The Sol-Ark 15K and EG4 FlexBOSS 18 Hybrid Inverter are two of the most talked-about hybrid inverters on the market right now. Both are powerful, UL-listed systems designed for serious solar power setups. However, if you’re looking at spending thousands of dollars on an inverter for your home backup or solar system, you’ll want to know which one gives you more value and better performance.
After putting both through plenty of real-world testing, it’s clear that one of these inverters has a massive edge in performance, features, and affordability. So, we’re going to put them head-to-head to break it all down.
Before diving into the comparison, it’s important to understand that the EG4 FlexBOSS and EG4 GridBOSS MID are two separate units that work together.
It sounds confusing, but it’s actually pretty straightforward, and having a basic understanding of how they work will be really helpful for the purposes of this FlexBOSS vs Sol-Ark comparison.
Here’s the basics:
Essentially, EG4 combined all of those components into one clean, code-compliant enclosure. The GridBOSS MID also adds advanced features like load shedding, a built-in 200A bypass, and support for up to three FlexBOSS inverters per GridBOSS unit.
Together, they make code-compliant installation easier, cleaner, and significantly more affordable than the traditional route. Unless you’re setting up a fully off-grid solar power system, you’ll most likely want to invest in a GridBOSS MID if you choose to go with one of the FlexBOSS Inverters (FlexBOSS18 or FlexBOSS21).
We have a full guide that breaks things down in full if you want a better understanding of how they work together: EG4 GridBOSS and FlexBOSS Explained
Now that we’ve cleared up the FlexBoss and GridBOSS combo, here’s how the EG4 FlexBOSS and Sol-Ark 15K compare on paper:
If you want to learn more about either of these hybrid inverters, we’ve tested and reviewed them individually:
➡️ EG4 FlexBOSS18 Hybrid Inverter Review
➡️ Sol-Ark 15K Hybrid Inverter Review
Time to jump into the ring and see how these two hybrid inverters stack up against each other head-to-head, starting with their outputs and inputs:
When it comes to raw output, Sol-Ark takes the early lead. As you can probably guess from the name, it delivers up to 15kW with solar connected (12kW from battery). On the other hand, the FlexBOSS18 pushes 13kW with solar (10kW from battery).
The Sol-Ark also supports a 1,500W higher solar input (19,500W vs. 18,000W). While that difference isn’t huge when you’re talking about a single inverter, it starts to really add up at scale. Still, even with only one unit, you could build a larger solar array—roughly four Sirius 415W Solar Panels extra, for example.
We open things up with an easy point for the Sol-Ark 15K-2P.
Score: EG4 FlexBOSS 0 - Sol-Ark 15K 1
Both units are built like tanks. They’re NEMA 4X rated for outdoor installation, they both run really quiet, and they’re surprisingly sleek for pieces of heavy-duty equipment. They also have the same snap-lock doors, which can be padlocked shut if you plan on keeping them outside.
However, Sol-Ark has one advantage: it includes a built-in touchscreen display right out of the box. It’s not flashy, and it almost feels like it runs on Windows 95, but it’s user-friendly and lets you configure everything directly from the unit.
The FlexBOSS doesn’t include a screen by default, which was a decision EG4 claims they made to help keep costs low. With that said, you can add purchase the optional EG4 LCD Screen Kit, which is a detachable touchscreen that can be added to any FlexBOSS inverter. Given it sells for just $90 through Signature Solar, it’s a fairly inexpensive upgrade that we think most people will go for.
Still, the Sol-Ark comes with a display screen at no additional cost, so we’re going to give it the point for this round.
Score: EG4 FlexBOSS 0 - Sol-Ark 15K 2
Safety-wise, both systems meet high industry standards. They’re UL-listed, feature arc fault and ground fault protection, and have rapid shutdown capabilities.
However, the FlexBOSS takes a lead here for one simple reason: it’s EMP-hardened straight from the factory at no additional cost. That means it’s better protected from surges caused by lightning or electrical spikes without needing any sort of add-on or EMP shield.
In fact, pretty much all EG4 batteries and inverters come EMP-hardened for free, which is one of many reasons why we are such big fans of the brand.
If EMP and external surge protection is something you care about, Sol-Ark charges about $2,000 extra to EMP-harden their inverters. That’s almost as much as a brand-new EG4 GridBOSS would cost you. It’s a really expensive upgrade that EG4 offers for free, so the FlexBOSS18 easily wins this round.
Score: EG4 FlexBOSS 1 - Sol-Ark 15K 2
Both inverters are easy to scale up and expand, but the FlexBOSS and GridBOSS combo makes it far easier and cheaper to do so.
With Sol-Ark, every time you add another 15K inverter, you’ll also need to pay for another manual bypass and transfer switch setup, which costs around $2,000 per unit.
With EG4, one GridBOSS unit supports up to three FlexBOSS inverters, so adding more power later is pretty much plug-and-play. With up to three FlexBOSS units and a single GridBOSS, there’s no need for extra wiring or switches.
Even though the Sol-Ark can be scaled up to 12 units, while the FlexBOSS can only run in parallel with 10, it’s so much easier and cheaper to expand your FlexBOSS setup, which is why we’re giving EG4 this round.
Score: EG4 FlexBOSS 2 - Sol-Ark 15K 2
In testing, both inverters handled demanding loads impressively. Still, the EG4 inverter pulls ahead in one key area: when we tested the FlexBOSS 18, it could start a 5-ton air conditioner straight out of the box.
Most inverters (including the Sol-Ark 15K) need a soft starter, like a SureStart device to handle that kind of demanding load, which adds cost and complexity. You’d assume better surge performance from the Sol-Ark, as it has a higher output, but the FlexBOSS18 outperformed it in this category. That’s enough to earn it another point.
Score: EG4 FlexBOSS 3 - Sol-Ark 15K 2
For any grid-tied hybrid inverter to pass code, you need a manual bypass and transfer switch setup.
The Sol-Ark doesn’t include these, meaning you’ll need to buy and install them separately. This will usually run you around $2,000 to $2,500 in extra parts and labor.
EG4 solved that with the GridBOSS MID, which includes:
Load shedding is a particularly clever feature. For example, if you tried to run your oven while your water heater was running, which could max out your system, you could have it set up so the GridBOSS would delay your water heater automatically to prevent overloading the inverter. Since your smart load shedding allows you to prioritize certain loads over others, as soon as you were done with your oven, your water heater would kick in by itself.
This allows for a whole new level of customization and smart home power management. Not only is it easier to install this setup and have it meet code, you unlock some genuinely valuable features. It’s another easy point to the FlexBOSS 18.
Score: EG4 FlexBOSS 3 - Sol-Ark 15K 2
Both companies have responsive support teams, but Signature Solar (the main EG4 retailer) really stands out.
Their tech team knows the EG4 product line inside and out, and you can call them for detailed installation or setup help. We’ve mystery shopped Signature Solar in the past, and their American-based support team crushed it.
When we called, the experience with Sol-Ark’s support was less consistent. They were helpful, but not always as quick or confident with their responses.
Score: EG4 FlexBOSS 4 - Sol-Ark 15K 2
This is probably the most one-sided round in our entire comparison. Let’s talk numbers:
Obviously, the FlexBOSS18 is significantly cheaper than the Sol-Ark Inverter, but even when you add the GridBOSS to make the FlexBOSS system fully code-compliant, you’re still roughly $3,000 cheaper than setting up a Sol-Ark 15K system with the necessary external switches and wiring, which will run you about $2,000 on top of the cost of the nearly $7k inverter.
This isn’t overly surprising, as affordability and saving people money was one of the main intentions behind the FlexBOSS and GridBOSS releases. Basically, EG4 wanted to give buyers the option to only pay for what they need. Even decisions like making the display screen an optional add-on helped drive down the up-front cost for a FlexBOSS inverter.
Once you consider all of the costs associated with installing these inverters and getting them up to code, the FlexBOSS + GridBOSS combination is easily the most budget-friendly route. That’s why we’re giving it the point.
Score: EG4 FlexBOSS 5 - Sol-Ark 15K 2
There’s no question that the Sol-Ark 15K is a proven, high-quality hybrid solar inverter. We love that it’s American-made, powerful, well-built, and that it has a proven track record for being one of the most reliable inverters out there.
But EG4 has listened to customers and innovated faster than Sol-Ark. The FlexBOSS 18 paired with the GridBOSS MID delivers the same performance, better expandability, has built-in surge and EMP protection, and it’s available at a far lower overall price. Plus, you unlock smart load management and get everything in what we think is a cleaner-looking package.
At the end of the day, if you’re setting up a hybrid solar system, the FlexBOSS18 + GridBOSS combo is the one to beat right now. We’d recommend it over the Sol-Ark 15K, as well as pretty much any other option out there right now, including the Rich Solar NOVA 12K or the EG4 18kPV Hybrid Inverter.
Check Price at Signature Solar: www.SignatureSolar.com
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Check Price at Shop Solar: www.ShopSolar.com
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