And Why We Prefer String Inverters For Most Systems
Solar systems can get complicated once you start digging into the technologies behind them. However, there are essentially two basic components of a grid-tied solar system: the solar panels (PV modules) and the inverter. The modules collect the sunlight, and the sunlight hitting elements in the solar cells within the module creates an electrical current. So why do we need an inverter? The electrical current generated by the photovoltaic effect at the module is Direct Current, or DC power. This DC power is not usable for most home appliances that require AC, or Alternating Current. DC energy is mostly used for low-voltage applications and appliances specifically built to run on DC. With grid power and modern appliances, our homes and appliances almost always need AC. The inverter converts the DC electrical current produced by the PV modules into conditioned power, or AC power, to run the home. Off-grid systems add in a few additional key components– batteries and charge controllers– but this brief overview is going to focus on the inverter, which is necessary for most any solar system.
There are three main types of inverter technologies in today’s residential solar energy market: string inverters, microinverters, and power optimizers. Each one is unique, yet they all convert DC energy from solar panels into AC for use in the home and to blend with your utility’s power.
Each inverter technology has its pros and cons. Weighing these can help you find the best-suited inverter for your unique needs. It is important to note that regardless of what inverter you choose, the inverter is the single highest point of failure in a system. In general, inverters have a .2-.6% annual failure rate. With all of that in mind, we’ll jump right into the three inverter options.
String Inverters
A string inverter is a central inverter that can accommodate a large array. Some of the advantages of a string inverter are:
Ideal for situations where the array has to be placed long distances from the inverter location, such as ground-mounted arrays, due to high DC voltage inputs.
Simplicity in design.
The secure power supply feature offered by SMA (our preferred string inverter) supplies up to 2,000 watts during a power outage without battery backup.
They are lower in cost without losing quality.
String inverters are mounted in a more protected and accessible location for easy servicing and monitoring.
They present a low probability of multiple failures.
The output of the inverter is not affected by the shading of a few panels.
There are a few cons with string inverter systems:
When a string inverter does go out, the entire array stops producing until the inverter is replaced. This usually can take up to a week to resolve. However, the inverter is at ground level, and if the homeowner is a DIY, it can be done by a single person on ground level with relative ease.
String inverters also have a shorter product warranty, but an extended warranty can be purchased for up to 25 years.
A few more things to note about string inverters
Unfortunately, there is a lot of misinformation on shading and string inverters. This is because a lot of old information is still out there. It used to be that if part of an array was shaded, the whole array would be reduced to the output of the shaded section. The shading was an issue when the original residential string inverter had a narrow operating voltage input window and only one PV input. Today’s string inverters have multiple inputs (up to three in some cases). Multiple string sizes can be installed in each input. Each input maximum power points to an optimal voltage and is adjusted to that. For example, if 4 panels in a string of 12 panels are shaded, the inverter’s MPPT (maximum power point tracking technology) simply adjusts to the new output voltage of 8 panels in a string. The other one or two strings in the same inverter would not be affected by the shading of the first, as they are independently maximum power point tracking to get the best results from their string of modules.
As mentioned, string inverters make a great fit for ground-mounted arrays that need to be placed a long distance away from the meter and inverter. Because of high DC input voltages and low amperages per string of panels, relatively small wire can be used to make the distance to the inverter with low losses. The functioning of the inverter is still sound, as the AC side is not experiencing any voltage drop.
For rooftop installs, if using a string inverter system, each panel will need to include a rapid shutdown device for fire safety and code compliance. These small devices are simply on-and-off switches that will cut power to each panel if triggered by a manual switch, and in some cases automatically when the grid power is lost. These devices rarely fail as they have no other functionality other than acting as a switch.
Microinverters
Microinverters are smaller inverters that are placed behind each solar panel, converting the DC energy to AC energy right at the module. Each device includes rapid shutdown, individual monitoring, and is easy to install.
Some of the pros to going with microinverters are:
They are easily deployable.
Microinverters are DIY-friendly.
They can tolerate heavy shading or roofs with multiple orientations.
They have individual module monitoring capabilities.
Microinverters come with a 25-year warranty.
Cons of microinverters:
They are more expensive.
They will not work optimally for ground-mounted arrays.
They have more parts to fail.
Have a low round-trip power conversion for battery backup.
They’re exposed to the elements under each panel, rather than in a protected area like string inverters.
Being that each module has a microinverter underneath it up on the rooftop, servicing is less convenient.
If you have a roof with multiple orientations or heavy shading, then maybe microinverters are the way to go, if you’ve got the budget for them.
Power Optimizers
The third and final type of inverter is called a power optimizer. Technically, power optimizers are not an inverter, but they are installed with each panel, almost like a microinverter. They have been described as a combination of a microinverter and a string inverter. These module-level power electronics (MLPEs) have an “optimizer” paired with each panel that optimizes and conditions the DC current before sending it on to the main string inverter. This technology is called maximum power point tracking (MPPT). The higher cost is still a disadvantage of this inverter option versus a string inverter, but it does fall below the cost of microinverters.
Pros of power optimizers:
Power optimizers allow individual module monitoring like microinverters.
They are less expensive than microinverters.
They enable optimal energy production for sites that have shading or multiple roof angles.
They have a built-in rapid shutdown safety feature.
They come with a 25-year warranty.
Cons of power optimizers:
They’re more expensive than string inverters.
They create more parts to fail.
Often not necessary at a site with good sun exposure.
New Farm Solar offers all three inverter options, however, we tend to favor using string inverters for most sites and situations. They are generally a better option for many of our customers after we’ve considered the design and needs of the customer, as well as their budget. Many solar companies are promoting microinverters. They are easy to install and have module-level monitoring capability, along with the other mentioned benefits. Microinverters do have their advantages, however, they are not necessary for most sites, and the cost is much higher. The same goes for power optimizers. There are unique situations and rooftops that would benefit from microinverters or power optimizers, but it is rare that string inverters don’t take care of the needs of the customer for a lower cost. If you'd like to see the specific inverters that we like to work with, visit our inverter page here.