EG4 FLEXBOSS 21 Hybrid Inverter Review: A Game-Changer That Could Save You Thousands of Dollars

Jan 10

The FLEXBOSS 21 All-in-One Hybrid Inverter is the latest product in the EG4 lineup and it’s already making waves in the solar industry.

Combining huge output power, versatility, a clean design, and an affordable price point, it’s a practical product that was designed to address the real-world needs of both off-grid and hybrid setups.

Let’s dive into this first look at the FLEXBOSS 21, the features it offers, and decide if it actually lives up to the hype. We’ll also take a look at how it stacks up against other EG4 products.

The EG4 FLEXBOSS 21 Overview

As mentioned, the EG4 FLEXBOSS 21 is the newest member of EG4’s lineup and it promises some really impressive features.

While there’s plenty of hype behind it, its launch has caused a bit of confusion, as it was released alongside the EG4 GRIDBOSS—a product with a similar name, but one that serves an entirely different purpose.

Unlike its counterpart, the EG4 GRIDBOSS—which requires pairing with an inverter to function—the FLEXBOSS 21 is a fully standalone hybrid inverter. It’s designed for both off-grid and hybrid solar setups, meaning it can function off-grid, grid-tied, or even as a net metering (grid sell back) inverter. This makes it a versatile option for pretty much any type of residential solar application.

EG4 already has a strong reputation for carrying practical products—we’ve even mentioned in previous product reviews how they really seem to have a genuine interest in solving real-world problems and reducing complexity for their customers.

This approach to product design is really evident even upon first glance of the FLEXBOSS 21, as it’s clearly a versatile and user-friendly piece of equipment. In fact, it might be the best example of EG4’s efforts to simplify solar equipment that we’ve seen so far.

Specs Breakdown

Here are some of the standout specs for the FLEXBOSS 21:

Weight: At 88 lbs, it’s light enough to just about pass the Tiny Baby Lift Test, meaning you should be able to lift it solo. However, it’s worth keeping in mind that it’s really awkwardly sized. We’d probably recommend getting a second person to help you out while you’re lifting and installing it.
Voltage and Power: It’s a 48V split-phase inverter with a 12kW continuous output when connected purely to batteries. It offers an on-grid AC output of 16kW continuous at 240V, and 13.8kW at 208V. You can also achieve this 16kW output when connected to a combined solar and battery setup.
Solar Input: It can handle a usable PV solar input of 21kW, with a maximum rating of up to 24kW (EG4 recommends pairing it with a 25kW solar array for maximum performance, as solar panels rarely generate their full output). These inputs are insane! Basically, there are very few residential solar setups that are going to exceed this thing’s maximum solar input.
Surge Capacity:

24kW for 0.5 seconds, which means it can briefly handle accidental spikes.
18kW for 1 second. This means it can even handle the start up requirements of a 5-ton air conditioner without the need for additional accessories—another huge win for the FLEXBOSS.

Parallel Capability: Up to 16 units can be connected for a combined output of 256kW. Again, absolutely crazy stuff!
Efficiency:

97% from PV to grid.
94.5% from PV to battery.

Weather Rating: NEMA 4X, which means it’s suitable for outdoor and partial outdoor installations. While we probably wouldn’t recommend using it fully outdoors, knowing that it’s not going to get damaged if you set it up in a garage or shed is a nice bonus.

Assessing the Design and Features

Now that we’ve taken a quick look at the specifications, let’s look at some of the things that make this unit such a practical hybrid inverter.

The Compact and Clean Layout:

To start things off, it offers a really compact and user-friendly layout. Even though it still weighs in at 88 pounds, the FLEXBOSS 21 is slightly shorter than the EG4 18KPV Hybrid Inverter, but maintains a similarly clean design for the internal layout.

With multiple PV input ports, battery terminals, and communication ports, it’s easy to wire and configure it for various setups. It also has punch-out wiring holes on the top and bottom of the unit, so you can neatly run your cables and wires in a way that makes sense for your specific setup.

No Built-in Display Screen:

This may sound like a bit of a head scratcher, but unlike the 18KPV, the FLEXBOSS 21 does not include a built-in display. This seems to be a decision EG4 made to help simplify the design and reduce costs for the buyer.

Instead of using a screen, users can monitor and control the unit via a mobile app. If you are someone who needs or just prefers a physical interface, they do offer an optional external screen, which is available for roughly $100 extra. The screen is also included for free during some of EG4’s promotional events and sales, so you might even get lucky and bag the screen for free.

Battery Compatibility:

The FLEXBOSS 21 is compatible with both lithium and lead-acid batteries, seamlessly allowing you to work with just about any type of battery. Again, this seems like a design decision that EG4 went with to help installers and homeowners enjoy a more flexible piece of equipment. The option to feed 90 Amps of grid power at either 120V or 240V also adds to the flexibility.

Comparing the FLEXBOSS 21 and Other EG4 Products

EG4’s product lineup now includes a variety of high-quality inverters, but we already think the FLEXBOSS 21 brings a lot of unique features to the table. Let’s see how it stacks up to some of its siblings:

EG4 18KPV Hybrid Inverter:

The FLEXBOSS 21 shares much of its internal design with the EG4 18KPV but offers several notable improvements:

Higher Solar Input: The FLEXBOSS 21 supports 21kW of solar input, compared to the 18KPV’s 18kW solar input rating.
Increased Output with Solar: It can output up to 16kW when solar is available, a boost from the 18KPV’s 12kW solar output.
Lower Price: Priced at $4,149 (even less with a discount code we’ll link below), the FLEXBOSS 21 is $750 cheaper than the 18KPV. Again, it seems EG4 was looking out for their customers, as they easily could have offered the two units at the same price point.

EG4 GRIDBOSS:

The GRIDBOSS Mico-Grid Interconnection Device is a complementary product designed for hybrid systems. While the FLEXBOSS 21 can function independently in off-grid setups, the GRIDBOSS adds features like a 200-amp service pass-through, which is particularly useful for grid-tied systems.

When you couple the FLEXBOSS 21 with the GRIDBOSS, you also enable AC coupling, smart loads, and generator pairing, so it’s a good option for those who need those features.

Basically, it helps you reduce the complexity of your system, replacing the breakers, transfer switches and other accessories you would need to add. However, for purely off-grid applications, the FLEXBOSS 21’s 90-amp pass-through is more than adequate, once again allowing EG4 to lower costs for users who don’t need the extra capacity.

If you’re unsure if you need to pick up the GRIDBOSS, don’t worry, we’ll review it in a separate article and video in the future.

What We Like and Dislike About the EG4 FLEXBOSS 21:

While we haven’t had our hands on the FLEXBOSS long enough to properly test it out and really put it through its paces, it’s pretty clear this thing offers excellent value and addresses some common pain points we’ve come across when setting up solar power systems.

Even though it’s still early, let’s take a look at some basic pros and cons.

The Key Advantages of the FLEXBOSS 21:

Let’s start by highlighting some of the things that stood out to us about this unit, then we’ll jump into a few of the minor limitations:

Affordability and Support:

At its current price point, the FLEXBOSS 21 offers top-tier performance at a price that we really can’t complain about. The value really stands out when you compare it to similar high-capacity inverters on the market right now. To the benefit of their customers, EG4 seems to have really undercut the competition.

We also like that EG4 backs this thing with a 10-year warranty, so you know that you’re getting something that will last. If you shop through someone like Signature Solar, you’re also getting really solid, American-based customer support.

Adaptability:

Whether you’re an off-grid enthusiast or setting up a hybrid solar system, the FLEXBOSS can do both, which is something that we think local installers and DIY homeowners will both really like.

Plus, the ability to connect up to 16 units means that it is scalable and can grow with you, rather than needing to be replaced if you expand your system in the future. Scalability is always something that we like to see in any piece of solar equipment.

Efficiency and Power:

With an industry-leading 21kW solar input capacity and robust output options, it’s capable of handling almost any residential solar array. The high solar input also means that you can rest assured that you will be getting the most out of your solar panels, which is a big plus for those looking to go fully off-grid, or even feed power back into the grid as part of a net metering setup.

User-Centric Design:

By eliminating unnecessary features, like the 200-amp pass-through for purely off-grid users, EG4 has created a product that’s not only functional but also cost-effective. It’s also really easy to mount and wire, and the fairly straightforward internal layout is not going to be nearly as intimidating to beginners as the more complex systems out there.

Even when you open the unit up and remove the plastic protective shield, all of the input and output ports are neatly laid out and clearly labeled. It’s just a really user-friendly piece of equipment.

Limitations Worth Considering

While the FLEXBOSS 21 is impressive, there are a few potential drawbacks:

No Built-In Screen:

The reliance on a mobile app for monitoring and control may be a drawback for some users. However, the optional screen upgrade is an easy solution. The unit we were sent didn’t come with a screen, but if we were planning to buy another, we’d probably add the screen just to make on-site monitoring that little bit quicker and easier.

Reduced Pass-Through Capacity:

The 90-amp pass-through is sufficient for most off-grid setups but may require the GRIDBOSS for grid-tied applications. It’s not exactly a drawback, as EG4 offers the solution, but it’s still something that’s worth consideration.

Our Final Verdict: Should You Buy the EG4 FLEXBOSS 21?

The EG4 FLEXBOSS 21 is a testament to EG4’s commitment to innovation and customer feedback. It’s cheaper and more powerful than the 18KPV, making it a solid choice for off-grid and hybrid solar enthusiasts. While it sacrifices a few features to lower the cost, these trade-offs are actually well thought out and pretty practical, aligning with the needs of real-world users.

For off-grid enthusiasts, it’s a no-brainer. And if you’re interested in using it for a hybrid system, pairing it with the GRIDBOSS unlocks its full potential. Basically, if you’re looking to maximize your solar setup without breaking the bank, the FLEXBOSS 21 is an excellent option.

Use our discount code through Signature Solar to save even more on EG4 products, including both the FLEXBOSS and the GRIDBOSS, which we plan on reviewing in the future. For now, the FLEXBOSS gets two thumbs up from us—it’s a really solid buy.

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