I have the EndurEnergy. they work fine. They are a related company (importer) to Sol-ark, so the installer gets a one-stop shop for any problems. They do have closed loop and UL 9540. I would get the ESP-BU20 and have them put in 3 batteries. Then you can add one later yourself. Tell them you want the low profile of the BU20.
The ESP-5100 are NOT DIY friendly. Support is non-existent (other than from your installer). No app to see what each individual battery is doing. etc. As far as I can tell, it is the USA version of the CFE-5100, but none of the apps work. Although it is a 5.1kWh battery, the BMS settings don't get you to the full 5.1kWh. Charges at 55.0v until batteries hit 54.5v, and then drops to 54.0v float.
I needed the "official" system because of the Energy Storage System rebates in my area. I did add a DIY using EVE LF280k, and it works great with the EE's.
I suggest biting the bullet and having them attach an ESP BU20 (with 3 batteries) to the Sol-Ark. That way you will have a fully permitted system.
That will use one of the two battery connections. DIY a 2nd battery to the other connection. EVE LF304 look nice. If you want, you can buy one more ESP-5100 and install it yourself in the empty slot. Or, just skip the EE battery and DIY one yourself. Note: The PowerPro is 300+ pounds. Not an easy DIY install. I wouldn't try to get it up or down stairs. At least with LF304 cells, you can move them yourself. The ESP-5100 are really heavy too.
