Micro Inverters and AC Solar Panels: The Future of Solar Power?
(If you don't particularly feel like reading my in-depth analysis of Micro Inverters, I've created an infographic summarising the pros and cons of microinverters here. I've also summarised my analysis in the video below. Otherwise, read on!)
In the beginning, there were solar panels which produced electricity as Direct Current. The same kind of electricity that you get out of batteries.
To make that electricity useful for powering appliances or connecting to the grid, it was converted to Alternating Current (AC) using a big box of electronics called an inverter.
In Grid Connected systems, the solar panels were connected together in series (called strings) to create higher voltage DC which is good for reducing losses. However, this also created some problems.
That’s why the Micro Inverter was invented.
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 from reliability issues and high prices. In the last few years, though, they have re-surfaced and are starting to really take off.
Around the world there are at least 21 different brands of Micro Inverters, and in Australia 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 Australia 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 produced by a series string system can create a risk of very high temperature arcing and potentially fire. Because Micro Inverters convert to 240V AC, the potential for this to occur is greatly minimised.
High voltage DC requires relatively expensive protective switches and fuses. By using AC, switchgear is more commonly available and thus cheaper.
To make this all a little more entertaining, I've put together a video discussing shading and solar panels. If you don't particularly feel like watching, feel free to skip it and just read the summary below.
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, and Micro Inverters do have some downsides:
They're 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 the USA 40% of all residential inverters installed in 2013 were Micro Inverters (90% of those were made by Enphase), so they are taking market share rapidly. They are growing in other countries too, but are best suited to residential markets where the price difference is less noticeable.
The Australian solar market is around 98% residential, so it is logical that we will see pretty strong growth here too. Already around 15% of the Aussie market is utilising Micro Inverters.
Who sells AC Solar Panels and Micro Inverters?
We know of 19 Micro Inverter manufacturers around the world including: Apparent, Delta, Sparq, Kaco, ABB, Array Converter, GreenRay Solar, Azuray Technologies, Petra Solar, Direct Grid, Accurate Solar, OKE/SMA, Exeltech, National Semiconductor, Larankelo, Enphase, APS, SWEA & Plug & Power.
But the only ones I've ever seen in Australia are Enphase and APS.
Enphase are undoubtedly the market leaders, with SMA and ABB (both huge inverter manufacturers) both having microinverters listed on their websites, but I don't know anyone who has ever seen one in the flesh. My guess is that ABB and SMA's hearts belong to central inverters and their micros will quietly disappear.
There is a growing list of big name PV companies around the world who have partnered with Micro Inverter companies to produce and sell AC Solar panels, including Trina Solar, BenQ, LG, Canadian Solar, Suntech, SunPower, NESL, Hanwha SolarOne, Sharp and probably more that we haven't heard of yet.
In Australia, we have (at the time of writing) one company producing AC Solar Panels. Adelaide's Tindo Solar manufacturer solar panels and use Enphase Micro Inverters providing our only true, locally made AC Solar Module. AC Solar Warehouse based in QLD also sell a good range of Micro Inverters.
Beyond these two companies, a growing number of Distributors and Dealers can access Micro Inverters and fit them to pretty much any solar panel you like.
The last word
A few readers have asked if AC inverters represent a "Do-It-Yourself" option and the short answer is no, although it is tantalisingly close. Because you are dealing with 240V AC power, a licenced electrician is still required to connect them and for rebates or FIT's you'll need an accredited installer.
Just how much of the market Micro Inverter equipped AC Solar Panels take remains to be seen but one thing is for sure; they do solve some hairy problems and have a big future.
If they can get the cost down just a little bit more, then they may well become the dominant solar inverter technology, with the big centralised inverters joining all those mainframe computers in the big electronic scrapyard in the sky.
Can you add batteries to a microinverter based system? The answer is here.
An example of microinverters and shading on my roof is here.
A cheaper alternative to microinverters are DC Optimisers
And finally, you can get 3 quotes for microinverter based solar systems here.