28 May 2014
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Solar
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Micro Inverters Vs String Inverters – which are better?

With the emergence of micro inverters in the last 18 months, there is now quite a debate about micro inverters vs string inverters – and which are best to use in a solar system?

Firstly – what is the difference between the two? Here is a quick summary.

String Inverters (or DC) up until around 2 years ago were simply called “Inverters” – because they were the only type of solar inverter available. They feature groups of panels wired together in an electrical series (or string) which feeds into the inverter as a DC input (around 400-600V DC), and comes out of the inverter as 240v AC – the same as the power that comes out of your powerpoint.

Micro-inverters (or AC) as the name suggests, are smaller, scaled down inverters that are connected to the back of each individual solar panel, in effect turning each panel into a mini-system of its own. Each panel produces 240v AC, straight into the main power supply of the house. This also allows the performance of each panel to be monitored individually, instead of just monitoring the system as a whole.

 

At Greenlife, we use SMA “Sunnyboy” inverters for our string inverters, and Enphase for our micro-inverter systems.

Here are the two head to head:

1) Each panel working individually Vs the weakest link principle: With a string inverter, if one panel is compromised, the other panels can only perform as well as the lowest performing panel. This is generally caused by shade. With micro-inverters, as each panel has its own inverter, only the compromised panels are effected, the others can work normally. This really only comes into play if there is noticeable shading on the roof, and also comes down to system design. Solar panels and shade are not a good mix, no matter the inverter setup.

Most of the time it comes down more to the placement of the panels themselves. Most modern string inverters now have 2 inputs, which generally halve the effect that shading would have in any case.

Be aware also, that the best micro inverters on the market are still lower efficiency than even the poorest string inverters.

SMA inverters are generally up around 98%, micro inverters are usually around 93-94% – an important factor in itself.

If the house has a very high amount of shade on the roof, micro-inverters will generally produce the most power over the year – but a formal inspection and shade test is always recommended.

2) 240V AC Vs 600V DC Micro-inverters are often presented as “safer” as they produce lower voltage than string inverters. And whilst you can’t argue that this is true – it’s not to the extent that it would appear. Lower voltage is never a bad thing, but that’s not to say that DC solar systems are dangerous. With the amount of disconnect switches and fail-safe points in solar systems under current standards – we don’t view either option, AC or DC, as a noticeable risk.

Using good brands of equipment, and a quality installation is a far more important factor in regards to safety than the voltage of the system alone.

3) Should the inverter be on the wall, or up on the roof?

String inverters are generally mounted on an external wall, often near the meterbox or switchboard. We always try to find a shaded area for the inverter, either on the south or east wall, under good eaves, or inside the garage. This keeps the inverter very cool, and helps to ensure the longest lifespan.

The biggest issue we have with micro inverters is that each inverter is up on the roof, under each panel, which is really not ideal when you think about it. The hotter an inverter gets – AC or DC – the less efficiently it works.

During last summer in Adelaide when temperatures of 40C+ were all too common, we were aware of some micro-inverters turning off altogether due to overheating, which came as no surprise when you think about how hot the roof will get on a Summer day. Even though the micro inverter is under the panel, the heat in and on the roof is still immense. We think it’s far better and more logical to have an inverter on the wall, in a shaded area.

Finally, if a micro inverter does fail, you need to get back on the roof to take off the panel in question, plus in most cases a lot of the other panels on either side – with a string inverter, you can just replace the inverter on the wall – a much quicker and easier job. We use SMA inverters because they are by far the best DC inverters in the world.

We have only replaced 2 SMA inverters in our history – an amazing record that speaks for itself. SMA also cover the cost of any replacement labour – many micro-inverter brands don’t.

4) System Monitoring

In this category, the micro inverters are the winner. Enphase systems come with computer monitoring that allows you to track each panel individually, to record output or to see if one panel or inverter has stopped making power. With string inverters, you can follow each of the two inputs, and see the performance of the system as whole.

SMA inverters do come with Bluetooth now so if you have a PC, you can still log system performance via your computer. Both types of inverter offer very hi-tech monitoring of your system, but the micros offer a higher level of detail.

5) Cost

The biggest factor for some people, and something always worth factoring into the equation. String inverters offer great economy of scale; a 5kw SMA inverter is only a few hundred dollars more than the 4kw model. With micro inverters, you don’t really get this scale, and each inverter (usually requiring 4 micro inverters per 1kW of solar) costs in the hundreds individually.

Although micro inverter systems are somewhat simpler and quicker to install, they are also noticeably more expensive than even the best (SMA) string systems. Generally in the $1000’s. Therefore, if your roof is suited to a string inverter system (as most are) they offer the best performance for your dollar.

 

In summary, we think string inverters are the way to go – most of the time.

They are the “tried and true” inverter technology, more reliable (in our experience), higher efficiency, better value for money, not to mention cheaper and easier to replace in the event of a failure.

Micro inverters are at their biggest advantage when there is a lot of shade over the roof, or if it is a roofline with multiple directions and angles – but in most cases, the string inverters are the best bet, and offer better peace of mind.

Contact Greenlife on 1300 365 378 or email info@greenlifesolar.com.au for more information.

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