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 Sol-Ark 18K-2P wins on value. The Fortress Power Envy Duo 21 costs $1,400 less ($4,898 vs $6,298).
Sol-Ark 18K-2P
For large homes that want seamless whole-house backup with no load shedding. Few inverters match its output, but it costs well over most rivals.
Shop Solar Kits $6,298$6,298
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 28,800 W of panels, 500 V max, 3 MPPTs).
- Choose the Sol-Ark 18K-2P for value ($350 per kW of continuous output vs $408 per kW of continuous output).
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 18K-2P |
|---|---|---|
| Overall rating | 4.3/5 | 4.1/5 |
| Value 30% | 58 $408 per kW of continuous output | 70 $350 per kW of continuous output |
| Solar input 20% | 100 25,000 W of panels, 550 V max, 3 MPPTs | 80 28,800 W of panels, 500 V max, 3 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 43 110 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 18K-2P | |
|---|---|---|
| Output | ||
| Output voltage | 120/240 V split-phase | 120/240 V split-phase |
| Continuous output | 12,000 W | 18,000 W |
| Surge | 24,000 W | 36,000 W |
| Peak efficiency | 94% | - |
| Max efficiency | - | 97.6% |
| CEC efficiency | 96.9% | 97% |
| Idle draw | 60 W | 110 W |
| Transfer time | 20 ms | 5 ms |
| Parallel units | Up to 10 units | Up to 12 units |
| Solar | ||
| Max PV power | 25,000 W | 28,800 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 | 36 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 | 350 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 | 34 x 19.4 x 12 in |
| Weight | 125.7 lb | 137 lb |
| Warranty | 10 years | 10 years |
| Released | 2026-01 | 2025-09 |