EG4 FlexBOSS21
For grid-tied homes that want whole-house backup and a big solar array from one inverter. It gives more output than the 18kPV for less money.
Signature Solar $3,599.99$3,599.99
The EG4 FlexBOSS21 is the better hybrid solar inverter overall, rated 4.4 to 4.3. The EG4 FlexBOSS21 leads on value. The Fortress Power Envy Duo 21 wins on solar input. The EG4 FlexBOSS21 costs $1,298.01 less ($3,599.99 vs $4,898).
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
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 EG4 FlexBOSS21 for value ($300 per kW of continuous output vs $408 per kW of continuous output).
- Choose the Fortress Power Envy Duo 21 for solar input (25,000 W of panels, 550 V max, 3 MPPTs vs 21,000 W of panels, 600 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 | EG4 FlexBOSS21 | Fortress Power Envy Duo 21 |
|---|---|---|
| Overall rating | 4.4/5 | 4.3/5 |
| Value 30% | 80 $300 per kW of continuous output | 58 $408 per kW of continuous output |
| Solar input 20% | 88 21,000 W of panels, 600 V max, 3 MPPTs | 100 25,000 W of panels, 550 V max, 3 MPPTs |
| Efficiency 20% | 65 94% peak efficiency, 65 W idle | 68 94% peak efficiency, 60 W idle |
| Features 15% | 95 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, 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
| EG4 FlexBOSS21 | Fortress Power Envy Duo 21 | |
|---|---|---|
| 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% | 94% |
| CEC efficiency | 97% | 96.9% |
| Idle draw | 65 W | 60 W |
| Transfer time | 20 ms | 20 ms |
| Parallel units | Up to 16 units | Up to 10 units |
| Solar | ||
| Max PV power | 21,000 W | 25,000 W |
| Max PV voltage (Voc) | 600 V | 550 V |
| MPPT range | 120 to 440 V | 120 to 440 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 |
| Max charge current | 250 A | 250 A |
| Battery communication | Yes | Yes |
| Grid sell-back | Yes | Yes |
| Generator input | - | Yes |
| UL 1741 | UL 1741 SB | UL 1741 SB |
| Build | ||
| App monitoring | Included | Included |
| Enclosure | NEMA 4X | IP66 (NEMA 4X) |
| Dimensions | 30.43 x 22.28 x 11.22 in | 22.3 x 35.2 x 11.2 in |
| Weight | 121 lb | 125.7 lb |
| Warranty | 10 years | 10 years |
| Released | 2024-11 | 2026-01 |