The Fortress Power eVault Max 18.5 is a 48-volt lithium iron phosphate battery that stores 18.5 kWh in one cabinet, according to Fortress Power’s datasheet. Fortress lists 180 A of continuous discharge (9.2 kW), a 10-year warranty, an 8,000-cycle rating at 80% depth of discharge and room for up to 20 batteries in parallel (370 kWh). For a complete home system, Fortress pairs it with its Envy True 12kW hybrid inverter. This review uses Fortress Power’s datasheets, product pages and warranty documents. We haven’t tested one ourselves.
Fortress Power eVault Max 18.5 Specs
| Spec | eVault Max 18.5 |
|---|---|
| Nominal voltage | 51.2 V (48 V class) |
| Nominal capacity | 360 Ah |
| Rated capacity at 0.5C | 18.43 kWh |
| Continuous discharge | 180 A (9.2 kW) |
| Peak discharge | 230 A (12 kW for 30 min) |
| Surge | 250 A (12.8 kW for 5 s) |
| Max charge current | 180 A (150 A recommended) |
| Depth of discharge | Up to 100% |
| Efficiency (0.5C) | 98% |
| Parallel units | Up to 20 (370 kWh) |
| Chemistry | LiFePO4, prismatic cells |
| Enclosure | IP55 aluminum, touch-screen display |
| Size and weight | 20.3 × 20.3 × 42.2 in; 520 lb |
| Certifications | UL 1642, UL 1973, UL 9540, UL 9540A |
| Warranty | 10 years; 8,000 cycles at 80% DoD |
Source: Fortress Power eVault Max 18.5 datasheet.
Fortress says the eVault Max can be AC- or DC-coupled to solar and supports off-grid, backup, self-supply, peak-charge reduction and demand response uses, with closed-loop communication to compatible inverters over CAN or RS485.
The Envy True 12kW Inverter
| Spec | Envy True 12kW |
|---|---|
| Continuous AC output | 12,000 W at 240 V (on- and off-grid) |
| Peak power | 14 kW for 10 min, 16 kW for 5 min, 20 kW for 500 ms |
| Max PV input | 21,000 W (18,000 W delivered to battery and AC) |
| AC pass-through | 200 A; 48,000 W to loads with grid or generator |
| Battery | 48 V lithium or lead-acid |
| Max efficiency | 97.5% |
| Parallel | Up to 10 units, 120 kW AC |
| Rapid shutdown | Built-in APsmart transmitter |
| Warranty | 10 years |
Source: Fortress Power Envy True 12kW product page.
Fortress says the Envy works with all its 48 V batteries, including the eForce, eVault MAX and eFlex MAX, and that it’s battery-agnostic with a published list of compatible lithium brands. It supports 120/240 V split-phase homes and three-phase commercial setups with two or more units.
The Smaller eFlex MAX 5.4
For smaller systems, Fortress sells the eFlex MAX 5.4, a 48 V LFP battery with 5.4 kWh total energy, 100 A continuous charge and discharge, 8,000 cycles at 80% depth of discharge and a 10-year warranty. It weighs 108 lb and has a saltwater-resistant aluminum enclosure rated for indoor or outdoor use. Fortress notes it isn’t compatible with the older eFlex 5.4 battery.
Warranty in Detail
Fortress Power’s 10-year limited warranty for the eVault Max covers defects until the first of 10 years, 8,000 cycles at 80% depth of discharge, or 3,000 cycles at 100% depth of discharge. The warranty document lists throughput equivalents of 87.2 MWh at 80% DoD, 48.9 MWh at 90% and 36.3 MWh at 100%, along with recommended charge currents for each tier. In short, limiting depth of discharge to 80% earns the most warranted energy. See home battery warranties compared.
What the Specs Mean
- One of the largest single cabinets. 18.5 kWh in one box is more than most home batteries, so many homes need only one or two.
- 48 V architecture. Low-voltage batteries pair with 48 V hybrid inverters like the Envy, which are common in off-grid and DIY-friendly designs. High currents mean cable sizing and fusing matter.
- 9.2 kW from the battery, 12 kW from the inverter. One eVault Max can’t supply the Envy’s full 12 kW continuously by itself. Two batteries can.
Monitoring, Grid Programs and Export Control
Fortress says the Envy True 12kW supports zero export and load shifting, so it can store solar instead of sending it to the grid and run the home from the battery during expensive hours, with or without solar panels. It’s approved by utilities across the US and Puerto Rico, according to Fortress, and is ready for virtual power plant programs that pay owners to discharge during peak demand. The Fortress Power mobile app shows solar production, battery state and grid use, allows remote firmware updates, and lets owners register the inverter warranty directly in the app.
Questions to Ask a Fortress Installer
- How many eVault Max batteries, and will the battery bank supply the inverter’s full 12 kW?
- Is the battery communicating closed-loop with the inverter?
- What depth of discharge will the system use, given the warranty’s cycle tiers?
- Where will a 520 lb cabinet sit, and is the floor rated for it?
- Will a generator be wired in, and how will it charge the batteries?
How It Compares to Powerwall 3, Enphase 5P and FranklinWH
| Spec | eVault Max 18.5 (+ Envy) | Tesla Powerwall 3 | Enphase IQ Battery 5P | FranklinWH aPower 2 |
|---|---|---|---|---|
| Energy per unit | 18.43 kWh (rated) | 13.5 kWh | 5.0 kWh | 15 kWh |
| Continuous power | 9.2 kW battery; 12 kW Envy | Up to 11.5 kW | 3.84 kVA | 10 kW |
| Voltage class | 48 V | High voltage | AC battery | AC battery |
| Max system | 370 kWh | 4 units + 3 Expansions each | Many units | 15 per aGate |
| Warranty | 10 yr or 8,000 cycles (80% DoD) | 10 yr, 70% (US) | 15 yr or 6,000 cycles | 15 yr or 60 MWh |
Figures from each maker’s US documents.
- Versus Powerwall 3: eVault Max stores more per unit and scales much further, but needs a separate inverter. See our Tesla Powerwall 3 review.
- Versus Enphase and FranklinWH: those are AC-coupled turnkey systems. Fortress is a modular 48 V ecosystem that suits large or off-grid builds.
Who It Suits
- Large homes, farms and off-grid properties that need 30 kWh or more.
- Installers building 48 V systems around the Envy or another compatible inverter.
- Owners who want generator integration through the inverter.
Who Should Look Elsewhere
- Homeowners who want a sealed, single-brand AC battery installed in a day.
- Tight spaces, since one eVault Max weighs 520 lb.
Sizing Example
Illustrative inputs: an off-grid cabin uses 20 kWh a day and wants two days of autonomy, cycling no deeper than 80%.
- Energy: 20 kWh × 2 days ÷ 0.8 = 50 kWh of battery.
- Batteries: 50 ÷ 18.43 = 2.7, so three eVault Max units (about 55 kWh).
- Power: three batteries supply up to 27.6 kW (3 × 9.2), more than one Envy’s 12 kW, so the inverter sets the limit.
- Solar: the Envy accepts up to 21 kW of PV. See our off-grid solar system guide.
Price and Incentives
Fortress sells through dealers and installers. We aren’t publishing a price we can’t verify. There’s no 30% federal credit for homeowner-owned systems installed after December 31, 2025 (IRS).
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
FAQ
How much does a Fortress Power eVault Max store?
It’s sold as 18.5 kWh, with a rated capacity of 18.43 kWh at 0.5C (51.2 V × 360 Ah).
How many eVault Max batteries can I connect?
Up to 20 in parallel, for 370 kWh, per Fortress.
What inverter works with the eVault Max?
Fortress pairs it with its Envy True 12kW, and the datasheet lists closed-loop communication with compatible inverters.
What is the eVault Max warranty?
10 years, or 8,000 cycles at 80% depth of discharge (87.2 MWh), or 3,000 cycles at 100%, whichever comes first.
Is the eVault Max LFP?
Yes. Fortress uses lithium iron phosphate prismatic cells.
Sources: Fortress Power, eVault Max 18.5 datasheet and 10 Year Limited Warranty; Envy True 12kW and eFlex MAX 5.4 product pages (fortresspower.com); Tesla Powerwall 3 Datasheet; Enphase IQ Battery 5P datasheet; FranklinWH aPower 2 datasheet; IRS, Residential Clean Energy Credit. Accessed Oct 2026.
Related: Home Solar and Battery Guide 2026, EG4 18kPV Hybrid Inverter and Home Battery, Generator vs Battery Backup for Your Home