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 4.0. The Fortress Power Envy Duo 21 leads on solar input. The SunGoldPower SGS-12K18MAX wins on value. The SunGoldPower SGS-12K18MAX costs $1,908 less ($2,990 vs $4,898).
SunGoldPower SGS-12K18MAX
A grid-interactive 12 kW whole-home hybrid with UL 1741 SB listing, for homeowners who want to sell solar and back up the house.
SunGoldPower $2,990$2,990
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 18,000 W of panels, 550 V max, 2 MPPTs).
- Choose the SunGoldPower SGS-12K18MAX for value ($249 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 SGS-12K18MAX |
|---|---|---|
| Overall rating | 4.3/5 | 4.0/5 |
| Value 30% | 58 $408 per kW of continuous output | 90 $249 per kW of continuous output |
| Solar input 20% | 100 25,000 W of panels, 550 V max, 3 MPPTs | 68 18,000 W of panels, 550 V max, 2 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 48 100 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 | 50 warranty not published (neutral score used) |
Specifications
| Fortress Power Envy Duo 21 | SunGoldPower SGS-12K18MAX | |
|---|---|---|
| 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 | 18,000 W |
| Peak efficiency | 94% | - |
| Max efficiency | - | 97.5% |
| CEC efficiency | 96.9% | 96.5% |
| Idle draw | 60 W | 100 W |
| Transfer time | 20 ms | - |
| Parallel units | Up to 10 units | Up to 6 units |
| Solar | ||
| Max PV power | 25,000 W | 18,000 W |
| Max PV voltage (Voc) | 550 V | 550 V |
| MPPT range | 120 to 440 V | 125 to 450 V |
| MPPT trackers | 3 | 2 |
| PV current per tracker | 26 A | 32 A |
| Runs without battery | Yes | - |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | Lithium | Lithium, lead-acid, user-defined |
| Max charge current | 250 A | 240 A |
| AC charge current | - | 200 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 |
| Dimensions | 22.3 x 35.2 x 11.2 in | 840 x 440 x 260 mm |
| Weight | 125.7 lb | 105.8 lb |
| Warranty | 10 years | - |
| Released | 2026-01 | 2025-09 |