EG4 FlexBOSS21 and 18kPV Side by Side: Where the Spec Sheets Separate Them
Summary
Both are 48 V hybrid inverters with three MPPTs, a 250 A battery connection and a 10-year warranty, and EG4 recommends the same 21 kW of solar on each. The FlexBOSS21 delivers 16 kW continuous with solar and grid present and 12 kW from the battery alone; the 18kPV is rated 12,000 W at 240 V. Peak ratings are where the sheets differ most: the FlexBOSS21 lists 24 kW for half a second down to 13.2 kW for 12 minutes and 195 A locked-rotor; the 18kPV lists 13,500 W for 10 minutes and 180 A. The FlexBOSS21 also asks for twice the battery, 600 Ah against 300 Ah.
The two sheets
| FlexBOSS21 (sheet v1.3.0) | 18kPV (sheet v1.4.3) | |
|---|---|---|
| Recommended max solar | 21 kW | 21,000 W |
| Continuous AC output | 16 kW (66.7 A) with PV and grid; 12 kW (50 A) battery only; 6 kW per leg | 12,000 W at 240 V; 10,400 W at 208 V; 8,000 W per 120 V line |
| Peak | 24,000 W (0.5 s), 18,000 W (1 s), 15,000 W (6 min), 13,200 W (12 min) | 13,500 W (10 min), without PV |
| Locked-rotor amps | 195 A | 180 A |
| MPPTs | 3; 26 A, 26 A, 15 A; inputs 2, 2, 1 | 3; 25 A, 15 A, 15 A; inputs 2, 1, 1 |
| MPPT voltage | 120 to 440 V operating; 250 to 440 V full power; 200 V start; 550 V protection | 140 to 500 V operating; 360 V nominal; 550 V protection |
| Battery current | 250 A charge, 250 A discharge | 250 A charge/discharge |
| Recommended battery | at least 600 Ah per inverter | at least 300 Ah per inverter |
| Grid pass-through / generator | 90 A pass-through from grid | 90 A AC bypass (generator) |
| Efficiency | 97% CEC weighted; 97% PV to grid or load | 97.5% PV to grid; 94% battery to grid; 99.9% PV to battery |
| Paralleling | up to 16 units | not captured from the sheet we read |
| Weight | 121 lb (55 kg) | not captured |
| Warranty | 10 years | 10-year standard |
Both sheets warn that exceeding 600 V DC on the PV input voids the warranty and that string sizing should follow the nominal MPPT voltage rather than the window's edges.
What 600 Ah means
At 51.2 V, 600 Ah is about 30.7 kWh of battery; 300 Ah is about 15.4 kWh. The FlexBOSS21's 12 kW battery-only output at 250 A works out to the same current limit as the 18kPV's, so the larger recommendation is about sustaining that output and absorbing the peaks, not about a different connector. Signature Solar's bundles reflect it: its FlexBOSS21 bundles list 14.3 to 28.6 kWh of WallMount battery.
Peaks are measured on different clocks
A 13,500 W figure for 10 minutes and a 13,200 W figure for 12 minutes are close; the FlexBOSS21's headline 24,000 W lasts half a second. The number that matters for a motor is the locked-rotor rating, 195 A against 180 A, both of which exceed a Powerwall 3's 185 LRA class by only a little or not at all; our guide on starting an air conditioner from a battery explains how to read it.
The name is not the solar limit
"18kPV" implies 18 kW of PV; the sheet we read recommends up to 21,000 W, the same as the FlexBOSS21. Our earlier guide on Signature Solar's 18kPV kit found the kit itself shipped with about 8 kW of panels, so the inverter's ceiling is rarely the binding constraint.
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Sources
- EG4 Electronics — FlexBOSS21 Spec Sheet (VERSION 1.3.0)
- EG4 Electronics — 18kPV Spec Sheet (VERSION 1.4.3)
- Signature Solar — Home Backup Kits
We have not tested these products. Specs, prices, stock and terms are as published on these pages on the date above and can change; check the manufacturer or retailer before you buy.