The Inverter you choose is one of the key decisions you will have to make when deciding on what type of photovoltaic system you want. The Inverters convert direct current (DC) electricity generated by solar panels into alternating current (AC) electricity that your home or business runs on. More important than the pv panels you select, the inverter(s) are the heart of the system and you want to get this right to maximize power production vs cost.

The three main inverter options available for residential and commercial solar installations are string inverters, micro-inverters and power optimizer systems. The string inverter is the least expensive option but least flexible in dealing with shading issues. The Micro-inverters and power optimizer systems are typically more expensive than string inverters, but offer a lot of flexibility in dealing with shade and maximizing power output.

What to read more? 

Shading on part of, or all of, a solar panel has the consequence of reducing the total systems overall power production when using a string inverter. Roof gables, trees, neighbors taller house, multiple roof faces for the solar panels, or angle to the sun, can all be factors that reduce production with string inverters. However much of these negative design aspects can be mitigated by using Micro-inverters or string inverters with power optimizers.

So why would you want to use a string inverter? They are the most cost-effective inverter option available in the U.S. If your roof is not shaded at any point during the day and does not face in multiple directions, (requiring solar panels being placed on roof sections facing different directions) then a string inverter could be your best choice. However, in recent years due to new electrical safety code requirements called “rapid shut-down” the string inverter cost advantage has been reduced somewhat.

The basics about how String Inverters work is that your solar panels are arranged into groups called strings. These strings connect to the inverter and function as a unit. Anything affecting a panel on a string affects the entire string. Thus a string of solar panels will only produce as much electricity as its least productive panel on the string. If one or more of your solar panel(s) is shaded during any part of the day, the power output from that entire string would be reduced to that lower level.

What to read more about Micro-inverters? 

Micro-inverters are more expensive than string inverters, but some of their upfront costs are offset over time by possible high productivity.

Micro-inverters are installed under each individual panel in a solar array. The individual micro-inverters convert the DC electricity from your solar panels into AC electricity right on your roof sending AC power directly to your electrical system. In some cases, pv panel manufacture integrate a micro- inverters into the solar panel itself, further simplifying the installation but complicating repair warranty work down the road.

One of the major advantages of micro-inverters is that partial or complete shading of one panel will not cause the entire string’s output to be reduced. Since each panel is now operating as an independent production device the shading losses are confined to just the panel(s) being shaded. So the choice to spend the extra dollars for micro-inverters may be warranted if the installation is where one or more panels may be shaded at various times of the day or year, or where panels are facing different directions.

Plus micro-inverters also allow you to monitor the performance of individual solar panels rather than just the system total as is the case with the basic string inverters. The down side to micro-inverters is the higher failure rate and need to get on the roof and under solar panels when doing corrective work. Some believe the failure rate is increased due to all the sophisticated electronics of an inverter in the harsh weather environment of extreme heat and cold experienced on the roof, while other claim that it is just newer technology teething pains.

Want to learn more about Power Optimizers? 

Power Optimizers offer many of the same benefits as micro-inverters, and when paired with string inverters tend to be slightly less expensive that micro- inverters. Power optimizers are often considered a compromise between more expensive micro-inverters and the standard string inverter and ofter the combo is called a hybrid inverter system.

Like micro-inverters, power optimizers are located under each panel or sometimes integrated into the panels themselves. However, the solar panels are still wired in strings and the system will still have a central inverter mounted on a wall in a garage or another easily accessible location. The big difference is instead of converting the DC electricity to AC electricity at the panels, the optimizers’ electronics adjust the DC electricity voltage to minimize the negative effects of shading on the string and send adjusted DC power to the inverter. This approach results in higher system efficiency than a string inverter alone and less complexity up under the roof than micro- inverters.

Similar to micro-inverters, power optimizers reduce the impact of panel shading on system performance, and also offer individual panel performance monitoring just like micro-inverters. Systems that use optimizers are typically more expensive than standard string inverters and slightly more affordable than micro-inverters.

What to read more about Micro-inverters vs Power optimizers?

Micro-inverters and the Hybrid string inverter paired with power optimizers are very comparable and are collectively referred to as “Module-Level Power Electronics,” or MLPEs. They both offer:

  • Individual solar panel performance monitoring.
  • Improved performance of solar panels on multi faced roofs, or roofs that experience shading.

The Hybrid system has one other notable advantage over the micro, one can increase the DC size of the pv system with the same inverter thus reducing the price per watt installed as the system size increases. One still has to buy additional optimizers and perhaps scale up to a larger inverter but scalability still works in the optimizer’s advantage. With micros, the greater number of pv panels, the greater number of inverted needed, and thus overall price goes up in a linear fashion.

The last major inverter class you should be aware of is battery ready. All NON battery ready inverters are designed to shut down whenever the grid is not functioning. To have a pv solar system that functions in grid outages or interfaces with a battery one must have an islanding inverter that can operate with or without grid power being present. These come in both micro and hybrid architecture so the choice comes down to DC or AC connected batteries. (A topic of another blog)

Warranties are the last item we will discuss. Typically micro inverters have a 25 year warranty and hybrid systems have 12 years on the inverter and 25 years on the optimizers. Often the 12 year can be extended to 25 year for a nominal cost. The gotcha to be aware of is that the micro inverter and optimizer warranties being 25 years sounds great, but that is only for materials. Labor to do any of this warranty work is not covered by the manufacture so to replace roof components it can be costly in the future even if they are covered under the manufacture’s warranty.