EG4 Flexboss 21 - Biggest hybrid solar inverter on the market!

Check out the EG4 Flexboss 21, the biggest hybrid solar inverter on the market! Paired with the Gridboss smart service entrance panel, it simplifies your solar installation process.

As of 4/1/25 here's a discount code you can use. The discount code UNPLUGGED100 to get $100 off the Flexboss21. This discount is only good until May 1st, 2025. Click the affiliate link below to purchase:


Here's an affiliate link to purchase the Flexboss21 hybrid inverter. By clicking on the link below you'll automatically get $50 off at the end of checkout.

Here's an affiliate link to purchase the Gridboss smart service entrance panel with smart loads. By clicking on the link below you'll automatically get $50 off at the end of checkout.

[dackjohnson]

I have the PowerPro battery installed. Flexboss going up later today along the Gridboss. I've hired an Electrician to wire the grid and breaker panels but I'm wiring the battery to Flexboss and Flexboss to Gridboss.. Love to see some install videos as this is my first installation of this type.

[andyfromtoystory]

I got two on the way, glad to see reviews to start coming out

[GavinStoneDIY]

The higher output with solar is what really interests me. I really like that feature.

[cedarpeaktrucking]

Great video, I'm encouraged by EG4's progress in the space. Cheaper than my Sol-Ark 15ks and seems to do everything I need. I'll be switching to EG4 in the future for all of my off-grid builds

[gerharddohne]

Thank you for this review and for taking the time to break down the 200A pass-through.

[brentjohnson]

So this is what you were talking about. You comment on the Boss (forget the full name) sent me searching for this product. Even with 90A pass through that should be enough for critical loads. Just have to figure out how to manage the non-critical loads. More to learn! Yay. I do enjoy learning so that is not a sarcastic yay. Haha. All the best from north Texas.

[chill-]

I'm currently working w/an independent designer to help me with all my documentation for interconnection and permits. Got the link thru Brian's e-mail subscription. Totally worth signing up for!

[offgridgarageUSA]

I just received my FlexBOSS 21 yesterday and the GridBOSS the day before. I'll be mounting them soon inside my big shop as well as the 40 410 watt panels. Hooking them up to 3 battery (300 amp hour) banks for a total of 900 amp hours! I'll add a 4th soon. I'll also hook up the 15 Kilowatt generator I have. Also, will be hooking up the new style chargeverter GC when needed.I love this stuff! ;-) Keep rockin' down there in Texas... maybe I'll join you some day! 🤓

[sneakypete]

Good stuff, looking forward to seeing how you wire up your system into the gridboss.. I had some concerns where my existing grid sell back meter would go as i also have my main meter.

[deanandrewjeski]

I was curious about the question (by others) raised runing high freqency inverters versus low frequency units so I asked AI for some answers and this was a response by " Grok"

You're raising a valid concern about safety with high-frequency inverters, particularly those without galvanic isolation like some of the EG4 models. Here's a breakdown of the issues you've highlighted:
Galvanic Isolation: This refers to the use of a transformer to physically separate the DC and AC sides of an inverter, preventing direct current (DC) from flowing into the alternating current (AC) side. Without this isolation, there's a risk that if the H-Bridge or other components fail, DC can leak into your AC circuits.
H-Bridge Failure: The H-Bridge in an inverter is crucial for converting DC to AC. If this fails, especially in a way that causes it to short to ground, it could lead to DC current being passed to AC loads.
Risk of Fire: When DC current is introduced into AC appliances not designed for it, it can lead to overheating, electrical fires, or damage to the appliances themselves due to the unintended current flow.
Safety Considerations:
Certifications: Check if the inverter has certifications like UL (Underwriters Laboratories), ETL (Intertek), or CE (Conformité Européenne) which indicate that the device has been tested for safety standards. However, not all certifications cover all types of failure scenarios.
Installation Guidelines: Always follow installation guidelines strictly. This includes proper grounding, use of appropriate circuit protection devices like breakers or fuses, and ensuring that your electrical system can handle any potential DC current leakage.
Monitoring: Regular monitoring of the inverter's performance, looking for signs of failure or unusual behavior, can prevent accidents. Some modern inverters come with built-in monitoring systems or can be integrated with home automation for real-time checks.
Professional Installation: Due to the potential risks, professional installation by someone knowledgeable with inverters and electrical systems is advisable. They can ensure proper setup and compliance with local electrical codes.
Upgrade Considerations: If you're using an inverter without galvanic isolation for cost reasons but are concerned about safety, consider the long-term costs of potential damage versus the initial investment in a safer, perhaps more expensive, unit with isolation.
Remember, while these inverters might be attractive for their price point, the safety implications are significant. If you're considering such devices for residential use, it's crucial to weigh these risks against the benefits or look into inverters that do provide galvanic isolation. Always consult with a professional electrician or engineer when dealing with high-risk, high-power applications in a home setting.
9.99s

So a solution to this problem would be an isolation transformer. Is that correct and what size is needed for example the EG4 Flexboss model.

25 web pages
Yes, an isolation transformer is indeed a solution to the problem of lacking galvanic isolation in high-frequency inverters. An isolation transformer physically separates the DC and AC circuits, preventing DC current from reaching your AC loads in case of an inverter failure like an H-Bridge short.
Determining the Size of the Isolation Transformer for the EG4 FlexBOSS:
Power Rating: The EG4 FlexBOSS21 inverter has a maximum AC output of 16kW (with solar PV gain) and a continuous output of 12kW from battery alone. For sizing the isolation transformer, you would generally match it to the highest continuous load you expect, plus a margin for safety. Here, a transformer rated for at least 16kVA would be appropriate.
Voltage: The FlexBOSS21 operates at a split-phase 120/240V AC output. Your isolation transformer should match these input and output voltages, so look for one that can handle 120/240V on both primary and secondary sides.
Current: Based on the power and voltage, you calculate the current. For a 16kVA transformer at 240V, you're looking at around 67A (16, 000VA / 240V). However, transformers are typically oversized slightly for safety, so you might go for one that can handle 75-80A or more per leg at 240V.
Type and Mounting: Choose an isolation transformer designed for residential or light commercial use, considering factors like noise, size, and installation requirements. Ensure it's suitable for indoor or outdoor use depending on your installation environment.
Practical Considerations:
Installation: This type of transformer can be heavy and large, so consider the installation space and accessibility. Professional installation is recommended to ensure it's done correctly, especially regarding grounding and connections.
Efficiency: Adding an isolation transformer will introduce some losses, so consider this in your system's overall efficiency calculation.
Cost: This is an additional cost to your system, but it's crucial for safety if you're using an inverter without built-in isolation.
Maintenance: Transformers require maintenance like checking for overheating or noise, which can indicate issues.
When selecting an isolation transformer for the EG4 FlexBOSS21 or any inverter, ensure it's UL listed or similarly certified for safety. Also, while this information provides a guideline, it's always best to consult with a professional electrician or an engineer familiar with solar installations to ensure all aspects of safety, compliance, and performance are covered.
13.65s

What KVA rating is therefore needed to handle the EG4 Flexboss model ?

For the EG4 FlexBOSS21, which has a maximum AC output of 16kW (with solar PV gain) and a continuous output of 12kW from the battery alone, you should consider a transformer with a kVA rating that can handle the maximum continuous load you expect plus a safety margin. Here's the breakdown:
Maximum Output: 16kVA is the highest output scenario with solar PV gain.
Safety Margin: It's common to add 10-20% over the maximum expected load for safety and to account for inefficiencies or unexpected increases in power demand.
Therefore, a 16kVA to 20kVA isolation transformer would be suitable for your needs. Going for a 20kVA transformer would provide a good safety margin and account for potential peak demands or future expansions of your setup. Remember, this is based on the assumption that you're looking to handle the maximum output capacity of the inverter. If your actual usage is consistently lower, you might consider a smaller transformer, but always keep some headroom for safety.
4.54s

[tjmooney]

Thanks 🎉 just ordered mine to replace my 18kpv and play with as im 💯 off grid

[Waldoe]

Sol-Ark has some tough competition here

[reviewsyou]

I agree the Flexboss seems like a good alternative to the 18kPV. It will be nice once they add a screen option, I prefer having a screen backup along w/ the app. It is curious that the MPPT efficiency from PV to battery and PV to grid is less efficient than the 18kPV though?

[bearupfarm]

The unit is very new and hasn’t proven itself yet. It better to see it up and running with hours to prove it worth buying. Just like anything new with all modifications and needs to run for a while to get all the bugs fixed

[jeremyallard]

Very nice, great presentation, I wonder when Will Prowse will be making a video in the Flexboss21?

[JoeCoolR]

Another great review. Thanks for all the information. I was going to do the 18Kpv, but now I'm going to install the BOSSes. My Grid and Flex are going to be separated quite a bit. The batteries will be near the Flex. I understand there's only one cable that needs to go between the Flex and Grid. Will you cover that in your review of the combo setup?

[TheComingGlory]

I was on the phone with Signature Solar a couple weeks ago going over the mppt ports on our Flexboss21 for 36 panels because the EG4 calculator determined I could have a string of 13, 13, 10. According to SS that will not work because mppt1 is 600v and mppt2 is like 440. They said I could do 12, 12 on mppt1 and 6, 6 on mppt2 or similar. I looked at the manual and did not see anything about mppt2 being 440, only that both mppt1 and mppt2 ports both have the same 26-amp limit. In the video at 1:26 it says all 3 ports can handle up to 600v, but I am not seeing this in the manual anywhere unless I missed it. The manual states that each mppt port will combine the two strings for each port, provided that they do not exceed the 26-amp limit (13 per string), so no combiner box is required. However, many combiner boxes have surge/back feed protection which I don't think the Flexboss21 has, so it may be wise to get one. Anyway, where can I find the information regarding the voltage limit for the mppt ports since SS had to of got that information from somewhere (unless they were looking at the wrong thing).

[michiganengineer]

Added as a sub so I can hear your take on the GridBoss. Looking at the EG4 site online that unit looks to be PERFECT for a hybrid setup that tries to be mostly off grid. As you might guess from my user name, I have to plan for a little bit more wintery weather than you and @WillProwse do, so being able to add more panels is ALWAYS a good thing!

[azpaquin]

Can’t wait for them to make a smaller one like the 8k

[ter]

Thanks for the review. I am curious if the application can work only using wifi, i.e., no internet connection. In a regional grid down scenario, this would be a concern since there is no longer a display.

I am looking forward to your schematic, especially since your prior video discussed placing an inverter bypass switch in place for system maintenance. I believe this would perform that function.