EG4 FlexBOSS18 vs FlexBOSS21
The two FlexBOSS models share a frame and a lot of features. The maker presents the FlexBOSS18 as a cost-effective way into hybrid solar and the…
EG4 FlexBOSS18Hybrid inverter10 kW (41.67 A), battery only output51.2 VDC battery18 kW solar input1 datasheet edges
EG4 FlexBOSS21Hybrid inverter12 kW (50 A), battery only output51.2 VDC battery21 kW solar input4 datasheet edgesDatasheet scoreboard: 8 lines where both datasheets list a number. It counts who has the bigger (or, for idle draw, smaller) figure; it is not a quality rating and says nothing about reliability.
The short version
Same frame: the 21 adds a third MPPT, more output and a 600 Ah battery minimum.
- FlexBOSS18: Your loads fit within 10 kW on battery and 13 kW with solar and grid.
- FlexBOSS21: You need 12 kW on battery and 16 kW with PV and grid.
The two FlexBOSS models share a frame and a lot of features. The maker presents the FlexBOSS18 as a cost-effective way into hybrid solar and the FlexBOSS21 as the larger, higher-output version. The differences are in power, MPPT count and how big a battery each expects.
EG4 FlexBOSS18 vs FlexBOSS21: specs head to head
| Spec | EG4 FlexBOSS18 | EG4 FlexBOSS21 |
|---|---|---|
| Model number | IV-13000-HYB-AW-FX-XX | IV-16000-HYB-AW-FX-XX |
| Continuous output | 10 kW (41.67 A), battery only | 12 kW (50 A), battery only |
| Continuous output with PV and grid (at 25 °C) | 13 kW (41.67 A) | 16 kW (66.7 A) |
| Peak power | 0.5 s: 20,000 W · 1 s: 15,000 W · 1 min: 13,500 W · 6 min: 12,500 W · 12 min: 11,000 W | 0.5 s: 24,000 W · 1 s: 18,000 W · 6 min: 15,000 W · 12 min: 13,200 W |
| Battery voltage (nominal) | 51.2 VDC | 51.2 VDC |
| Max battery charge current | 208 A | 250 A |
| Max DC input voltage | 600 VDC | 600 VDC |
| MPPT voltage high protection (inverter faults here; plan strings below it) | 550 VDC | 550 VDC |
| MPPT full-power voltage range | 250 – 440 VDC (360 VDC optimal) | 250 – 440 VDC (operating range 120 – 440 VDC) |
| Number of MPPTs | 2 | 3 |
| Rated PV current per MPPT | 26 A (MPPT 1) | 26 A (MPPT 2) | 26 A (MPPT 1) | 26 A (MPPT 2) | 15 A (MPPT 3) |
| Recommended max solar array power (STC) | 18 kW | 21 kW |
| Idle consumption (standby) | <60 W @ 25 °C | <65 W @ 25 °C |
| Efficiency, battery to grid/load (max) | 94% | 94% |
| Max inverters in parallel | 16 | 16 |
| Warranty (as listed) | 10 years | 10 years |
| Enclosure rating | NEMA 4X | NEMA 4X |
| Weight | 115 lbs (52 kg) | 121 lbs (55 kg) |
| Dimensions (H×W×D) | 30.43×22.28×11.22 in (773×566×285 mm) | 30.43×22.28×11.22 in (773×566×285 mm) |
| Recommended minimum battery capacity per inverter | not on datasheet | 600 Ah |
Highlighted = the better figure where both datasheets give a number (higher is better, except idle draw). More is not always better: a larger solar input, for example, needs a bigger battery and wiring to use it. Sources: EG4 FlexBOSS18 datasheet · EG4 FlexBOSS21 datasheet. Checked September 30, 2026. “Not on datasheet” means the sheet does not list that line, not that the product lacks it.
What the FlexBOSS18 and FlexBOSS21 numbers mean
- Output on battery alone: 10 kW (41.67 A) for the FlexBOSS18, 12 kW (50 A) for the FlexBOSS21. With PV and grid at 25 °C: 13 kW against 16 kW.
- MPPTs and solar: two MPPTs and up to 18 kW recommended on the FlexBOSS18; three MPPTs and up to 21 kW on the FlexBOSS21.
- Peak power: 20,000 W for half a second on the FlexBOSS18, 24,000 W on the FlexBOSS21.
- Battery: charge current limit of 208 A against 250 A. The FlexBOSS21 datasheet recommends a minimum of 600 Ah per inverter; the FlexBOSS18 sheet gives no minimum.
- Shared: 51.2 V battery, NEMA 4X, 10-year warranty, up to 16 units in parallel, UL 3141 and UL 1741.
FlexBOSS18 or FlexBOSS21: who should pick which
Pick the FlexBOSS18 if…
- Your loads fit within 10 kW on battery and 13 kW with solar and grid.
- You want the lower entry point into hybrid, as the maker positions it.
- Your array tops out around 18 kW.
Pick the FlexBOSS21 if…
- You need 12 kW on battery and 16 kW with PV and grid.
- You want three MPPTs and room for up to 21 kW of solar.
- You are building a big bank (600 Ah or more per inverter) and may add a GridBOSS.
EG4 FlexBOSS18 vs FlexBOSS21: questions people ask
Do both FlexBOSS models carry the same warranty?
Yes, both datasheets list 10 years.
Which has more MPPTs?
The FlexBOSS21 has three; the FlexBOSS18 has two.
Specs change with new revisions; check the current datasheets before you buy. Checked 2026-09-30. Next: size a system or compare any two models.