How do solar inverters work?
Inverters do the important task of converting your solar power into usable alternating current. Here’s how to choose the right one 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.
Microinverters are small, self-contained inverters that attach to the back of each solar panel and manage the power output for that panel.
Just like with power optimizers, shading or panel failures affect only individual panels, not the entire system. This means that microinverters let you harvest the maximum amount of power from your system.
This also gives you maximum flexibility to expand the system in the future, because you aren’t limited by the capacity of a central inverter. You can simply add as many panels as you want.
Also, if one microinverter fails, you only have to replace one, not the entire system.
Just like with power optimizers, you get individual panel monitoring with a microinverter system.
You pay for this flexibility: microinverters are the highest priced of all the different types of inverter systems.
One possible downside is that even though microinverters are protected from direct exposure to rain and sun by being on the backside of the panel, they are still exposed to the elements. Unlike a string inverter which can be inside your home or garage, microinverters need to be able to stand up to wind, moisture, heat and cold. Vendors do quality testing, but time will tell whether the electronics inside will stand up to 25 years of outdoor service.
Enphase is the largest microinverter company, and they provide a 25 year warranty, which provides some peace of mind. (Read more about warranties below.)
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 panels factory-integrated with microinverters
There are vendors that now sell solar panels that ship from the factory with integrated microinverters. Examples include the SunPower Equinox and the awkwardly capitalized LG NeON 2 ACe line. In both these cases, the inverters are actually supplied by Enphase.
A possible advantage of this approach is lower installation costs. You’ll want to discuss with your installer whether they believe this would be a better or most cost-effective solution for you.
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.
If you are interested, efficiency is listed on the product datasheet of the inverter. Just like how the California Energy Commission certifies the power ratings of solar panels, they do the same with inverters. You can look up the CEC rating on the datasheet, or visit the Go Solar California for a list of approved and tested solar inverters.
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||• 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
Inverter sizing: input and output power
Inverters have two power specifications: the input power that it will it handle, and its maximum output power (in AC).
This means that if your string inverter lists a maximum input power of 5,000 watts, you can connect a maximum of 5,000 watts of solar panels to the unit. However, remember that in the previous article we learned that those STC panel ratings are not really achieved in the real world, and a panel’s true maximum is going to be around 25% less than that.
SMA, an inverter company, specifically recommends that you size your system so that the solar array’s STC size is larger than the inverter’s rated input capacity so that you can save money.
If future system expansion is a possibility for you - say, maybe you’re planning to buy an electric microbus and you want to charge it with solar electricity - and you are getting a string inverter, you may want to hedge against that possibility by going with a larger inverter or asking your installer to plan for adding a second inverter in the future.
Also, this is a situation where your best choice may be to go with microinverters.
How much do solar inverters cost in 2019?
Just to give you a ballpark idea of what the retail price for inverters are in 2019, the table below gives an example for each of the major inverter categories. Keep in mind that your installer is working in bulk quantities and may have direct relationships with suppliers, so their cost will almost certainly be lower.
|SMA Sunny Boy 6.0||string||$1,400|
|SolarEdge SE6000H HD-Wave||string inverter for power optimizers||$1,400|
|SolarEdge P320||power optimizer||$67 (per panel)|
|Enphase IQ7+||microinverter||$140 (per panel)|
|Enphase IQ Envoy||microinverter remote monitoring||$500|
You should check with online retailers for latest pricing.
Remember that inverters are part of the total system cost, which means that they are covered by the incentives available to you.
Monitoring your solar output
Once you’re up and running with a home PV system, you’ll naturally want to be able to monitor it to find out how your investment is performing. First of all, it’s a great feeling to check your system on a sunny day and see thousands of watts of power being generated. But it’s also important to periodically check that the components of your system are working correctly. Some inverters will do this automatically, and email you if power drops out in any part of the system.
Any inverter will provide some method of monitoring the system, and many provide web or mobile apps. Here are some public or demo accounts from some of the major manufacturers that you can view to get a better idea of the functionality provided.
Keeping the power on during a blackout
If you don’t have a battery system, grid-tie inverters do not supply power to your house when the grid loses power. Even if it’s bright and sunny, in a blackout your inverters will switch off when they can no longer sense power from the grid. This is because they need to synchronize with the 60 hz grid frequency, and also to prevent your system from putting electricity into downed power lines, posing a hazard to utility repair workers.
There is one exception on the market, however: the SMA Sunny Boy inverter series comes with a feature they call Secure Power Supply, which can provide up to 2,000 watts of power to a special outlet connected to the inverter.
This means that the inverter won’t seamlessly switch over your home to solar power in a blackout, as would happen with a battery backup system. Instead, you have to go and manually plug in the devices you want powered into the inverter.
Also, the manual for the Sunny Boy inverter mentions that this feature isn’t suitable for anything that requires a stable electricity supply, which means that plugging in your computer would be a bad idea. The inverter in this case is powered directly by the panels with no batteries to buffer the supply, so a dip in power from a passing cloud would cause the power to fluctuate.
Still, this could be a handy feature to keep some key appliances running during the daylight hours of a blackout - your refrigerator, for example. 2,000 watts is also just enough juice to run one space heater or a small air conditioner, so it could help keep you comfortable while you wait for the utility to effect repairs.
Inverter companies and their warranties
The list of inverter companies is shorter than the list of solar panel manufacturers, so it’s likely that you will end up with an inverter from a company on this list. These are some of the more popular ones in the US market.
The warranties listed below are for residential products.
|SolarEdge||12 years on inverters, 25 years on power optimizers|
|Enphase||25 years on microinverters, 5 years on IQ Envoy (monitoring)|
As we mentioned in the previous article about solar panel warranties, a 25 year warranty does you no good if the company is out of business. For example, there have been concerns about the financial health of Enphase, although they appear to have turned things around in recent quarters.