A Micro Inverter is simply a miniaturised inverter, sized to suit individual solar panels rather than a string of solar panels.
They aren’t new; they first appeared in the late 1990’s but arguably it was a bit too early and the technology suffered a bit from reliability issues and high price. In the last few years however, they have re-surfaced and are starting to really take off.
Around the world there are at least twenty one different brands of Micro Inverters and in New Zealand, about 8 brands of Micro Inverters are approved for use with more on the way.
Here's what one of the little fellas looks like next to its conventional alternative, a big, bad central inverter.
AC Solar Panels
An AC Solar panel is simply a solar panel that has been fitted with a Micro Inverter so that it produces Alternating Current instead of Direct Current.
A typical "Series String" array
Most of the solar panels installed in New Zealand right now are configured like this, with one big inverter and one big DC voltage. If that 600V DC arcs then there's going to be a bang! And possibly a fire. (Which is why you should never skimp on installation cost)
A typical "AC Solar Panel or Micro Inverter" array
What's good about AC Panels and micro inverters?
There are a number of complexities caused by the traditional way of connecting solar panels together (in a series string) which Micro Inverters can help overcome including:
High Voltage DC
High Voltage DC can create a risk of very high temperature arcing and potentially fire. Because Micro Inverters convert to AC the potential for this to occur is greatly minimized.
High voltage DC requires relatively expensive protective switches and fuses. By using AC, switchgear is more commonly available and thus cheaper.
When solar panels are connected together in a series string, shading just one of them can dramatically affect the entire array; (kind of like standing on a hose).
As an example: Here's an array of 3 solar panels connected to a conventional, central inverter. One of the panels has been pooed on by a bird, which could easily reduce its output by 50%. However, it will also reduce every other panels' output by the same amount:
But by having a Micro Inverter on each solar panel, the outputs are completely independent of each other. So that bird poo is going to only affect the soiled panel:
According to the US National Renewable Energy Laboratory, this effect can yield as much as 12% more energy.
Solar panel mismatch
When solar panels are made they each have slightly different electrical characteristics due to imperfect manufacturing tolerances. When you connect them together in series string, this effect is called "mismatch". Micro Inverters can adapt to the individual characteristics of each panel, avoiding mismatch.
Maximum Power Point Tracking
Like the effect of mismatch, different electrical characteristics also create different Maximum Power Points for each solar panel. The maximum Power Point is the perfect point for extracting maximum power from a solar panel and Micro Inverters attached to individual solar panels can therefore target this point better.
Monitoring and fault finding
Almost all inverters have some level of monitoring and fault finding however; it can only see the combined output from every solar panel in the series string. A Micro Inverter however, can monitor each solar panel individually, allowing you to easily identify exactly what's happening more quickly and easily.
Assembling and connecting components in a factory environment is inevitably a more controlled environment and can potentially save time and money. A number of solar panel manufacturers now factory assemble Micro Inverters to produce AC Panels.
If your series string inverter develops a fault, your entire solar array stops producing power until it is fixed. If a Micro Inverter develops a fault, the remaining Micro Inverters can continue to operate, so you should have a more reliable system.
Series string inverters can only accept specific number of solar panels per inverter so it's not always possible to simply add a few more panels at a later date. AC Solar Panels however, can be added much more easily because they are independent of each other.
In a series string, all your solar panels need to be connected in the same orientation so they are combining to produce the right voltage at the same time to fire up the inverter. Because they operate independently, AC Solar Panels can be oriented in any direction and will not affect the operation of other solar panels.
What's bad about AC panels and micro inverters?
Nothing is perfect however, and Micro Inverters do have some downsides including:
On the roof
If your Micro Inverter develops a fault, someone has to get up on the roof and disconnect it from under your solar panel. This can add time and cost, compared to simply taking a series string inverter off the wall.
Because Micro Inverters are on the roof (albeit under the solar panels) they do suffer from more extremes of weather including heat, cold and moisture. This means they have to be really carefully built and in many cases, use electronic components that are more robust than would otherwise be required. As a general rule, extremes of temperature reduce the efficiency of electronic devices and shorten their life.
Although they are getting close, Micro Inverters have not yet reached the same efficiency levels of series string inverters, so they can't convert as much solar energy into electrical energy.
Again, they are getting close, but Micro Inverters remain about forty to fifty percent more expensive than high quality series string inverters and almost three times the price of low cost series string inverters.
When is an AC Solar Panel or Micro Inverter a better choice?
As you can see, there are a number of advantages to AC Solar Panels using Micro Inverters. The most common reason people choose them is because they have shading or they need to use different orientations on their roof to generate the power they require.
An increasing number of people are also choosing them because they are prepared to pay a premium to avoid mismatch, increase their redundancy and allow for future expansion. The other benefits described tend to strengthen the case and some people just love the idea of having the latest/newest technology.
Clearly, if you have shading or sub optimal orientation Micro Inverters are an ideal choice and could end up being cheaper in the long run because they will produce more energy.
Beyond this, Micro Inverters become a personal choice about how much you are prepared to pay balanced against the extra features and advantages.
How popular are they?
A few years ago there were virtually no Micro Inverters or AC Panels installed around the world. However, in California it is estimated that around 40% of all inverters installed in 2012 were Micro Inverters, so they taking market share rapidly, particularly in the US. They are growing in other countries too, but are best suited to Residential markets where the price difference is less noticeable.