The Envy Duo 21 is a 48V hybrid inverter built for larger residential and light commercial solar-plus-storage systems. It's designed for homeowners or installers who need high PV input capacity, whole-home backup, and the option to scale up as energy demands grow.
The Envy Duo 21 is a hybrid architecture inverter: it operates grid-tied with battery backup capability, not an off-grid-only unit. It draws PV directly (25kW max array power) and integrates with 48V battery banks for continuous and backup power. Note: If you would like to review the manufacturer Product Specifications, Warranty, Brochure, etc., for this system, please refer to the "Resources (System Manuals and Warranty)" Tab above on this page.
Critical Power & Interconnection Requirements
- AC Output: 240V split-phase or 208V three-phase (three-phase requires two or more inverters in parallel)
- DC Battery Voltage: 48V — matched to Fortress Power 48V battery systems
- Continuous Backup Output: 12kW across both voltage configurations
- Max Export Power: 16kW
- Max PV Input: 21,000W (unit rated); Max PV Array Power: 25,000W
- Number of MPPT trackers: 3, with inputs per MPPT of 2:2:1
- MPPT operating voltage range: 120–440V; DC voltage range 100–550V; start voltage 100V
- Nominal output current: 50A @ 240V, 57.7A @ 208V
- Parallel scalability: up to 10 units for a combined 120kW total output
- NOT an off-grid-only inverter — this is a hybrid unit designed for grid-interactive operation with battery backup
- NOT compatible with 12V or 24V battery banks — 48VDC only
- Three-phase 208V operation is NOT available on a single standalone unit — requires parallel configuration
Pro-Installer Tip
On multi-MPPT hybrid inverters like the Envy Duo 21, mismatched string lengths across the 3 MPPT inputs are a common source of underperformance flags, especially when arrays face different orientations. Confirm each string's Voc stays within the 120-440V MPPT operating window before energizing, particularly in cold-climate installs where Voc rises at low temperatures and can push a string toward the 550V absolute DC ceiling. Use a digital multimeter to verify open-circuit voltage per string prior to commissioning, and log the readings against the 100-550V DC voltage range spec. This step, combined with correct labeling of the integrated rapid shutdown transmitter per NEC Article 690, prevents nuisance overvoltage faults and keeps the install passing AHJ inspection on the first visit.
Key Benefits
- High PV input capacity — accepts up to 25kW max array power for larger residential or light commercial arrays
- Dual voltage compatibility — supports 240V split-phase and 208V three-phase configurations
- 12kW continuous backup output with 16kW max export power when paired with Fortress Power 48V batteries
- Parallel scalable to 120kW total output across up to 10 units
- Integrated AC/DC breakers and rapid shutdown transmitter for streamlined, code-oriented installation
- Remote monitoring and firmware updates via the Fortress Power mobile app
| Spec | Value | Unit |
|---|---|---|
| Continuous Backup Output | 12 | kW |
| Max Export Power | 16 | kW |
| Max PV Input (Unit) | 21000 | W |
| Max PV Array Power | 25000 | W |
| Number of MPPT | 3 | count |
| Inputs per MPPT | 2:2:1 | ratio |
| Max Usable Input Current | 26/26/15 | A |
| Max Short Circuit Input Current | 31/31/19 | A |
| Start Input Voltage | 100 | V |
| MPPT DC Nominal Voltage | 360 | V |
| DC Voltage Range | 100-550 | V |
| MPPT Operating Voltage Range | 120-440 | V |
| Nominal Output Current | 50@240V, 57.7@208V | A |
| DC Battery Voltage | 48 | VDC |
| Parallel Stacking | Up to 10 units, 120 | kW total |
Note: See "Resources (System Manuals and Warranty)" Tab above for full specifications.
Compatibility & Fitment
- Compatible with Fortress Power 48V battery systems for whole-home or light commercial backup
- Supports both 240V split-phase and 208V three-phase grid configurations
- Three-phase 208V mode requires a minimum of two or more inverters in parallel — NOT achievable with a single unit
- Parallel scalable up to 10 units (120kW combined output)
- Includes integrated rapid shutdown transmitter compatible with module-level shutdown requirements
- NOT compatible with 12V or 24V battery banks — 48VDC systems only
- NOT designed as an off-grid-only inverter — this is a grid-interactive hybrid unit
- Remote monitoring via Fortress Power mobile app — [NEEDS VERIFICATION] whether a WiFi/cellular module is included or sold separately
Compliance & Environment
| Category | Detail |
|---|---|
| Rapid Shutdown | Integrated rapid shutdown transmitter included |
| Breakers | Integrated AC/DC breakers |
| UL 1741 Listing | [NEEDS VERIFICATION] — not specified in manufacturer data provided |
| UL 9540 | Not applicable — this listing applies to certified system-level ESS pairings, not a standalone inverter |
| NEMA Enclosure Rating | [NEEDS VERIFICATION] — not specified in manufacturer data provided |
FAQ
Can this inverter sell power back to the grid?
The manufacturer data provided specifies a max export power of 16kW, indicating grid-export capability as part of its hybrid design. Verify local utility interconnection agreements before enabling export.
What battery voltage is required?
The Envy Duo 21 requires a 48VDC battery bank. It is matched with Fortress Power 48V battery systems. It is NOT compatible with 12V, 24V, or other DC battery voltage banks.
Can I parallel multiple units together?
Yes. The Envy Duo 21 supports parallel scalability up to 10 units for a combined output of 120kW. Note that 208V three-phase configurations require a minimum of two or more inverters in parallel.
Is this UL 1741 listed for utility interconnection?
[NEEDS VERIFICATION] UL 1741 listing status was not specified in the manufacturer data provided for this model.
Does this unit include an integrated rapid shutdown transmitter?
Yes, the Envy Duo 21 includes an integrated rapid shutdown transmitter and integrated AC/DC breakers for code-compliant installation.
Warranty & Registration
Manufacturer warranty terms for the Envy Duo 21 were not detailed in the data provided. [NEEDS VERIFICATION] Refer to the "Resources (System Manuals and Warranty)" Tab above on this page for exact warranty length, closed-loop battery communication terms, and registration requirements before purchase. Contact us if you have questions — we always reply back.
