Product Description

The EG4 FlexBOSS21 is the higher-output model in EG4's FlexBOSS hybrid inverter line. Per EG4, it utilizes up to 21,000W of solar across three MPPTs, delivers 16,000W of continuous split-phase 120/240V output with PV available (12,000W from battery alone), and charges a 48V bank at up to 250A — with grid-tied, backup, and off-grid operation from a NEMA 4X outdoor-rated enclosure. It is designed to pair with the EG4 GridBOSS MID for 200A whole-home backup.

Who This Is For

Larger homes and properties that run heavy simultaneous loads — central air, well pump, range, EV charging — or arrays spanning multiple orientations that need more MPPT capacity and string inputs. If your loads and array are more moderate, the FlexBOSS18 uses the same platform, batteries, and GridBOSS integration.

Key Features

  • 16,000W continuous split-phase output with PV (12,000W battery-only), with surge to 24,000W for 0.5 s per EG4's spec sheet
  • Up to 21,000W of utilized PV — per EG4's spec sheet, three MPPTs with seven string inputs (3/2/2), rated 26A / 26A / 15A, 600 VDC max input
  • 48V battery charging/discharging up to 250A with closed-loop communications for EG4 batteries
  • Grid-interactive certifications per EG4: UL 1741 SA/SB with PCS CRD (EG4's product page also lists UL 3141), IEEE 1547, California Rule 21, Hawaii Rule 14H
  • GridBOSS-ready: connects to the EG4 GridBOSS MID for whole-home backup, smart loads, AC coupling, and generator management
  • NEMA 4X enclosure, -13 to 140 °F operating range, EMP-hardened per EG4
  • EG4 Connect monitoring via Wi-Fi (dongle included per EG4), Ethernet, or cellular

Why EG4

EG4 Electronics is a US-based manufacturer with published spec sheets, manuals, and wiring diagrams for every product and US-based technical support. The FlexBOSS platform anchors EG4's current residential ecosystem: FlexBOSS inverters, WallMount batteries, and the GridBOSS MID are engineered and documented to work as one system with closed-loop communications.

Specifications

Specifications below are per EG4's published FlexBOSS21 spec sheet.

Continuous AC output 16,000W (66.7A) with PV and battery; 12,000W (50A) battery-only
Surge capacity 24,000W (0.5 s); 18,000W (1 s); 15,000W (6 min)
Output voltage Split-phase 120/240VAC or 120/208VAC (L1/L2/N)
PV input 21,000W max utilized; 3 MPPTs, 7 string inputs (3/2/2)
MPPT voltage range 250–440 VDC full power (120–440 VDC operating; startup 200 VDC; max input 600 VDC)
PV current per MPPT 26A / 26A / 15A rated; 31A / 31A / 19A Isc
Battery voltage 51.2V nominal (40–60 VDC range)
Max charge / discharge current 250A / 250A
AC input Grid input 50A / 12,000W; 90A pass-through
Transfer time 20 ms default (10 ms configurable)
Efficiency 97% CEC; 97% peak PV-to-grid; 94% battery-to-load
Idle consumption <65W @ 77 °F
Parallel capability Up to 16 units
Certifications UL 1741 SA/SB with PCS CRD; California Rule 21 Phase I–III; Hawaii Rule 14H; CSA C22.2 107.1 & 330; IEEE 1547:2018 / 1547.1:2020 (UL 3141 also listed on EG4's product page)
Enclosure / environment NEMA 4X; -13 to 140 °F
Dimensions / weight 30.43 x 22.28 x 11.22 in; 121 lb
Communications / monitoring Wi-Fi (dongle included), Ethernet, cellular; EG4 Connect app
Warranty 10 years per EG4 (registration required within first year)
SKU INV027

Grid-Tied, Backup, or Off-Grid

Grid-tied: solar carries the house and charges batteries, and surplus can be exported where your utility approves the interconnection — export capability and compensation vary by state and utility. Backup: on grid failure the FlexBOSS21 transfers to battery in 20 ms per EG4 (10 ms configurable); with a GridBOSS at the service entrance, backup covers the full 200A service with automatic smart-load management. Off-grid: the same hardware runs without any utility connection. Operating mode and utility-required settings are configured at commissioning.

Battery Compatibility

The FlexBOSS21 manages a 48V-class bank with closed-loop communications to EG4 WallMount batteries. With 250A of charge/discharge current, plan a substantial bank: the EG4 WallMount 314Ah indoor battery (16 kWh, UL9540A-tested, self-heating) stacks in 16 kWh increments toward the 30+ kWh most 16 kW systems carry.

Installation

Licensed-electrician work: the FlexBOSS21 interconnects with your main panel and, in grid-tied modes, with the utility — an interconnection application is required before export. Residential energy storage is subject to local permitting and inspection. The NEMA 4X enclosure permits outdoor mounting; plan for 121 lb and EG4's published clearances. For whole-home backup at the service entrance, install it with the EG4 GridBOSS 200A MID.

Warranty & Support

Per EG4, the FlexBOSS21 carries a 10-year warranty; registration within the first year (or proof of purchase from an authorized distributor) is required. OffGrid Horizon is a family-owned authorized EG4 dealer — call (302) 496-3554 for sizing, commissioning, or warranty help.

Shipping

Ships within the contiguous United States. At 121 lb, the FlexBOSS21 ships LTL freight on a pallet. Inspect packaging before signing the delivery receipt and note any damage with the driver; call us right away if anything looks wrong.

Frequently Asked Questions

FlexBOSS21 or FlexBOSS18?

Per EG4, the FlexBOSS21 delivers 16 kW with PV (12 kW battery-only), 21 kW of PV, and three MPPTs with seven string inputs; the FlexBOSS18 delivers 13 kW with PV and 18 kW of PV on two MPPTs. Choose the 21 for heavier simultaneous loads or a larger, multi-orientation array.

Why is output higher with PV than on battery alone?

Per EG4's spec sheet, the FlexBOSS21 produces 16 kW when solar is contributing and 12 kW from battery alone — size your always-available backup power around the 12 kW battery-only figure.

Can it sell power back to the grid?

It carries EG4's grid-interactive certifications (UL 1741 SA/SB), but actual export depends on your utility approving your interconnection application. Rules and compensation vary by state and utility.

Do I need a GridBOSS?

Not strictly — the FlexBOSS21 can back up a critical-loads panel on its own. The GridBOSS adds 200A whole-home backup switching, four smart-load ports, and a 125A generator input per EG4.

How many strings can I land?

Per EG4's spec sheet: seven string inputs across three MPPTs (3/2/2), 600 VDC max input, MPPT ratings of 26A / 26A / 15A.

Can I parallel multiple units?

Yes — per EG4, up to 16 units.

Is it OK outdoors?

Yes — NEMA 4X enclosure with a -13 to 140 °F operating range per EG4.

Speccing a whole-home system? Call OffGrid Horizon at (302) 496-3554 — family-owned, authorized EG4 dealer — and we will help you size the FlexBOSS21, GridBOSS, and battery bank together.

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