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 4.0. The EG4 FlexBOSS21 leads on solar input. The SunGoldPower SGS-12K18MAX wins on value. The SunGoldPower SGS-12K18MAX costs $609.99 less ($2,990 vs $3,599.99).
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 EG4 FlexBOSS21 for solar input (21,000 W of panels, 600 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 $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 | SunGoldPower SGS-12K18MAX |
|---|---|---|
| Overall rating | 4.4/5 | 4.0/5 |
| Value 30% | 80 $300 per kW of continuous output | 90 $249 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 | 48 100 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 | 50 warranty not published (neutral score used) |
Specifications
| EG4 FlexBOSS21 | 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 | 97% | 96.5% |
| Idle draw | 65 W | 100 W |
| Transfer time | 20 ms | - |
| Parallel units | Up to 16 units | Up to 6 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 | 125 to 450 V |
| MPPT trackers | 3 | 2 |
| PV current per tracker | 26 A | 32 A |
| Battery and grid | ||
| Battery voltage | 48 V | 48 V |
| Battery types | - | 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 |
| UL 1741 | UL 1741 SB | UL 1741 SB |
| Build | ||
| App monitoring | Included | Included |
| Enclosure | NEMA 4X | IP65 |
| Dimensions | 30.43 x 22.28 x 11.22 in | 840 x 440 x 260 mm |
| Weight | 121 lb | 105.8 lb |
| Warranty | 10 years | - |
| Released | 2024-11 | 2025-09 |