Inverters do the important task of converting your solar power into usable alternating current. Here’s how to choose the right solar inverter for your home.
Solar inverters are the part of your photovoltaic system that converts the direct current from your photovoltaic panels into the alternating current that your home uses. They’re tucked away and less prominent than solar panels, but they’re just as important to the operation of your system, so it’s worth taking the time to understand the different types and tradeoffs so you can make the right choice.
What does a solar inverter do, exactly?
Direct current is electricity that flows in one direction. It’s the type of electricity used by batteries and portable electronics, like your phone and laptop. This is what your solar modules produce, but your home can’t use DC power directly. The North American electric grid uses alternating current (AC) at 60 hertz, which means that it switches direction 60 times a second.
It’s the job of the solar inverter to perform this DC-to-AC conversion.
Types of solar inverters
In addition to doing DC-to-AC conversion, inverters for PV systems that are interconnected to the electric grid (like most home systems) must be able to manage the flow of electricity to and from from the grid. These are called grid-tie inverters. As the name suggests, grid-tie inverters allow your photovoltaic system to seamlessly connect with the grid, and provide clean, uninterrupted power no matter how much or little power your panels are generating.
Think of the different situations that can occur: your panels may be producing more electricity than your house is using, so you have to send power into the grid. Or you might be producing only some of your power needs, so your house is partly powered by solar, and partly by the grid. Or it might be nighttime, and all of your power is coming from the utility. It’s the job of a grid-tie inverter to handle all of these scenarios without so much as a light flickering.
There are also off-grid inverters that are used with battery systems and do not connect with the grid at all. These are outside the scope of this article.
There are a few major types of grid-tie inverters available. Choosing an inverter is one of the important product decisions you’ll make, so it’s important to get familiar with these.
Solar panels on your roof are wired together in one more more “strings” which can be connected to a type of inverter that handles the power output from all of them. This type is called a string inverter, and is the least expensive option.
Lower cost does come with some drawbacks, however.
String inverters suffer from a problem where a power drop in one panel causes power to drop across the entire string. (In a way, this is similar to how a single broken bulb can cause a whole strand of Christmas lights to go dark.) A power drop could be caused by shadows from any number of things: a chimney, nearby buildings, trees, leaves, soiling from dirt or birds. Or, it could also be caused by a faulty panel.
Most solar arrays will experience shading for at least some parts of the day, so with a string inverter you’ll likely experience reduced power collection compared with other inverter types.
Some string inverters manage this problem better than others. An inverter has a unit called a Maximum Power Point Tracker (MPPT) that handles the output from a string of panels. Some have inverters have more than one MPPT, each of which can individually optimize the power output of a string.
So, if your inverter has two or more MPPT units, your solar array can be wired into multiple strings and the inverter can manage each one individually. This means that if your inverter has two MPPT units, and one of your strings gets some shade during the day, the other string won’t be affected.
Another downside of string inverters is the inability to monitor the performance of individual panels in the system. This means that if you lose power in the array, you won’t know if it’s a single faulty panel or a more system-wide issue.
Finally, a string inverter is a single point of failure. A ten year warranty is typical, but a failure after that could mean an expensive replacement. This also places a limit on future expansion. For example, if you decide in the future to buy an electric car and want to add more panels to charge it with, you will be limited by the capacity of the inverter, and might be forced to upgrade or add another inverter.
Keep in mind that these drawbacks may not be severe enough to warrant the higher cost of a more sophisticated inverter system. It’s true that a string inverter may not harvest as much electricity as a more expensive option, but you’ll need to consider whether the value of that extra electricity you’d gain over the lifetime of the system would be greater than the added cost.
A power optimizer-based inverter system is basically a string inverter with an MPPT unit (power optimizer) attached to the back of each solar panel. With this setup, because each panel has its own power optimizer, a power drop at one panel affects only that panel, not the rest of the system. This mitigates the major flaw of string inverters.
Power optimizers also give you panel-level monitoring, so you can see the power output of each panel and know if there are problems with any of them.
The cost of this type of system is higher than a regular string inverter, but generally not as high as microinverters.
A hybrid inverter is a type of string inverter that incorporates a charge controller for managing a battery system. A charge controller can also be purchased as a separate unit, so a hybrid inverter gives you the convenience of an all-in-one device. Apart from that, it has the same characteristics as a standard string inverter.
Solar inverter efficiency
When inverters perform the DC-to-AC conversion, there’s always a little bit of energy that is lost as heat. Cheap DC-to-AC inverters, like the type that plug into your car’s 12v socket, can have very poor efficiency, but solar inverters from the popular manufacturers are highly efficient.
For example, Enphase microinverters are listed as 97% efficient, and SolarEdge inverters are between 97-99% efficient. So the differences are quite minor, and are small enough that you don’t really need to worry about them. Other factors, like shading, have a much larger role in power losses than inverter efficiency.
So, which solar inverter is best?
As you can see, there isn’t a single best inverter for any application. You need to understand the tradeoffs that come with each type, and work with your installer to weigh the pro and cons.
Here’s a table that summarizes the advantages and disadvantages of string inverters, power optimizers, and microinverters:
|String Inverter||Least cost||• Least effective in shade|
• Single point of failure
• May limit future system expansion
|Power optimizers||• Handles shading well|
• Panel-level monitoring
|• Expansion limited by string inverter|
• String inverter is single point of failure
• Some components are exposed to the elements
|Microinverters||• Handles shading well|
• Panel-level monitoring
• Single failures do not impact system
• Easiest to expand in the future
|• Highest cost|
• Units are exposed to the elements
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