at National Energy Foundation. His is an active member of the Institution of Engineering and Technology and the Energy Institute. He's an Electrical Engineer by training with expertise in the theoretical and practical application of renewable energy and low and zero-carbon technologies and energy efficiency measures.

He provides training and technical dispute resolution services to various clients including the provision of expert witness services for both the police and trading standards.

Cliff is a Member of the Institution of Engineering and Technology, an independent MCS auditor for biomass and solar thermal technologies, an Independent Report on Metering Arrangements assessor (required for the Renewable Heat Incentive) and a certified Display Energy Certificate assessor.

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

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4 comments - read them below or add one


JamesMacComment left on: 9 June 2015 at 2:59 pm

I have heard similar things about solar edge systems rich. a good point at the end there, as usual sales people don't always know enough.

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LozzarkComment left on: 8 June 2015 at 3:22 pm

I have a 4kwk system with 12 panels facing SE & 4 SW.  The company installing said I'd need two inverters at first but then changed their minds.  I don't know what governs it.  The system works well returning what MCS said it would.

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richmcComment left on: 30 May 2015 at 1:30 pm

I have a friend also with a Solar edge system, unfortunatly he hasn't had the same positive experence that pmburton had, in the two years he's had it he has had four power optimisers fail, each time requiring full scaffolding and loseing his PV during peak generating months, he isn't happy. the more bits that go on the roof means theres more to go wrong.

After moving home two years ago, I got solar for my new home, having had experence with PV at my previous home. I had quotes from 4 or 5 companies, one of whitch insisted I NEEDED micro inverters, my house is SW facing with no shading or obstructions, needless to say they didnt get the contract. My system has a dual string Solis inverter. I would guess thats what whould work in your location, Talk to as many installers NOT salesmen who seem to have disaterously poor knowledge of their product.

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pmburtonComment left on: 20 May 2015 at 2:24 pm

You may also want to investigate a Solar Edge based system - a single inverter, with "power optimisers" on each panel.

I have such a system, with panels on a South facing and West facing roof, and it works very well. Another benefit of such a system is that shading of individual panels only affects the output from that panel, rather than the entire string.

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