Fortress Power Envy Duo 21
For homes with a large roof array that want whole-house backup and grid export from one outdoor unit. It takes more solar than the Envy True 12 for the same price.
Wind & Sun $4,898$4,898
4.2/5
For grid-tied or off-grid homes that want one outdoor inverter to back up the whole house and export to the grid, with US-based support.
- 12 kWOutput
- 48 VBattery
- 21,000 WSolar input
- 70 WIdle draw
- 10 yearsWarranty
Check price at Current Connected$4,898Check price at Wind & Sun$4,898
Price checked October 23, 2026. As an Amazon Associate we earn from qualifying purchases.
Pros
- Runs a whole house: 12 kW of 120/240 V and a 200 A passthrough from one unit
- Takes 21,000 W of panels on three trackers
- Sells power back to the grid with UL 1741 SB
- Outdoor-rated case and a ten-year warranty
Cons
- About 70-80 W idle draw costs nearly 2 kWh a day off grid
- Weighs 127 lb, a two-person mount
- Costs more per kW than the EG4 and Growatt alternatives
How it scores
The rating comes from 5 measured or published factors, each scored 0-100 against fixed reference points for its output. How we rank explains each one.
Independent measurements
Efficiency and idle draw measured by independent reviewers.
| Tested by | Efficiency | Idle draw | Conditions |
|---|---|---|---|
| The Solar Lab | - | 80 W | About 80 W idle measured off-grid on battery only. |
Specifications
| Model number | FP-ENVY-12K |
|---|---|
| Output | |
| Output voltage | 120/240 V split-phase |
| Continuous output | 12,000 W |
| Surge | 20,000 W |
| Peak efficiency | 94% |
| CEC efficiency | 96.5% |
| Idle draw | 70 W |
| Transfer time | 20 ms |
| Parallel units | Up to 10 units |
| Solar | |
| Max PV power | 21,000 W |
| Max PV voltage (Voc) | 600 V |
| MPPT range | 230 to 500 V |
| MPPT trackers | 3 |
| PV current per tracker | 25 A |
| Runs without battery | Yes |
| Battery and grid | |
| Battery voltage | 48 V |
| Battery types | Lithium, lead-acid |
| Max charge current | 250 A |
| Battery communication | Yes |
| Grid sell-back | Yes |
| Generator input | Yes |
| UL 1741 | UL 1741 SB |
| Build | |
| App monitoring | Included |
| Enclosure | IP65 (NEMA 4X) |
| Dimensions | 34.2 x 20.5 x 11.2 in |
| Weight | 127 lb |
| Warranty | 10 years |
| Released | 2023-04 |
Alternatives in this output
EG4 18kPV
For grid-tied homes that want whole-house backup and grid sell from one proven inverter. Compare against the FlexBOSS21 first, which costs less for more power.
Signature Solar $3,999.99 Shop Solar Kits $3,999.99$3,999.99
EG4 12kPV
For grid-tied homes that want battery backup and net metering from a mid-size inverter. Pay the premium over the XP line only if you need grid sell or an outdoor install.
Signature Solar $2,999.99 Shop Solar Kits $2,999.99$2,999.99
Compare
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