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 3.4. The Fortress Power Envy Duo 21 leads on warranty, solar input and features. The SunGoldPower FR12K-B wins on value. The SunGoldPower FR12K-B costs $3,108 less ($1,790 vs $4,898).
SunGoldPower FR12K-B
A lot of split-phase power for the money for off-grid homes. Buyers who need UL-listed gear should look at the SPH10048P.
Amazon SunGoldPower $1,790$1,790
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 1-year warranty), solar input (25,000 W of panels, 550 V max, 3 MPPTs vs 13,200 W of panels, 500 V max, 2 MPPTs) and features (UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication vs Wi-Fi add-on, parallel stacking, generator input, battery communication; not UL 1741 listed).
- Choose the SunGoldPower FR12K-B for value ($149 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 | SunGoldPower FR12K-B |
|---|---|---|
| Overall rating | 4.3/5 | 3.4/5 |
| Value 30% | 58 $408 per kW of continuous output | 100 $149 per kW of continuous output |
| Solar input 20% | 100 25,000 W of panels, 550 V max, 3 MPPTs | 48 13,200 W of panels, 500 V max, 2 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 38 92% peak efficiency |
| Features 15% | 100 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, generator input, battery communication | 45 Wi-Fi add-on, parallel stacking, generator input, battery communication; not UL 1741 listed |
| Warranty 15% | 100 10-year warranty | 0 1-year warranty |
Specifications
| Fortress Power Envy Duo 21 | SunGoldPower FR12K-B | |
|---|---|---|
| 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% | 92% |
| CEC efficiency | 96.9% | - |
| Idle draw | 60 W | - |
| Transfer time | 20 ms | 10 ms |
| Parallel units | Up to 10 units | Up to 6 units |
| Solar | ||
| Max PV power | 25,000 W | 13,200 W |
| Max PV voltage (Voc) | 550 V | 500 V |
| MPPT range | 120 to 440 V | 125 to 425 V |
| MPPT trackers | 3 | 2 |
| PV current per tracker | 26 A | 22 A |
| Runs without battery | Yes | Yes |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | Lithium | Lithium, lead-acid, user-defined |
| Max charge current | 250 A | 200 A |
| AC charge current | - | 120 A |
| Battery communication | Yes | Yes |
| Grid sell-back | Yes | No |
| Generator input | Yes | Yes |
| UL 1741 | UL 1741 SB | No |
| Build | ||
| App monitoring | Included | Optional add-on |
| Enclosure | IP66 (NEMA 4X) | IP20 |
| Dimensions | 22.3 x 35.2 x 11.2 in | 28.7 x 19.1 x 6.7 in |
| Weight | 125.7 lb | 61.7 lb |
| Warranty | 10 years | 1 year |
| Released | 2026-01 | 2026-01 |