fortress battery review

The Fortress Power battery complete review

Founded in 2016, Fortress Power is a Southampton based manufacturer of lithium ion solar batteries, offering both businesses and homeowners safe, reliable and sustainable energy storage solutions. Their storage solutions support multiple applications, are easy to install, and are competitively priced, with customer satisfaction as the company’s core value. Fortress Power offers four home battery products: the eVault 18.5, the eVault Max 18.5, the eFlex 5.4  and the LPF-5K-48V. In this article, we’ll explain what you need to know about their various offerings. 

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Key takeaways

  • Fortress power offers four lithium ion batteries: the eVault 18.5, the eVault Max 18.5, the eFlex 5.4, and the LPF-5K-48V.
  • Fortress Power’s energy storage units are compact and compatible with most solar systems.
  • Fortress Power batteries are modular and can be stacked to increase storage capacity.
  • They are known for their wide operating temperature range (-4 to 140 F) and high power rating and safety. 

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How do the Fortress Power batteries work?

The Fortress Power batteries pair well with solar panel systems, especially if your utility has reduced or removed net metering, introduced time-of-use rates, or instituted demand charges for residential electricity consumers. Installing a storage solution like a Fortress Power  battery with a solar energy system allows you to maintain a sustained power supply during both day and night, as long as you store enough power from your solar panels when the sun is shining.

As with many other home battery products, Fortress Power batteries are sized for day-to-day use at your home and are primarily designed to be paired with a solar panel system. When your solar panels produce more electricity than you can use in your home, you can store the excess electricity in the battery system instead of sending it back into the grid. Later, when your panels aren’t producing enough electricity to meet your home’s needs, you can use the stored energy in your battery instead of having to buy it from your utility company.

Key things to know about Fortress Power batteries

Fortress Power aims to provide affordable and user-friendly lithium ion storage solutions based on the latest technology. Their energy storage units are compact and compatible with most solar systems, allowing homeowners and businesses to minimize grid dependency and maximize solar storage capacity. Fortress Power’s batteries come in four models: the eVault 18.5, the eVault Max 18.5, the eFlex 5.4 and the LFP-5K-48V.

When evaluating Fortress Power batteries, there are various important metrics and technical specifications to keep in mind. Among the most important are the size of the battery (power and capacity), its chemistry, depth of discharge, and roundtrip efficiency


Fortress Power offers three sizes of batteries: 18.5 kilowatt-hours (kWh), 5.4 kWh, and 5.12 kWh. Two important metrics to keep in mind when comparing Fortress Power batteries to other home storage options are power and usable capacity. Power (measured in kilowatts, or kW) determines the maximum amount of electricity that can be output at a single time, while usable capacity (measured in kilowatt-hours, or kWh) is a measure of the maximum amount of electricity stored in your battery on a full charge. See the table below for Fortress Power’s power and usable capacity ratings for the various products.  

Fortress Power battery sizes

eVault 18.5eVault Max 18.5eFlex 5.4LFP-5K-48V
Maximum power (kW)9.6 kW12.8 kW6.6 kW9.2 kW
Continuous power (kW)4.8 kW9.2 kW3.5 kW4.0 kW
Usable capacity (kWh)18.5 kWh18.5 kWh5.4 kWh5.12 kWh

The Fortress Power Battery battery is modular. The eVault 18.5 can be connected by stacking 12 units to reach 222 kWh, the eVault Max 18.5 can be connected by stacking 20 units to reach 370 kWh, the eFlex can connect 15 units for a maximum of 81 kWh and the LFP-5K-48V can have a maximum of three units stacked together for 15.36 kWh. 

Deeper dive into power and usable capacity

Think of your battery like water running through a pipe. The usable energy capacity is the amount of water available to push through the pipe, while power is the size of the pipe itself. Larger pipes allow more water to flow through at once, which depletes the water faster. Similarly, a battery with a high power rating can deliver more electricity at one time, but will burn through its available energy capacity faster too.

A battery’s power determines what appliances you can run with it at the same time, while usable capacity determines how long those appliances can be run. Batteries with a higher power rating are capable of powering more robust appliances or many appliances at once, while batteries with a higher usable capacity can store more total energy and thus can run your appliances for longer periods of time without needing to recharge.

Basic functionality

The functionality of one solar power battery next to another can vary; some batteries have excellent off-grid capabilities, while others offer software solutions specific to rate arbitrage. Here are the important qualities of the Fortress Power: 

Backup power

Fortress Power batteries come with backup power capabilities. Most solar batteries require an external transfer switch to allow for off-grid operation. Fortress Power batteries are able to have a full home backup and achieve grid independence during power outages. This component connects your home, the grid, your storage system, and your solar energy system all together so that you can automatically power your home appliances if the grid goes down.


Fortress Power batteries use lithium iron phosphate (LFP) chemistry. This is one of the most common lithium-ion battery technologies, and for good reason: LFP batteries are know for their high power rating and safety. Fortress Power batteries can also operate in a wide temperature range of -4 to 140 F.To learn more about how different lithium-ion battery chemistries stack up against one another, check out our overview of battery chemistry differences.

Performance metrics

Two key ways to evaluate the performance of a solar battery are its depth of discharge and roundtrip efficiency

Depth of discharge (DoD) indicates the percentage of a battery’s energy that has been discharged relative to the overall capacity of the battery. Because the useful life of a battery decreases each time you charge, discharge, and re-charge it, many battery manufacturers specify a maximum DoD level for optimal battery performance. In general, batteries with a higher depth of discharge are considered higher quality products. Fortress Power batteries boast a depth of discharge of up to 100 percent across all battery sizes, reflective of their safe and advanced LFP battery chemistry.

Roundtrip efficiency is a measure of the electrical losses involved with charging and discharging a particular battery. The higher the percentage, the more efficiently the battery is able to convert incoming electricity into stored electricity and then back into usable electricity. Fortress Power batteries have a roundtrip efficiency of more than 98 percent; this means that for every 10 kilowatt-hours (kWh) of electricity you put into the battery, you’ll receive 9.8 kWh of output at least.

Fortress Power warranty coverage

Fortress Power warranties

  • Warranty period: 10 years
  • Warrantied cycles: 6,000 (eVault 18.5 & LFP-5K-48V) or 8,000 cycles (eVault Max 18.5 or eFlex 5.4)
  • Guaranteed end of warranty capacity: 70%

In most cases, homes with a Fortress Power will charge and discharge their battery every day. So, how long does the Fortress Power last? The Fortress Power comes with a 10-year, or 6,000 or 8,000 cycles (depending on the model) warranty, and Fortress Power guarantees that the battery will maintain at least 70 percent of its capacity to hold a charge during that time period. 

Fortress Power battery technology is similar to other rechargeable batteries both large and small: as time goes on, the battery loses some of its ability to hold a charge. Think of how the battery life of a brand-new smartphone compares to one that is a few years old. As you continually charge and drain your phone’s battery, it starts to lose some of its ability to hold a charge.

The battery life of your Fortress Power will deteriorate in the same way. That isn’t an indicator of a product flaw – all batteries lose some of their ability to hold a charge over time after extended usage, whether it’s an electric vehicle battery, a home energy battery, or a rechargeable AA battery. This is why Fortress Power offers a warranty that guarantees a certain percentage of storage capacity.

If you want to compare individual battery models side-by-side, our battery Buyer’s Guide lets you select products and compare them based on efficiency, capacity, power, and more.

What to know about the Fortress Power Guardian

In fall 2022, Fortress Power launched a new gateway device called the Guardian to connect their batteries to the cloud. With a Guardian device, you’ll be able to easily remotely monitor your battery’s charge, voltage, current, and temperature. It will also allow you to share this monitoring data with Fortress Power so they can quickly diagnose any issues and fix the issue. The Guardian enables Fortress Power to perform remote firmware updates and if you have any questions or need to submit a warranty claim, you can easily do so right in the app. Best of all, you can install the Fortress Power Guardian all by yourself – check out their setup guide for step-by-step instructions.

How much does the Fortress Power cost?

A solar battery installation isn’t as simple as a list price for a component – depending on your electrical setup, among other factors, installation costs can vary widely. As a rough estimate, you can expect a 5 kWh battery to cost about $6,500 and a 18.5 kWh to cost about $24,000 with a full system installation. 

If you want to install a Fortress Power battery as part of a solar-plus-storage system, battery costs are just one part of the equation. A 5 kilowatt (kW) solar energy system costs anywhere from $9,000 to $15,000 depending on where you live and the type of equipment you choose.

That may sound like a lot of money, but installing a solar-plus-storage system can be a worthwhile investment. Whether or not a Fortress Power battery makes sense for you is determined by the way that your electric utility structures its rates, as well as your reasons for installing a solar battery.

In some cases, depending on where you live, you may have access to financial incentives that can reduce your home energy storage installation costs. For instance, if you live in California, you could get a cash rebate that covers most of your home battery costs through the Self-Generation Incentive Program (SGIP). Other states (such as Massachusetts) are in the early stages of evaluating battery storage performance incentives as well, and several states already provide cash rebates.

Can you go off-grid with Fortress Power batteries?

Installing a solar-plus-storage system at your home is a great way to take control of your electricity bill, but it doesn’t mean that you’re completely disconnected from your utility. Going “off the grid” with solar batteries is actually a more expensive and complicated proposition than you might think. Most home batteries, including the Fortress Power, only have enough capacity to store a few hours of electricity. That being said, the battery could serve as a temporary backup when the grid goes down if you have a solar panel system to provide power. If you’re really set on going off-grid, the scalability of Fortress Power batteries allows you to stack batteries in parallel to provide enough back-up power for your whole home – but you should expect a high price tag.

Where to buy Fortress Power batteries

Whether you want to install a Fortress Power battery or another home battery, you will most likely need to work through a certified installer. Adding energy storage technology to your home is a complicated process that requires electrical expertise, certifications, and knowledge of the best practices required to install a solar-plus-storage system correctly.

A qualified EnergySage-approved company can give you the best recommendation about Fortress Power batteries and other energy storage options available to homeowners today. If you are interested in receiving competing installation quotes for solar and energy storage options from local installers near you, simply join the EnergySage Marketplace today and indicate what products you’re interested in your profile’s preferences section.

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About Ananyaa Mehra

Ananyaa is a researcher in electrification, renewable energy, and green technology, with a focus on policy and equity. She’s a junior at Tufts University pursuing a Bachelor's degree in Environmental Studies. On campus, Ananyaa is involved with the Energy Club and Sustainability Council and serves as an organizer of the annual Tufts Energy Conference. Ananyaa is especially passionate about protecting our oceans, where she loves to spend time.