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Micro inverters vs string inverters - which works best?

Posted by Paul Hutchens on 19 April 2011 at 8:45 am

The yield and efficiency of solar PV panels is a constant topic of debate. Talk of kWp and kWh as well as percentage efficiencies of solar PV modules rings out at energy clubs across the land.

This is just the headline information though. Just as an obsession with a car's top speed or the time it takes to get from 0 to 60 does not really relate to how long it takes to complete a journey - it does not relate to how well the "system" performs and how much useful energy is actually transmitted. 

The inverter is the main component of the transmission process. Firstly, it goes without saying, that the inverter needs to be chosen carefully. To install the best one, you need to consider the size of the PV array, the type of modules, orientation and pitch, location and potential shading problems.

Many inverter manufacturers have made significant developments including the inclusion of multiple Maximum Power Point (MPP) tracking which is the automatic adjustment of electrical load to achieve the greatest possible power harvest, during moment to moment variations of light level, shading, temperature, and photovoltaic module characteristics.

PV cells have a single operating point where the values of the current and voltage of the cell result in a maximum power output. MPP trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell.

Therefore multiple MPP trackers allow this to be done at multiple points in the solar array which will maximise yield even if part of the array is shaded.

Micro inverters, such as those from Cambridge based Enecsys, take this a step further.The micro inverter is a compact unit, installed at the back of each solar module. It converts DC to AC power for connection to the electricity grid without the need for a string or central inverter.

Apparently a micro inverter maximizes the power harvested from each solar module and therefore the entire PV array. This is accomplished by performing MPP tracking for each module ensuring maximum harvest of energy even under partially shaded conditions. Degradation in the performance of any one module, due to module mismatch, shadows from trees or chimneys does not degrade the performance of other modules and therefore has minimal impact on the power harvested from the PV array.

According to Enecsys this results in improved power harvest of 5% to 20% from the solar PV system. The system is also claimed to be safer as power conversion from DC to AC is done at each solar module, high voltage DC wiring is eliminated and a lifespan of up to 25 years is quoted.

As you would expect there is a cost implication to this and, although pricing for the UK is not formally available yet, micro inverters are expected to work out more expensive than string inverters. At Ecobuild I was told of a new version which effectively shares one micro inverter between 2 PV modules which may bring pricing to a par with string inverters.

In conclusion there are advantages to micro inverters but there may be a cost penalty which needs to be taken into consideration. I would also suggest that string inverters, particularly with advances in technology such multiple MPP trackers, would probably still be suitable for many applications.

More information about solar PV and inverters from YouGen

Inverters for solar PV panels: your questions answered

How important is voltage for a solar PV inverter?

Solar PV information page

Find a solar PV installer

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around 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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5 comments - read them below or add one

Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 3 August 2016 at 7:31 am

Hi Fi

You were right to be skeptical about the proposal form solar plants. It does not make sense to connect things in parallel as you suggest.

Inverters last between 5 and 15 years so it probably does not make sense cost wise to replace it until it fails.

Then, I would suggest calling an installer to discuss options. If you planned to install micro inverters or a Solar Edge system with voltage optimisers, bear in mind that the roof will need to be accesed again meaning scaffolding.

This is likely to increase the cost from a few hundred pounds for a simple inverter replacement taking an hour or two to a job taking a day and costing thousands!

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fiComment left on: 2 August 2016 at 9:32 pm

I have just been contacted by a company (solar plants) offering a replacement inverter, guaranteed for 20 years, and claiming to boost production by 17-25% (solar edge).

They claim that the panels will be connected in parallel, thereby maximising their output, and removing the issues caused by shading etc lowering output of all panels, rather than only those affected. 

Was visited by a salesman, who was unalbe to comment on our question about increased current resulting from parallel connection.

I will not be purchasing, but as my system is 5 years old, think it wise to start considering my options for when my current inverter (a Sunny Boy)  fails, and would value knowledgeable comments about replacement inverters. 

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sim0nComment left on: 6 December 2015 at 7:17 am

FYI, I just clicked on the Enecsys link above and ended up on a cyber-squatted domain. It looks like Enecsys is no more:

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Lee Higson

Lee HigsonComment left on: 19 October 2011 at 1:23 am

We had a 3.98kw system installed with 14 x 250w and 2 x 240w  Sanyo Panels.  Each panel is fitted with an Enecsys micro inverter.  After the first three months, we've received our first cheque for £580.  As far as I see it, there are multiple advantages to having micro inverters fitted :-

1) Failure of one panel or inverter will not degrade the rest of the system.

2) Each inverter is fitted with a WiFi transmitter giving constant data stream of the power being generated.  This can be monitored online at any location in the world.  You can see at a glance how each individual panel is performing, what the lifetime yield is etc.  If a panel or inverter goes into fault, an alert is immediately sent to the installer.

3) We have slight shading issues with a chimney during morning hours.  Micro inverters ensure that this does not bring down impair the performance of the remaining panels if only 1 or 2 are in shade.

We paid £17k for the system including micro-inverters.  The installer said the cost difference to a standard system was an extra £1.5k


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GM Renewables

GM RenewablesComment left on: 20 May 2011 at 9:22 am

My biggest concern with micro inverters is what happens when one of them fails.

Full scaffolding, remove the offending panel, which is bound to be in the middle, replave the inverter, refit the panels and then pay the bill.

SMA's Optitrac is a far better proposition.  then search for optitrac.

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