Pros and cons of string inverters vs. microinverters

What are microinverters?

Microinverters perform the same basic function as string inverters, except that they are installed underneath each solar panel on your roof. Each of these microinverters is about the size of an Internet router. 

The big difference between microinverters and string inverters is that a solar panel installation with microinverters will typically have the same number of microinverters as there are solar panels, although there are microinverters that accept two or four solar panels.

Enphase is a leading manufacturer of microinverters. Enphase microinverters have been on the market since 2009, and have become an integral part of its booming business. Its researchers have been studying how to best apply principles of maximum power point tracking (mppt) into components for enhanced solar photovoltaic production. 

While standard string inverters will cap the electricity production of each panel by the lowest producing panel on your roof, microinverters do not have this problem, since they function in a parallel circuit. 

A microinverter will take full advantage of the production of each individual panel. It will convert the power generated by each panel to the grid voltage. Each combination of solar panel and microinverter can “do their best” and contribute as much power as they can. 

Power optimizers are an option for standard string inverters, as well. They function very similarly to microinverters, though electricity is not converted into AC behind each panel. That work still gets done at the string inverter. 

Optimizers are able to smooth voltage and combat production differences, thus resulting in enhanced solar power output, even in challenging, partially-shaded solar installations. Solar Edge is a leading manufacturer of power optimizers.

Advantages of microinverters

Rapid shutdown capability 

New electrical codes require rapid solar system shutdown, so that first responders or firefighters are safe from high voltage when they need to be on rooftops or servicing power lines. Microinverters comply with these rapid shutdown requirements, and have this capability embedded into each module. 

More electricity 

The core advantage of using microinverters is that, theoretically, you can yield more solar electricity. The reason for this is that there are slight differences in currents between solar panels. When solar panels are in a string, the current is reduced to that of the least-producing panel in the string.

Suitability for challenging installation conditions 

If a solar system is facing multiple angles, meaning that some panels are facing south, some east, and some west, then microinverters are the way to go. Or, if you have shading issues from trees or a large chimney, again microinverters would be your best option. 

In these situations, the solar panels will be producing different amounts of electricity at different times of the day, but microinverters will ensure that you harvest all of the energy, while with a standard inverter you will lose some of this production. 

Lifespan 

Microinverters typically have 25-year warranties while standard inverters typically have 8 to 12-year warranties. The reliability of microinverters was in question several years ago, but the technology now has caught up with the industry, and the long warranties on microinverters demonstrate the confidence that the manufacturers have in their products.

Panel-level monitoring 

Microinverters and the add-on optimizers have the ability to track the production of each individual panel, while with a standard inverter you can only track the production of the whole system.

System expansion ease 

If you were to expand your system in the future, microinverters are simple to add one at a time. Each panel and microinverter pair can be easily added to your existing solar array with no need to worry about purchasing, siting and installing additional string inverters. 

To sum it all up, microinverters are definitely a value-add, but are only recommended if you need to comply with rapid shutdown requirements, have panels facing multiple orientations or have shading issues. Otherwise, the less-expensive standard inverter is usually more cost-effective.

Disadvantages of microinverters

Cost 

The main disadvantage of microinverters is their price. They are typical $1,000 or so more expensive than a string inverter on a standard 5kW residential solar installation.

Maintenance 

If one of your microinverters were to fail, not only would detecting which one has failed be a challenge, but once you did, servicing and replacing the component would not be as easy as plopping in a new string inverter on the side of your house. 

Your solar installer would need to, once again, get up on your roof, work with your racking system, unbolt the solar modules, and replace the microinverters to re-establish the AC conversion capability. 

Amount of hardware on your roof 

Since there is a microinverter attached to every solar panel on your roof, that is a lot of expensive metal equipment up there. 

Microinverters may act as miniature lightning rods. If you are in a storm-prone area and have a historical wooden structure with shake roofing materials, you might want to think twice before installing them. 

Microinverters vs. string inverters: which is best for your home?

Most microinverters manufacturers claim very low failure rates.

The most complex electronic device in a solar system is the inverter. And based on my decades-plus experience in the solar installations business, the most common thing to fail in a solar setup is the inverter. I would be very reluctant to put 20 inverters up on a job rather than putting one string inverter. I would only do this where there were severe shading issues.

Although microinverter manufacturers sell the ability to monitor each panel as a benefit (and it is!), they do noit include the monitoring that allows the customer to do this. They only allow the installer to see the panel-level data from your system, and not you as a customer, unless you agree to buy a higher level of monitoring as an upgrade.

This means that you, as a customer, only see system-wide monitoring information, and not what is going on with each panel. A skeptic would say that the reason why they do this is to protect them and their installers from support calls related to broken inverters. It is very difficult with only system-wide monitoring data to determine if only 1 or 2 inverters out of 25 or so have failed.

Having said this, I still like microinverters—but if I were a client purchasing a system with one, I would pay the extra money for the level of monitoring that gives you panel-level analysis. At least, when a string inverter fails, the whole system stops and this is easy to notice. 

My gut feeling is that there are a lot of failed microinverters out on rooftops where the client does not even know that they have failed.

Key takeaways

• Inverters convert DC electricity to AC electricity, which your home appliances use.
• String inverters are stand-alone boxes ideally suited to unshaded solar panel arrays on roofs with uniform pitch.
• Microinverters are affixed to the back of every solar panel and maximize the output of each solar panel independent of the production of any neighboring panel, thus making them smart to use on partially-shaded solar installations.
• String inverters do not have native rapid shutoff capabilities, whereas microinverters do.
• String inverters, while more affordable, do not have panel-level monitoring capabilities and come with a shorter lifespan of 8 to 12 years.