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PV systems need an efficient inverter

Posted by Graham Eastwick on 15 May 2009 at 11:07 am

Photovoltaic (PV) cells and modules are only part of the picture when you consider the efficiency of a solar electricity generating station. Key to an effective PV system is an efficient inverter, which converts the direct current (DC) electricity produced to AC (alternating current or mains equivalent).

For small systems connected to the national grid (rated up to around 4 kWp) you will need a G83/1 approved inverter. This makes connection to the grid simple and makes sure that the system operates safely. If the inverter detects a mains failure, your system will shut down so that there is no danger to anyone carrying out repairs on the electricity network.

Grid connected inverters are typically in the region of 93 per cent efficient. This will depend on the design of the inverter. To maximise efficiency, inverters typically have a MPPT feature (this stands for maximum power point tracking). It optimises the operating voltage point of the inverter so that the maximum power is extracted from the PV system as the light levels vary.

When putting a system together you need to make sure that under fluctuating light levels the output voltage range from the string of PV modules  is greater than the inverter's minimum input voltage range. If it is lower for a significant amount of the time the system will lose efficiency. It is equally important never to exceed the maximum voltage permitted by the inverter, as this may permanently damage it.

If you plan a large system, you may need to connect under G59/ regulations. The procedure is much more complex, so it is worth discussing this with the local electricity network operator (DNO) as they will often make exceptions. I have come across cases where they have allowed 10 kWp generators to be connected on a single phase.

photo by mountain ash

About the author: Graham Eastwick is a director of Encraft, and manages renewable energy installations for home owners, community organisations and small businesses across the UK.

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

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Comments

14 comments - read them below or add one

Savalot

SavalotComment left on: 12 June 2015 at 12:12 pm

Well, this is interesting. Further to my posted question on the suitability of an inverter to handle 4KWp from a single string of 16 panels, I have two opposite opinions from two MCS approved installers!

One says: "Each string can take up to 750V DC. I have never heard of an inverter that cannot take 480V from a single string" 

The other says: "Each MPPT can handle the following, so if you are designing as two strings in parallel you need to connect to on on the inverter inputs or MPPT trackers only:

Each input or MPPT tracker on the inverter can handle:

3000W max total

Input voltage 140-530V

16 amps

Having two strings of 8 in series would be OK for the voltange but over the 3000W max power input and also over the 16A current input as each panel is producing 8.2A each string.

Obviously having two strings of 8 panels would be a 2000W power input, around 250V voltage input and 8.2A current which would work perfectly.

And this is standard for 100% of the inverters on the market."

So, who is correct? The inverter data sheet that is being used for reference is the ABB (ex Power-One) single phase 4.2KW model.

Perhaps one of the inverter manufacturers would like to weigh in, or perhaps there is a consumer out there who has a similar installation.

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Savalot

SavalotComment left on: 12 June 2015 at 2:55 am

I am planning a 16 x 250W panel 4KWp installation with all panels in a single string. The panels will all face SW on the unshaded roof of an outbuilding. I would like to transmit the output in a single 480DC cable to the incoming meter location about 110m distant to minimise line losses and cable size. However, the inverters I have looked at have a dual (rather than single) feed, and imply a maximum of 8 x 250W panels per string. Can you recommend an inverter (for exterior mounting) that could handle the 480V (peak) input?

 

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Cathy Debenham

Cathy DebenhamComment left on: 6 January 2015 at 3:28 pm

The short answer to your questions Mikey and Chris is that a 3.6kW inverter with a 4kW array of solar panels is pretty much standard. If you want to read more about why, you can do so here and here.

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Milkeysufc

MilkeysufcComment left on: 6 January 2015 at 2:17 pm

Hi,

My question is similar to Chris 100's back in Nov 2011. I have just had a 4kWp system installed and the inverter is an Afore HNS3600TL-1 (3.6Kw).

Is this going to affect the performance of my system? Should I have a 4Kw inverter instead?

Thanks.

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Chris 100

Chris 100Comment left on: 23 November 2011 at 1:05 pm

Hi

I am about to have a 4kW PV system installed but, because of the forthcoming changes in the tarrif, some of the equipment quoted has been changed - the panels are changing from 16no 250W to 16no 245W (that doesn't seem to be much of a change), but more importantly the supplier is changing from an SMA Sunny Boy 4000TL inverter to a Kostal Pico 3.6.  The former being a 4kW inverter (ie same rating as the original 16 250W panels) but the latter being a 3.6kW inverter.  Is the overall system output now restricted by the output of the inverter - should I insist that I have an inverter which is at least the same kW rating as the panels?

Any help would be much appreciated.  Many thanks

 

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Cathy Debenham

Cathy DebenhamComment left on: 11 September 2011 at 9:15 pm

Hi CarlGray

Have you seen this blog about solar panels across different roofs? It might help answer your question. 

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carlgray

carlgrayComment left on: 11 September 2011 at 9:36 am

I am hoping to have a solar PV panel system installed soon. The main issue is that I have 3 roof slopes that I can use: a west facing one which could have 7 panels, another west which will only be big enough for 1 panel and a south slope which could have 4 or 5 panels. I have heard that some inverters can cope with different facing slopes but that 5 panels on the south slope might not be enough for them to work properly. It would help if somebody could shed light on this matter in laymans terms and, if necessary, suggest an inverter than might be suitable. Many thanks

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Richard

RichardComment left on: 12 June 2011 at 3:41 pm

The blog suggests inverters are typically 93% efficient. PV Solar UK suggests that its Power 1 Inverter is 99% efficient and lasts longer because it doesn't use a fan. Has anyone got any comments about PV Solar UK or, more particulary, the Power 1 Inverter?

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Cathy Debenham

Cathy DebenhamComment left on: 16 May 2011 at 10:06 am

This is a great example of the sort of problem solving I envisaged when I started YouGen. Thank you to David and Chris for your input.

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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 15 May 2011 at 12:04 am

Hi Bob0260. It is accepted that the grid voltage SHOULD be under 250v, but in rural areas this 'on load' voltage can rise higher. This is far more evident with houses near a substation which has been adjusted for voltage drops over long cable runs.

However, the voltage will be checked during the commission process, and trained installers will be able to adjust the inverter parameters for your local conditions if outside standard program G83/1 settings. 

We have found that most of our 'problem' sites with fluctuating grid voltage going over 250v are rural sites adjacent to a substation or transformer. Usually over  weekends, or evenings the grid voltage will rise as the local circuit may have less of a load, (is there a local office or industrial unit), causing grid voltage to rise.

Chris 

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Bob0260

Bob0260Comment left on: 14 May 2011 at 4:20 pm

It was a DNO voltage issue. DNO measured the voltage at 250 v average and peaking at 257 v. They reduced the voltage yesterday and since then the inverter has been working normally. It think installers should measure the house voltage as part of their survey.

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David Hunt

David Hunt from Comment left on: 10 May 2011 at 2:18 pm

This can be an issue with your domestic electricity supply. We have had a customer in Cumbria with the same problem, when we came to do fault finding the grid power supply to the house was 265 volts.  The inverters in the UK are set to G83 standards and to cope with a 230-255 volts (230v + or - 10%). The other issue was due to a surge from a local dairy farm their domestic supply spiked up to 271 volts.  Your installer should be able to do this analysis for you, and you may need to contact the DNO (national Grid).

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Bob0260

Bob0260Comment left on: 10 May 2011 at 9:44 am

What causes an inverter to go into standby when the sun shines? My inverter switches off for most of the afternoon during the recent sunny spell. Last Saturday started off cloudy and dark. Then early afternoon there was a break in the cloud. The sun came out and the watts increased from 500 w rising to 3000 then the watts decreased to 90 watts even thought the sun was shining brightly. The inverter stayed in standby for 3 minutes and would only stay on if the sun was behind a cloud. A similar system to mine that is on the other side of town produced 200 watts more than me last month.

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Anonymous

AnonymousComment left on: 5 November 2009 at 8:41 am

Graham, check with this inverter manufacturer and see

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