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 warranty, efficiency and features. The EG4 12000XP wins on value. The EG4 12000XP costs $2,798.01 less ($2,099.99 vs $4,898).
EG4 12000XP
For off-grid homes that need whole-house 240 V power and a big solar array at a low price per kW. Grid-tied owners who want to sell power should look at a hybrid instead.
Signature Solar $2,099.99 Shop Solar Kits $2,099.99$2,099.99
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 warranty (10-year warranty vs 5-year warranty), efficiency (94% peak efficiency, 60 W idle vs 90.2% peak efficiency, 70 W idle) and features (UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication vs UL 1741 listed, Wi-Fi app, parallel stacking, generator input, battery communication; no grid sell-back).
- Choose the EG4 12000XP for value ($175 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 | EG4 12000XP |
|---|---|---|
| Overall rating | 4.3/5 | 4.1/5 |
| Value 30% | 58 $408 per kW of continuous output | 100 $175 per kW of continuous output |
| Solar input 20% | 100 25,000 W of panels, 550 V max, 3 MPPTs | 92 24,000 W of panels, 500 V max, 2 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 38 90.2% peak efficiency, 70 W idle |
| Features 15% | 100 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication | 85 UL 1741 listed, Wi-Fi app, parallel stacking, generator input, battery communication; no grid sell-back |
| Warranty 15% | 100 10-year warranty | 44 5-year warranty |
Specifications
| Fortress Power Envy Duo 21 | EG4 12000XP | |
|---|---|---|
| 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 | 15,360 W |
| Peak efficiency | 94% | 90.2% |
| CEC efficiency | 96.9% | - |
| Idle draw | 60 W | 70 W |
| Transfer time | 20 ms | 10 ms |
| Parallel units | Up to 10 units | Up to 16 units |
| Solar | ||
| Max PV power | 25,000 W | 24,000 W |
| Max PV voltage (Voc) | 550 V | 500 V |
| MPPT range | 120 to 440 V | 120 to 385 V |
| MPPT trackers | 3 | 2 |
| PV current per tracker | 26 A | 35 A |
| Runs without battery | Yes | Yes |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | Lithium | Lithium, lead-acid |
| Max charge current | 250 A | 250 A |
| Battery communication | Yes | Yes |
| Grid sell-back | Yes | No |
| Generator input | Yes | Yes |
| UL 1741 | UL 1741 SB | Yes |
| Build | ||
| App monitoring | Included | Included |
| Enclosure | IP66 (NEMA 4X) | IP20 |
| Dimensions | 22.3 x 35.2 x 11.2 in | 34.25 x 20.87 x 5.91 in |
| Weight | 125.7 lb | 104.7 lb |
| Warranty | 10 years | 5 years |
| Released | 2026-01 | 2024-11 |