The solar inverter is the heart of a domestic solar PV installation. Without it the electricity generated by the solar panels couldn’t be used by home appliances or exported back to the grid.
This crucial piece of the solar system has undergone many changes over the past 10 years. Originally, back in the days when most solar installations were large-scale affairs, UPS (uninterruptible power supply) inverters were used.
Bespoke solar inverters didn’t exist at that time, so existing technology was used. However, they had drawbacks. UPS inverters were used 20 years ago to provide AC power from large banks of lead-acid batteries and were used as a single, central inverter.
The development of string inverters and later micro-inverters focused more and more on solar panel-level DC to AC conversion. Micro-inverters operate at the panel level and are more efficient than string-inverters, which can accommodate up to 10 panels connected together in a string.
Video – Microinverter Vs String Inverter
What is a solar inverter?
All inverters have the same basic function, which is to convert the DC electricity generated by solar panels or other generator, into AC power that can be used by domestic appliances. However, the differences between a basic UPS inverter used 20 years ago and a modern solar generator is huge.
On a top range solar central or strong inverter you might find the following features:
- reverse polarity protection
- anti-islanding circuitry (automatically isolates solar system during power outage for safety)
- integrated Wi-Fi
- phone app monitoring
- ability to grid-tie (connect to power grid)
What are the different types of inverters?
Inverters can be split into main categories, according to their internal operating frequency and their output waveforms. The different types are low-frequency, high frequency, modified sine wave and pure sine wave.
Inverters can either be low frequency or high frequency, which refers to high fast the IGBT circuitry chops up the DC voltage before re-shaping it into the sine wave home appliances need. Low frequency inverters were much-preferred before advancements around 2010, as they handled current surges very well.
However, to do this, they incorporated a large inductive coil, which was heavy and expensive. As newer designs appeared without the coil, so-called ‘transformer-less’ inverters, their use faded out. These new design made possible all kinds of improved features, such as dual-MPPT technology (Maximum Power Point Tracking) which improved installation efficiency.
Modified vs Pure Sine Wave Output
Both types of inverters have their place. I use a 1000 watt modified sine wave model for running motors and appliances that are generally out of earshot, because they generate a high-pitched buzz when operating.
These devices use less circuitry when chopping up the DC input power, and so the output waveform isn’t very smooth. In addition, they can interfere with electronic devices and other sensitive equipment.
Pure sine wave inverters are more expensive but generate a very smooth sine-wave, so there are no issues with interference. It’s a matter of choosing well with the application in mind.
Central Inverter $
Converts DC to AC in one location.
No local panel monitoring.
Whole system output reduced by shading
String Inverters $$
Converts DC to AC for several panels.
String can be monitored.
Only string output reduced by shading.
Micro Inverters $$$
Converts DC to AC at panels level.
Panel level monitoring and optimization.
Only individual panel output affected by shading.
String Inverter + Power Optimizer $$$$
Converts DC to AC for several panels.
Adds panel level MPP function – DC optimization at panel level.
Only individual panel output affected by shading.
What is a string inverter?
For many years just one central inverter was used, both in industrial and domestic solar installations. Most domestic solar systems now use string inverters, which convert the output of several panels connected together called a ‘string’.
It has to be said though, that prior to 1990 there were not that many domestic solar systems installed.
In the early days of home solar solar panels were expensive and people didn’t go for the big arrays that we see now. In this case a central inverter was quite adequate. However, two things happened that changed the viewpoint:
- electric PV prices per watt came tumbling down and
- commercial installations expanded in size and more efficient inverters were needed.
When a central inverter is used all of the AC current runs through one cable, which means bigger, more expensive wiring and a lack of redundancy. By redundancy I mean that if there is a problem with a central inverter, or the cable carrying the AC, the whole system is down until it’s fixed.
How does a string inverter work?
The way that solar panel arrays are configured is called the ‘topology’ and it was time for the central inverter topology to get upgraded! The solution was to connect several solar panels together in series or parallel, depending on the DC voltage required, and dedicating one inverter to those panels.
In the commercial world these groups of panels were known as ‘strings’ and the name stuck for domestic systems. So a dedicated solar inverter became known as a string inverter. They are smaller and generally more efficient, although it’s a little more expensive as there are more of them on a large system.
String inverter vs central inverter
Advantages of string inverters are reduce wiring costs and improved redundancy. There are also operational advantages – if one string or array is shaded for any reason, then other strings will not be affected.
They also have a maximum energy efficiency approaching 98%, which is not too shabby and reduces power losses. Reduced DC cabling costs is also an advantage. String inverters are ideal for domestic situations where consumer voltage is low and distribution cable lengths are short.
What are micro inverters?
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A micro-inverter is fixed to the rear of an individual solar panel and converts its DC output to AC for transmission for use by home appliances. Some micro inverters can be connected to several solar panels.
Are micro inverters better?
Solar panels with micro inverters built in have certain advantages over string inverters.
They control individual panel outputs, which means that if one panel is underperforming due to shade or anything else reducing sunlight exposure, the rest of the system won’t be affected. This is the biggest advantage – microinverters maximize system power output.
Individual panel monitoring – a central monitoring station connected by Wifi can immediately detect problems with individual panels. Mobile phone apps can also monitor microinverters with either Wifi technology or Bluetooth connectivity.
Which is better micro inverter or string inverter?
In certain circumstances microinverters are superior to string inverters, but they are also subject to advertising hype, just like any new technology.
In my opinion, for most home-owners, it isn’t worth the extra money. The only exception is if you have a particular set of circumstances on you property. If you have shading problems where your solar panels are sited, then it makes sense to regulate each panel individually.
Another scenario may arise in which you have a large installation spread across a complex roof with different angles and orientations, some point South, Some East, some West, for example. In this case each panel needs to be regulated as the Maximum Power Point (MPP) will be different for each one.
Are solar optimizers worth it?
Solar optimizers combine MPPT function with the advantages of microinverters, regulating and maximizing the DC power production at the panel level. Unlike microinverters, they don’t function as an inverter – this is done at the string level.
Are micro inverters better than optimizers?
There is very little difference between the output performance of microinverters compare to power optimizers. Both add up to 20% cost to an installation. Overall, for the vast majority of domestic users, a standard string inverter is the best option. Claims of up to 25% improved performance of both devices compared to string inverters are over-inflated, in the opinion of most solar industry experts.)
Does tesla use string inverters?
Are micro inverters worth the extra money?
Microinverters can add up to 20% to the cost of a solar installation compared to string inverter topology. For most domestic solar installations they are not worth the extra expense, unless there are particular shading issues or several arrays pointing in different directions.
What do power optimizers maximize?
Power optimizers maximize the DC power output from an individual solar panel using the Maximum Power Point function.