If we look at quality inverters for domestic PV setup today you might find the following features as standard:
- dual MPPT
- integrated Wi-Fi interface
- data logging control
- monitoring through phone apps
- grid-tie ability
- ART fault protection
- ground fault protection
- DC reverse polarity protection and
- anti-islanding protection.
When were these advanced solar inverters for domestic applications developed? Let’s have a look at that. Twenty years ago you had almost none of these features in a domestic solar inverter. In fact, most home inverters back then were UPS inverters, which are designed mainly for using large lead acid batteries as a source to provide uninterrupted power in case of power cuts.
Even today many UPS inverters are still used for solar installations in the developing world as they cost very little compared to purpose-built solar inverters. Going back to early 2000 the solar installations were mainly commercial and utility scale. There were high tech commercial solar inverters in the market that were being made by a handful of companies.
What is a string inverter?
These included SMA, Siemens and ABB. There were two different topologies for electrical systems that large scale solar installations could use. One was the centralized electrical system which used a single inverter for the whole setup. This was called the ‘central inverter‘ topology.
The other topology in solar farms utilized several small inverters instead of a single large one. For every string of panels there was one inverter handling the power and fed it directly to the grid. This inverter was called the ‘string inverter‘.
With time it became clear that the configuration with several string inverters was much better than the central inverter configuration in terms of efficiency, even though the upfront costs were higher. About 15 years ago, in around 2005, string inverters gained popularity in commercial installations and were produced in mass. Their cost dropped and this allowed them to find a foothold in domestic installations.
Our modern domestic inverters are nothing but commercial string inverters with a few modifications and this is where the term string inverter comes from. Now let’s look at inverter technology development in the last few years.
Which is better low frequency or high frequency inverter?
Before 2010 the inverters found in the market were classed as low frequency inverters and high frequency inverters. Low frequency inverters had an advantage of handling higher power spikes and were more reliable. They had a large inductor inside them which made them comparatively heavy and more expensive.
In the end of 2010 SMA released their first transformer-less inverter for residential installations. The abbreviation TL was used to classify them. Because of their versatility they were widely adopted by the solar industry in a small amount of time. They had superior efficiency and could be used with power tools and pumps, and were lighter.
The transformer-less inverter architecture was such that it allowed for not just one but two MPPT inputs, thus they became ideal for domestic applications. Not only it was easier to mount them because of their lower weight, but could also allow for two different strings of panels which worked really well on pitched roof with limited space.
Today the majority of solar and inverter manufacturers are producing transformer-less inverters. Another popular development in the solar products is the micro-inverter, so let’s look at their re-emergence:
What is a micro inverter?
Micro-inverters were conceived in the 1970s. They were first produced in the 1990s but soon disappeared. They made a comeback in 2010 and by the year 2011 micro-inverters were the fastest growing segment of the solar PV inverter market, seeing a hundred and eighty percent growth.
Instead of a centralized inverter that looked after a whole string or a couple of strings of panels, the micro inverters took the power management down to a single panel and directly fed its power to the grid. Every panel in the system has its own micro inverter.
This meant better power management and throughput, particularly in the case of partial or complete shading. What has to be remembered is that with a string inverter a single partially shaded panel can bring down the power level of the whole string of panels, so micro-inverters allow panels to operate at their maximum power point independent of all other panels in the system.
Essentially, micro-inverters did the same thing to string inverters that the string inverters did to central inverters a decade ago. That is, they became a better, more reliable and more efficient replacement for their larger counterparts. However, micro-inverters haven’t been able to completely dislodge string inverters from their perch, mainly because of their higher up-front system cost.
The other problem they post was the complex wiring, where instead of a single AC circuit leading to the grid, there had to be several AC circuits leading up to the roof and connecting to the grid. Having said, that they lowered the voltage the wires were carrying which can be very advantageous for safe operation in several scenarios.
How do power optimizers work?
As micro-inverters gain popularity a third solution has also emerged, which was the use of power-optimizers. Reportedly, it manages power even better than micro-inverters. This solution was invented by the company Solar Edge in 2009 and became mainstream by the year 2013.
The idea behind it is simple. If you look at a micro-inverter it is essentially carrying out two functions. One is the MPPT or the maximum power point tracking, which is to maximize power from the panel for a given amount of sunshine falling on the panel. The second is the inversion, which is the converting of DC to AC and feeding it to the grid.
A power-optimizer based system only performs MPPT at the panel level. It doesn’t do the inversion. It then feeds the DC current to a single centralized inverter. This inverter converts the DC into AC and feeds it to the grid. Both micro-inverters and DC optimizers come with 25 year warranty.
However, the central inverter used by DC optimizers and the string inverter have 10 to 12 year warranty. One problem that has a resentment this technology is that because it was patented there was no other choice but to buy the system from Solar Edge.
As the build quality of the power-optimizer system wasn’t top-notch many of their installation failed, which led to people dropping this technology despite its technical superiority. Chinese manufacturers Huawei tried to launch their own power optimizers but were prevented by lawsuits.
In terms of the output, considering all things equal, a DC optimizer on average would give 4 kilowatt hour extra per year per kilowatt of installation compared to the micro inverters.
In the year 2018 small solar panels started appearing in the market. These were panels that were sold with Solar Edge’s power optimizer embedded at the back. The cost for them was lesser than the two items purchased separately.
Panasonic, Canadian Solar are some of the brand names that are producing smart modules. The other technology development in inverters is mainly software and not hardware. You might be aware that many string inverters can be controlled by remote phone apps.
There are concerns that systems may be hacked in the future and for this, robust firewall systems are being designed for the inverter software. Modern inverters now generate fault codes which makes them easier to service and repair.
Which is better UPS or solar inverter?
Lastly, I would like to suggest to people in the developing world that are buying UPS-based inverters for their PV systems to perhaps invest a little more and buy string inverters, the prices of which are falling precipitously.
UPS inverters are designed to use battery input therefore they are designed for input voltages of 12 to 24 volts. Now small panels which were around 80-200 watts could have been used with them because these panels have lower output voltages of 18 to 24 volts but a single large panel today such as the 360 watt panel is almost always 48 volts of output voltage.
This means that it is impossible to use UPS inverters, particularly if you want to connect your panels in a series string. Given that a solar PV system lasts a lifetime and the most likely thing to fail in the system is the inverter, it is best to invest in a quality product.