Fortress Power eFlex MAX 5.4
A tough 5 kWh module for off-grid cabins, boats and small systems that need an outdoor-rated case. Larger inverters need three or more to run at full power.
Current Connected $1,449$1,449
The Fortress Power eFlex MAX 5.4 is the better 48V solar battery overall, rated 4.2 to 3.6. The Fortress Power eFlex MAX 5.4 leads on value and features. The Briggs & Stratton SimpliPHI 6.6 wins on output. The Fortress Power eFlex MAX 5.4 costs $1,913.99 less ($1,449 vs $3,362.99).
Briggs & Stratton SimpliPHI 6.6
For homeowners and installers who want a neat, outdoor-rated stack that goes up in minutes with no battery cabling. It costs more per kWh than server-rack batteries, so it suits buyers who value the clean install and the Briggs warranty.
Off Grid Stores $3,362.99$3,362.99
Prices were last checked October 24, 2026 and are subject to change; the price and availability shown by the retailer at the time of purchase apply.
Which should you buy?
- Choose the Fortress Power eFlex MAX 5.4 for value ($270 per rated kWh vs $505 per rated kWh) and features (inverter communication, app; no self-heating vs inverter communication; no self-heating).
- Choose the Briggs & Stratton SimpliPHI 6.6 for output (130 A continuous (about 6.7 kW) vs 60 A continuous (about 3.1 kW)).
Scores side by side
Each factor is scored 0-100 against the same fixed reference points for every wall and floor mount 48V solar battery; a clear win is highlighted. How we rank explains each one.
| Factor weight | Fortress Power eFlex MAX 5.4 | Briggs & Stratton SimpliPHI 6.6 |
|---|---|---|
| Overall rating | 4.2/5 | 3.6/5 |
| Value 35% | 73 $270 per rated kWh | 21 $505 per rated kWh |
| Output 15% | 39 60 A continuous (about 3.1 kW) | 100 130 A continuous (about 6.7 kW) |
| Warranty 15% | 100 10-year warranty | 100 10-year warranty |
| Safety listing 15% | 100 UL 1973, UL 9540 listed | 100 UL 1973, UL 9540 listed |
| Cycle life 10% | 100 8,000 cycles at 80% depth, maker rating | 30 4,500 cycles at 100% depth, maker rating |
| Features 10% | 70 inverter communication, app; no self-heating | 40 inverter communication; no self-heating |
Specifications
| Fortress Power eFlex MAX 5.4 | Briggs & Stratton SimpliPHI 6.6 | |
|---|---|---|
| Capacity | ||
| Case | Wall or floor mount | Wall or floor mount |
| Nominal voltage | 51.2 V | 51.2 V |
| Capacity | 105 Ah | 130 Ah |
| Energy | 5.4 kWh | 6.7 kWh |
| Chemistry | LiFePO4 | LiFePO4 |
| Cycle life | 8,000 at 80% depth | 4,500 at 100% depth |
| Power | ||
| Continuous discharge | 60 A | 130 A |
| Peak discharge | 130 A | 240 A |
| Max charge current | 100 A | 100 A |
| Modules in parallel | Up to 16 | Up to 18 |
| Safety and communication | ||
| UL 1973 | Yes | Yes |
| UL 9540A | Yes | Yes |
| UL 9540 | Yes | Yes |
| Communication | CAN, Modbus, Wi-Fi | CAN, RS485 |
| Works with inverters | Fortress Envy, Schneider, Sol-Ark, Midnite Solar, OutBack, Morningstar, Phocos, Victron, Growatt | EG4, Sol-Ark, Growatt, Megarevo, Solis |
| Self-heating | No | No |
| App | Yes | No |
| Build | ||
| Display | No | - |
| Enclosure | IP65 | IP20 (IP65 when stacked) |
| Dimensions | 17.5 x 7.2 x 24.2 in (446 x 184 x 616 mm) | 726 x 440 x 170 mm (28.6 x 17.3 x 6.7 in), 480 mm with tabs |
| Weight | 112 lb | 125 lb |
| Warranty | 10 years | 10 years |
| Released | 2024-05 | 2024-04 |