Micro Inverters versus traditional string inverter systems
Aussie Made Solar offers both conventional grid connected DC solar systems as well as micro inverter solar systems; both have advantages and disadvantages over one another, many of which only effect the installer, but a basic understanding of the workings of each type will help you, the consumer, decide which system is best suited for your needs.
Conventional DC solar systems
Conventional grid connected DC solar systems typically only have one inverter often referred to as a ‘central string’ inverter which is usually mounted on the wall near your property’s switchboard. Due to their problematic nature which is discussed in detail below, the design of these systems can be quite complex, and if not done correctly by an experienced professional, the result is often an under-performing and disappointing system.
- Cheaper: The initial cost for these systems is however considerably cheaper than micro inverter systems, which is their main advantage. For small systems however, the price difference may not be much at all.
- Easily replaceable inverter: The second main advantage is that in the event of an inverter failure, the inverter is usually in an easily accessible location, allowing for a quick, safe, and easy replacement by one person only.
- Dangerous high DC voltages: In DC solar systems, the solar panels are electrically connected together in series to form an array or arrays which are known as ‘strings’. It is quite common for an installation to consist of just one string, and the resultant lethal DC voltage is typically between 300 to 600V DC for a domestic installation, and much higher for larger commercial systems.
The main problem with DC voltages in solar systems is that they cannot be circuit protected like regular 240V AC circuits, as the DC circuit is designed to operate at the full short-circuit current of the solar panels.
This means that in the event of a DC arc forming as a result of vermin chewing through the insulation of the DC wiring for example, at no point will any circuit breaker trip; the arc will continue to grow and potentially cause a fire. Click here for further reading regarding the dangers of DC arcs.
New legislation introduced in Australia in late 2012 in the form of Australian Standards AS5033 2012 has addressed this issue somewhat by stipulating that all new installations of grid connected DC solar systems have their DC wiring entirely enclosed within Heavy Duty conduit when run internally, in order to better protect the cabling.
- The Christmas light effect: We are all familiar with the problems with Christmas lights; if one bulb blows, the entire string goes down.
This is the same for conventional DC solar systems; if one solar panel fails, the entire string or array stops working.
And the bad news doesn’t stop there…if just one panel is partially shaded and only producing 50%, potentially the entire array could only produce 50%. The system is therefore only as strong as its weakest link.
Bypass diodes within solar panels can sometimes assist in these circumstances, but are often ineffective. Complete failure of an array is quite easily detectable by the system owner, but what about a partial failure of a solar panel resulting in a total system power degradation of 10 or 20%? The system owner may never be aware of it, and has no system monitoring capability at the individual module level. And if a problem was suspected, an electrician may have to test every solar panel in order to find the problem; this could be quite time consuming and expensive, especially on large commercial installations consisting of hundreds of solar panels.
- Higher maintenance costs: Conventional DC solar systems require higher maintenance costs in order to keep the system operational for the lifetime of the solar panels.
These costs are usually associated with inverter replacements and subsystem component failures such as rooftop DC isolators and string fuses.
Micro Inverter solar systems
Grid connected micro inverter solar systems, also referred to as AC solar systems or Intelligent solar systems, differ radically to conventional DC solar systems via the elimination of the ‘central string’ inverter concept and instead, have a small inverter known as a micro inverter installed under each and every solar panel in the installation.
This enables many previously only dreamed of features and advantages over conventional DC solar systems, and eliminates virtually all of the major problems associated with those systems.
These advancements obviously come at a cost, and micro inverter systems also have other disadvantages that are avoided be conventional DC solar systems.
- Safer: The system does not employ dangerous, unprotected high DC voltages, eliminating the risk of potential fires as outlined above. The system’s cabling through the roof to your switchboard is regular 240 AC, the same as all other household cabling, and is circuit protected via a circuit breaker in your switchboard.
- Shade tolerant and fault redundancy: Each solar panel operates independently, as it has its own dedicated micro inverter. The result is that the system does not suffer from the ‘Christmas light effect’ explained above; one shaded or soiled panel will result in a loss of power from that panel only, and will not drag down the performance of the others. The same goes for any fault; in a conventional DC solar system, a complete failure of a solar panel or central string inverter will usually result in an entire system failure.
- Longer warranty: Most microinverters have a minimum warranty of 10 years; some have as high as 25 years. This is in large contrast to only 5 years for most string inverters. Even the best string inverter manufactures freely admit that in the life-time of your solar panels you will need to replace your string inverter at least once. The reasons why micro inverters have a longer life expectancy and warranty is due to radical differences in the electronic componentry. String inverters deal with kilowatts of power and high DC input voltages typically ranging from 300V to 600V DC for domestic systems, and even higher for larger commercial systems. As a result, these types of inverters require electrolytic capacitors and high power transistors – components that tend to give up the ghost after 5 years of constant operation. Micro inverters however, are only dealing with around 250 watts of power and only around 40V DC, subsequently avoiding the need for troublesome electronics, and instead often employ breakthrough thin-film capacitor technology and solid state ICs which make lengthy warranties possible.
- Extra electricity yield: Micro inverter solar systems boast higher average long term electricity yields between 5 to 25% more than conventional DC solar systems. This is due to a number of reasons including:
(a) shade tolerance and fault redundancy as explained above,
(b) individual Maximum Power Point tracking (MPPT) of each solar panel,
(c) lower de-rating of output power at high temperatures,
(d) less down-time due to higher system reliability,
(e) lower operational DC input voltages, meaning that at first light, the micro inverters will produce power long before a conventional central string inverter will. Similarly at last light.
- Real-time individual panel monitoring: Internet based, the system owner is able to log on from anywhere in the world and view the performance of each and every single solar panel on their roof in real-time. The installation company as well as the inverter manufacturer are also able to do this. Technical support is therefore readily available and any faults are relatively easily detected and often indisputable. The larger the system the more advantageous this feature, as far less on-site troubleshooting is required in order to find a potential fault with a solar panel. In a conventional DC solar system, any suspected fault with a solar panel may result in an electrician having to disconnect and test each and every solar panel in order to find any fault.
- Easily expandable: The system size is not ultimately dictated by the capacity of a wall mounted string inverter, allowing for additional panels to be added at any time and only limited by your roof space and budget.
- Flexible and simple system design: Unlike conventional DC solar systems, the design for micro inverter solar systems is relatively simple and overall flexible. Solar panels employing micro inverters can be positioned to face several different directions in the one installation, and at several different tilt angles without any system parameter limitations, making the design almost child’s play. For split level, multi-gabled, multi-angled, and multi-pitched rooves, this design flexibility can mean the difference between having no system at all installed, and a well working micro inverter solar system.
- No unsightly inverter on your wall: This is a small point but one worth mentioning nonetheless. All wall mounted string inverters also radiate heat and many generate undesirable noises such as humming, buzzing or whirling from cooling fans.
- Cost: The initial cost for a quality micro inverter solar system can be considerably higher than that of a quality conventional DC solar system.
- Requires an Internet connection: Although the system will still produce power, without an internet connection and a computer or smart phone, many of the advantages and benefits of the micro inverter system are moot.
- Difficult to replace a micro inverter: In the event of a failure of a micro inverter, the electrician will need to climb on the roof and remove a solar panel in order to change out the micro inverter.
This is a dangerous maneuver which may require two people, adding to time and cost.
So which type of system is best for you?
We would all like a chauffeur-driven Rolls Royce, but most of us have to settle for a Holden, Ford, Toyota or similar. If budget constraints are a major factor than perhaps a conventional DC solar system is the way forward; after all, solar power in any form is a step forward in the long run, both economically and environmentally.
A smaller than desired micro inverter system should also be considered if your budget is tight, as the system can easily be added to later. If you do not have a computer or internet, then a conventional DC solar system is the way to go, as many of the advantages and benefits of the micro inverter system would be null and void. If you are considering obtaining an internet connection at your property just for the purpose of having a micro inverter solar system, then the ongoing internet costs should also be factored into the price of the system.
If you have partial shading on your roof however, than the micro inverter system is the clear choice, as a cheaper conventional DC solar system may not work very well at all under such circumstances. If you have a difficult roof (i.e. a combination of split level, multi-gabled, multi-angled, and multi-pitched), then a micro inverter solar system may be the only option you have.
Whichever type of system you choose, quality components should always be on your agenda. Always keep in mind that not all solar panels are created equal, and the same goes for central string inverters and micro inverters.
Take the time to research the product and the company selling or installing it. After-all, you are not buying a tooth brush; few can afford to simply throw it out when it stops doing the job and buy a new one.