Types of Solar Inverter and how they work?

string-inverter

What does solar inverter do?

A solar power inverter converts DC current to AC current. This conversion is needed because solar panels only produce DC but our home and our appliances use AC. The power that is imported from the grid is also an AC power.

But if I claimed that DC -> AC is all a modern solar power inverter like the SMA sunny boy does (shown above in red), it would be a lie. Scientists and engineers have been adding more and more features each year inside the inverter that makes it more powerful, useful and smart. One simple example, all modern inverters can communicate to a smart phone app through your home Wi-Fi.

A modern solar inverter can “communicate” with solar panels, batteries and power grid, all at the same time. This type of inverter is gaining popularity in recent years. Which brings me to the next topic I want to discuss.

Three types of solar string inverter

Before I talk about the types of inverter, I want to clarify “string” part of the solar inverter naming. The term “string” comes from the orientation of solar panels. Please see below..

Power produced from a string of solar panels is fed into this inverter. Hence the inverter is commonly also known as String inverter.

string-inverter-setup

Great..now to the types of string inverter. 

There are three types of solar string inverter designed for three types of solar electric systems. So, to understand the kinds of inverter, you’ll also have to understand how each of the solar electric system works as well. In this post, I will explain both side by side. 

Without further delay, let’s get started..

1. Hybrid solar string inverter:

A string inverter that supports a hybrid system is a hybrid string inverter.

What’s considered “Hybrid” system in solar?

Let me illustrate a situation to explain what a Hybrid solar system is.

Solar panels produce electricity when the sun is out. As it is being produced, you use that electricity and reduce your grid import. Now, you’ve decided you also want to offset your nighttime power usage with solar energy because the grid electricity is so expensive.

Only way the solar power can be used at night is by storing that power in batteries. You’d then discharge the batteries in the night, as you need it. Then, recharge again the next day when the sun is out.

However, because you are sane, you don’t want to go complete off-grid. Meaning you want to remain connected to the grid to avoid situations like when your batteries are discharged and the weather has been bad for few days. Sounds like a smart idea.

At this point, basically what you are looking at is solar panels + batteries + grid connect – aka Solar Hybrid System.

First thing with ANY solar PV system, you need an inverter to convert DC from panels to AC for the appliances. That’s simple and any inverter will do that. In addition to doing that, now in our hybrid system we also want to store some of that solar power in the battery. Then, come night, discharge the batteries first, and finally fallback to using the grid power once the batteries are completely discharged. A hybrid solar inverter will enable you to do just that. Here is a diagram that illustrates how a hybrid inverter may work..

hybrid-inverter-system

Let’s look at other types of inverters.

2. Grid-tied solar inverter:

A grid-tied solar inverter is less complex than a hybrid inverter because it has one less thing to worry about – the batteries. 

This is the most common type of solar PV string inverter installed by households and businesses today, largely due to the fact that only few people are installing batteries because of their prices.

In the diagram above, if you removed the battery from the system, you end up with a grid-tied solar system. You can choose to keep a hybrid inverter for your grid-tied system or you can just go with a cheaper grid-tied string inverter. Which one should you really choose? I talk about this in the last paragraph.

3. Off-grid solar inverter:

Off-grid system means you are not connected to the main power grid. This is common for standalone homes in remote areas where the transmission line hasn’t reached. An off-grid inverter manages DC to AC power conversion and battery charge/discharge.

Few households are voluntarily converting to an off-grid system by cutting off their ties to the grid. In recent years, falling prices of solar panels and batteries have made this possible for people who want complete power autonomy. Off-grid inverters are also popular for application such as mobile homes.

There you have it.. the three types of solar string inverter that are available in the market today.

Next, I would like to discuss about two key features that are common in modern string inverters. It would be unfair if I left them out of this article.

Feature#1: Maximum power point tracking (MPPT)

MPPT is a small circuit installed in almost all the modern solar string inverters. Its function is to maximize the power harvesting from the solar PV system. Power is a function of current and voltage. MPPT circuit optimizes those two parameters for the maximum output from solar panels for all weather conditions through out the day.

Single MPPT vs. Dual MPPT

single-dual-MPPT-inverter

Single MPPT has only one input for power optimization. A single set of solar panels placed in a string can be optimized for power output using Single point MPPT string inverter. What if you have two strings of solar panels on your roof?

As long as both strings have equal number of solar panels, are on the same side of the roof and shading is not an issue on either string, they will receive the same amount of sunlight, have the same inclination, receive the same wind speed and also the temperature of the panels in both the strings will match very closely at any given point in a day. So, technically, the two strings can be treated like one. A single MPPT can very likely optimize power output for both strings without an issue.

A single MPPT tracker can still optimize the power output from two sets of strings that are under two different conditions at any given point in time. However, optimizing the output of such systems from single MPPT is not as effective and efficient. In such cases, string with less power output will dominate and drag down the higher producing string. For example: if an east facing and west facing strings are fed into a single MPPT inverter, west facing string will drag down the east string in the morning, and vice versa in the evening. The amount of power lost in such cases can be significant.

The solution?

Usually more expensive in price, dual MPPT inverters are worth the additional cost especially if you are going to have two strings of panels that face in different direction.

A dual MPPT inverter has two separate inputs. Each input work independently to optimize each string.

Likewise, some suggests that a single MPPT may be used in cases where a dual MPPT is required – as long as the magnitude of the differences between the two strings of panels can be minimized. This may be a difficult task, but when you can have dual MPPT, why have single MPPT?

Feature#2: Anti-islanding

Anti islanding is a feature common to any solar inverter that is connected to the grid. So, a grid-tied and hybrid inverter both will have this feature built-in because both these systems are connected to the power grid. Off-grid inverter won’t have this feature.

What anti-islanding means is in case when there is blackout, the inverter shuts off for safety purpose, for you and people who may be working in the transmission lines. Once the grid power is available again, the inverter turns on and operates as normal.

Wrapping up..

A typical string inverter comes with 5 years of warranty but some have up to 10 years of manufacturing warranty as well. If you plan to add batteries to your solar system in the future, it is probably wise to spend a bit extra and get the hybrid inverter instead. Even if you don’t add batteries today, you’ll have a solar power system that is battery ready.

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