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Fortress Power Presents Their New Flex Tower

Flex Tower open (Courtesy of Fortress Power)

George Harvey

Fortress Power (FP) is a company that makes batteries with some fairly important advantages. It has been described in Green Energy Times in a couple of articles already, as we have looked into questions of mobile energy systems and battery backup for homes with solar systems. An article on the latter was “Working Together: Fortress Battery and Green Mountain Solar” (). That article was of particular interest for the present one, because it discussed FP’s eFlex 5.4 kilowatt-hour (kWh) battery.

Now, building on its success with the eFlex 5.4 lithium iron phosphate (LFP) batteries, FP has introduced a new product, the FlexTower All-in-One Energy Storage System (FlexTower). To understand this, we should do a quick review the eFlex 5.4 battery. It has a capacity of 5.4 kWh, weighs 108 pounds, has an efficiency of over 98%, and a discharge rate under 1% per month. It is far safer than old-fashioned lithium-ion batteries, has a 10-year warranty, and is rated for 8,000 cycles.

The FlexTower builds on the eFlex 5.4 battery by providing an enclosure for up to four batteries along the charge controller, an LCD control panel, and a remote monitoring app, so, for example, the user can tell what the state of charge is at all times. These are integrated in one DuraRack unit that also has space for an inverter, which the customer can choose from a respectably sized list. A 200-amp pass-through eliminates a backup panel requirement. Basically, that means it can include all the major equipment needed for an average household or commercial solar photovoltaic system but the panels themselves. (FP also has a larger product for even larger systems.)

It supports operation off-grid and for back-up power, and it has an app for remote monitoring. But it goes far beyond those. It can be used for time-of-use and demand response applications. It supports peak shaving, and can function as part of a virtual power plant.

For household and commercial applications, planning for a FlexTower should be really simple. It can be installed in all sorts of places. Since there is no danger of gas or fire, indoor installations are as convenient as we might hope for. But the FlexTower is also rated for outdoor installations. Its normal operating temperature is 32°F to 114°F (0°C to 45°C), but it can have internal heating with modest power requirements. It can withstand any extreme temperatures that might be seen in most parts of the Northeast because it has a thermostat and optional temperature controls to keep everything within its very wide operating range. We might note that for off-grid applications, the extra energy used by a heater should be considered before deciding to install the unit outdoors.

The FlexTower can be purchased with or without an inverter. The inverter can come from any of several companies, and a list of inverters is available at the FP website, The list has a wide variety of products to choose from, made by nine companies, Magnum, Midnite Solar, Morning Star, Outback, Schneider, SMA, Sol-Ark, Studer, and Victron.

We contacted Zach Reagan at Sol-Ark about his company’s experience with FP’s batteries. He told us, “Sol-Ark inverters actually have extra levels of communications and certifications with some batteries that have higher quality Battery Management Systems. Fortress is a great example of this type. The Sol-Ark hybrid inverters have settings for different batteries, and one of the battery companies we have the highest level of communication with is Fortress.” He also stressed the fact that Sol-Ark and FP know each other’s products. He said, “If you’re on the phone with us about an inverter question, and you also have a question about the Fortress battery setup, odds are that we will be able to answer that question without you having to hang up and spend any extra time on the phone by calling another company for that same question.” It seems that we live in a time of increased communications, both for equipment, and for people.

The FlexTower is a relatively new product. But Fortress Power batteries are well known. One of the solar installers we know who has had a fair amount of experience with the equipment is John Blittersdorf. He uses Fortress Power LFP batteries for his own home and has installed them elsewhere. In one case, two systems that were very near each other were brought down by a single lightning strike, and this led to his having some experience dealing with FP’s customer service department and repairing the units in the field. “Fortress Power is a good company,” he told us, “And the owners seem eager to help.” He also noted that customer service is really good, the equipment is well made, and it is easy to repair.”

Fortress Power’s website is

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