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Solar Microinverters vs. String Inverters
Traditionally, we have always used string inverters to convert the solar-generated Direct Current (DC) into Alternating Current (AC) – for use with everyday household appliances – because getting a designated inverter for every single solar panel was considered to be obtuse and costly.
The bad news is, getting an individual inverter for each solar panel is still a bit more costly endeavor to undertake. The good news, however, is there are a few exceptions now since the time 'microinverters' hit the solar markets somewhere around 2007-2008. Even if they do cost more most of the time, they're definitely worth the extra cost in terms of output of electricity and troubleshooting. But first, let us try to understand the need for an inverter in the first place. Why not just use DC to power our appliances?
See, the flow of electric charge is unidirectional in DC, but for AC the flow of electric charge periodically reverses direction. DC voltages also tend to vary over time and require the use of a voltage regulator to get the same voltage over a period of time. For these reasons, DC carries energy more slowly and tends to lose some of it along the way. Moreover, DC installations require the use of completely different sets of sockets, connectors, switches and fixtures, mostly due to the low voltages used, so it complicates DC usage in a world dominated by AC designs.
AC, on the other hand, uses both positive and negative voltages that rise and fall - just like smooth sinusoidal sine waves – and is therefore more efficient for transmission of power. It carries energy faster without losing any of it. This is the reason why most electrical appliances use AC and it remains the standard for power suppliers as well. This is also the reason why we must use an inverter to make use of the solar energy our solar power systems produce.
Coming back to the issue of how string inverters are less efficient than their ‘micro’ counterparts, a fair question to ask if your knowledge of Physics is a little foggy could be, "You mean I should get a separate inverter for each solar panel just to convert DC into AC. Is that really wise when we’re considering efficiency?"
It does seem counter-intuitive, but for good reasons. What a string inverter does is, it combines the power generated from all the solar panels in a string, together, which can sometimes be as high as 600 volts (DC), and converts it into AC electricity so that we can use it for electrical appliances in the house. It is usually much simpler to install as well since it is available as a single unit, and results in a lower cost for every Watt produced. Yes, this does mean greater efficiency in a cost-benefit analysis.
The only major downside to using string inverters – apart from their size and the noise they produce – is that a single solar panel’s failure can result in a failure of the entire solar power system. Because it uses the power generated from all the solar panels in a string, one tiny tree branch shading just one solar cell – say, 9% of the total setup – can decrease power output of the entire system by as much as 54%!
String inverters are optimized for the weakest link in the entire system. They do not optimize separately for each solar panel, which is precisely what microinverters are able to offer – individual optimization. Manufacturers of microinverters claim they are able to increase power output by 5-25%, which can mean a lot if you pragmatically consider efficiency of a solar system over a long period of time.
To better understand optimization, we must consider one of the trickiest things in solar power system installations – Maximum Power Point Tracking (MPPT). For every solar panel or entire system installed, its voltage needs to be adjusted according to the level of sunlight it receives for maximum output of power. This is important because the voltage load applied by the inverter determines the performance of a solar panel. MPPT is merely the technique used by installers to determine the right voltage setting.
In the case of string inverters, the MPPT is applied across the entire system, which is connected in a string array. This is why any kind of shading – dirt or snow or tree branches, or even a slight disorientation of one of the panels can have a profound effect on the output of an entire solar power system.
In most cases, microinverters are little black boxes installed at the back of each solar panel. The MPPT is applied for each microinverter, allowing every solar panel to perform at its maximum potential. So just one or even two underperforming solar panels will not drag down the average performance of the entire solar array.
So microinverters are easier to troubleshoot, save you money over a long period of time, support multiple orientations, and help increase power output. What else? Well, apart from a longer warranty, from 10 to 25 years (compared to the usual 5 years for string inverters), there are some downsides to getting microinverters as well – they’re not just costlier.
Since they’re mounted on rooftops with the solar panels, incorrect installation might expose microinverters to high heat from the sun which could significantly reduce their life.
But if you think about how manufacturers are giving warranties spanning 25 years, the downsides hardly seem to matter. It is clear that microinverters have helped solar technology enter a new era. Moreover, it can be reasonably predicted that the price per watt for microinverters will continue to drop, until it becomes cheaper to use microinverters over string inverters.
For now string inverters are still the standard component for solar power systems, but more and more New Zealand solar installers are offering microinverters as an option. The choice is yours!
Article by Kristy Hoare of MySolarQuotes.co.nz