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
4.3/5
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.
- 12 kWOutput
- 48 VBattery
- 25,000 WSolar input
- 60 WIdle draw
- 10 yearsWarranty
Check price at Wind & Sun$4,898
Price checked October 23, 2026. As an Amazon Associate we earn from qualifying purchases.
Pros
- Takes up to 25,000 W of panels, more than most single hybrids
- 12 kW of 120/240 V backup with export under UL 1741 SB
- Lower 60 W idle draw than the Envy True 12
- IP66 outdoor case and a ten-year warranty
Cons
- Lithium batteries only, no lead-acid
- Weighs about 126 lb, a two-person mount
How it scores
The rating comes from 5 measured or published factors, each scored 0-100 against fixed reference points for its output. How we rank explains each one.
Specifications
| Model number | FP-DUO-21 |
|---|---|
| Output | |
| Output voltage | 120/240 V split-phase |
| Continuous output | 12,000 W |
| Surge | 24,000 W |
| Peak efficiency | 94% |
| CEC efficiency | 96.9% |
| Idle draw | 60 W |
| Transfer time | 20 ms |
| Parallel units | Up to 10 units |
| Solar | |
| Max PV power | 25,000 W |
| Max PV voltage (Voc) | 550 V |
| MPPT range | 120 to 440 V |
| MPPT trackers | 3 |
| PV current per tracker | 26 A |
| Runs without battery | Yes |
| Battery and grid | |
| Battery voltage | 48 V |
| Battery types | Lithium |
| Max charge current | 250 A |
| Battery communication | Yes |
| Grid sell-back | Yes |
| Generator input | Yes |
| UL 1741 | UL 1741 SB |
| Build | |
| App monitoring | Included |
| Enclosure | IP66 (NEMA 4X) |
| Dimensions | 22.3 x 35.2 x 11.2 in |
| Weight | 125.7 lb |
| Warranty | 10 years |
| Released | 2026-01 |
Alternatives in this output
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
EG4 18kPV
For grid-tied homes that want whole-house backup and grid sell from one proven inverter. Compare against the FlexBOSS21 first, which costs less for more power.
Signature Solar $3,999.99 Shop Solar Kits $3,999.99$3,999.99
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