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
The Fortress Power Envy Duo 21 is the better hybrid solar inverter overall, rated 4.3 to 4.1. The Fortress Power Envy Duo 21 leads on solar input. The Fortress Power Envy Duo 21 costs $101 less ($4,898 vs $4,999).
Sol-Ark 15K-2P
For homeowners and installers who want a long-proven whole-home hybrid with strong support. It costs more than EG4 for similar output.
Amazon Shop Solar Kits $4,999$4,999
Prices were last checked October 23, 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 Envy Duo 21 for solar input (25,000 W of panels, 550 V max, 3 MPPTs vs 19,500 W of panels, 500 V max, 3 MPPTs).
Scores side by side
Each factor is scored 0-100 against the same fixed reference points for every 120/240 V split-phase hybrid solar inverter; a clear win is highlighted. How we rank explains each one.
| Factor weight | Fortress Power Envy Duo 21 | Sol-Ark 15K-2P |
|---|---|---|
| Overall rating | 4.3/5 | 4.1/5 |
| Value 30% | 58 $408 per kW of continuous output | 57 $417 per kW of continuous output |
| Solar input 20% | 100 25,000 W of panels, 550 V max, 3 MPPTs | 81 19,500 W of panels, 500 V max, 3 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 52 90 W idle |
| Features 15% | 100 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication | 100 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication |
| Warranty 15% | 100 10-year warranty | 100 10-year warranty |
Specifications
| Fortress Power Envy Duo 21 | Sol-Ark 15K-2P | |
|---|---|---|
| Output | ||
| Output voltage | 120/240 V split-phase | 120/240 V split-phase |
| Continuous output | 12,000 W | 12,000 W |
| Surge | 24,000 W | 24,000 W |
| Peak efficiency | 94% | - |
| Max efficiency | - | 97.5% |
| CEC efficiency | 96.9% | 96.5% |
| Idle draw | 60 W | 90 W |
| Transfer time | 20 ms | 5 ms |
| Parallel units | Up to 10 units | Up to 12 units |
| Solar | ||
| Max PV power | 25,000 W | 19,500 W |
| Max PV voltage (Voc) | 550 V | 500 V |
| MPPT range | 120 to 440 V | 150 to 425 V |
| MPPT trackers | 3 | 3 |
| PV current per tracker | 26 A | 26 A |
| Runs without battery | Yes | - |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | Lithium | Lithium, lead-acid |
| Max charge current | 250 A | 275 A |
| Battery communication | Yes | Yes |
| Grid sell-back | Yes | Yes |
| Generator input | Yes | Yes |
| UL 1741 | UL 1741 SB | UL 1741 SB |
| Build | ||
| App monitoring | Included | Included |
| Enclosure | IP66 (NEMA 4X) | IP65 / NEMA 3R |
| Dimensions | 22.3 x 35.2 x 11.2 in | 33 x 19.4 x 12 in |
| Weight | 125.7 lb | 135 lb |
| Warranty | 10 years | 10 years |
| Released | 2026-01 | 2022-08 |