EG4 FlexBOSS18
For grid-tied homes that want net metering and whole-home backup at a lower price than the FlexBOSS21. Choose the 21 if you need more battery-only output or a third tracker.
Signature Solar $3,199.99$3,199.99
The EG4 FlexBOSS18 is the better hybrid solar inverter overall, rated 4.4 to 4.1. The EG4 FlexBOSS18 leads on value and solar input. The EG4 FlexBOSS18 costs $699.01 less ($3,199.99 vs $3,899).
Fortress Power Envy 10K
For grid-tied homes that want an outdoor hybrid with export, backup and US-based support at a lower price than the Envy True 12.
Wind & Sun $3,899$3,899
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 FlexBOSS18 for value ($320 per kW of continuous output vs $390 per kW of continuous output) and solar input (18,000 W of panels, 600 V max, 2 MPPTs vs 15,000 W of panels, 600 V max, 2 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 FlexBOSS18 | Fortress Power Envy 10K |
|---|---|---|
| Overall rating | 4.4/5 | 4.1/5 |
| Value 30% | 76 $320 per kW of continuous output | 62 $390 per kW of continuous output |
| Solar input 20% | 82 18,000 W of panels, 600 V max, 2 MPPTs | 68 15,000 W of panels, 600 V max, 2 MPPTs |
| Efficiency 20% | 68 94% peak efficiency, 60 W idle | 63 94% peak efficiency, 70 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 FlexBOSS18 | Fortress Power Envy 10K | |
|---|---|---|
| Output | ||
| Output voltage | 120/240 V split-phase | 120/240 V split-phase |
| Continuous output | 10,000 W | 10,000 W |
| Surge | 20,000 W | 20,000 W |
| Peak efficiency | 94% | 94% |
| CEC efficiency | 97% | 96.5% |
| Idle draw | 60 W | 70 W |
| Transfer time | 20 ms | 20 ms |
| Parallel units | Up to 16 units | Up to 10 units |
| Solar | ||
| Max PV power | 18,000 W | 15,000 W |
| Max PV voltage (Voc) | 600 V | 600 V |
| MPPT range | 120 to 440 V | 210 to 500 V |
| MPPT trackers | 2 | 2 |
| PV current per tracker | 26 A | 25 A |
| Runs without battery | - | Yes |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | Lithium, lead-acid | Lithium, lead-acid |
| Max charge current | 208 A | 210 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 | IP65 (NEMA 4X) |
| Dimensions | 30.43 x 22.28 x 11.22 in | 29.5 x 20.5 x 11.23 in |
| Weight | 115 lb | 110 lb |
| Warranty | 10 years | 10 years |
| Released | 2025-03 | 2023-06 |