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 4.2. The EG4 FlexBOSS21 leads on warranty, solar input and features. The Growatt SPE 12000 US wins on value. The Growatt SPE 12000 US costs $1,830.99 less ($1,769 vs $3,599.99).
Growatt SPE 12000 US
For off-grid homes that need whole-house 240 V power and a big solar array at a very low price per kW.
Self2Solar $1,769$1,769
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 warranty (10-year warranty vs 5-year warranty), solar input (21,000 W of panels, 600 V max, 3 MPPTs vs 18,000 W of panels, 550 V max, 2 MPPTs) and features (UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, battery communication vs UL 1741 listed, Wi-Fi add-on, parallel stacking, generator input, battery communication).
- Choose the Growatt SPE 12000 US for value ($147 per kW of continuous output vs $300 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 | EG4 FlexBOSS21 | Growatt SPE 12000 US |
|---|---|---|
| Overall rating | 4.4/5 | 4.2/5 |
| Value 30% | 80 $300 per kW of continuous output | 100 $147 per kW of continuous output |
| Solar input 20% | 88 21,000 W of panels, 600 V max, 3 MPPTs | 68 18,000 W of panels, 550 V max, 2 MPPTs |
| Efficiency 20% | 65 94% peak efficiency, 65 W idle | 67 95% peak efficiency, 75 W idle |
| Features 15% | 95 UL 1741 listed, Wi-Fi app, parallel stacking, grid sell-back, battery communication | 82 UL 1741 listed, Wi-Fi add-on, parallel stacking, generator input, battery communication |
| Warranty 15% | 100 10-year warranty | 44 5-year warranty |
Specifications
| EG4 FlexBOSS21 | Growatt SPE 12000 US | |
|---|---|---|
| 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 | - |
| Peak efficiency | 94% | 95% |
| CEC efficiency | 97% | 94.5% |
| Idle draw | 65 W | 75 W |
| Transfer time | 20 ms | 20 ms |
| Parallel units | Up to 16 units | Up to 9 units |
| Solar | ||
| Max PV power | 21,000 W | 18,000 W |
| Max PV voltage (Voc) | 600 V | 550 V |
| MPPT range | 120 to 440 V | 60 to 480 V |
| MPPT trackers | 3 | 2 |
| PV current per tracker | 26 A | 27 A |
| Runs without battery | - | Yes |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | - | Lithium, lead-acid |
| Max charge current | 250 A | 250 A |
| AC charge current | - | 250 A |
| Battery communication | Yes | Yes |
| Grid sell-back | Yes | - |
| Generator input | - | Yes |
| UL 1741 | UL 1741 SB | Yes |
| Build | ||
| App monitoring | Included | Optional add-on |
| Enclosure | NEMA 4X | IP20 |
| Dimensions | 30.43 x 22.28 x 11.22 in | 550 x 465 x 150 mm |
| Weight | 121 lb | 47.4 lb |
| Warranty | 10 years | 5 years |
| Released | 2024-11 | 2025-09 |