Product Description

This kit pairs BigBattery's stackable ETHOS 48V LiFePO4 battery platform with the EG4 FlexBOSS21 hybrid inverter. Per EG4, the FlexBOSS21 offers 21 kW of usable PV input across three MPPTs, delivers 12 kW of AC power from battery during non-sunlight conditions and up to the full 16 kW of AC output with solar PV contribution, and is UL 3141 / UL 1741 listed. Per BigBattery's kit page, the base configuration pairs one FlexBOSS21 with two ETHOS modules (10.24 kWh), and this listing supports storage up to 30.72 kWh with closed-loop communication between battery and inverter.

Who This System Is For

Larger homes and heavier simultaneous loads: central air, well pump, range, and shop tools at the same time. The FlexBOSS21 is the output and PV-capacity step up from the FlexBOSS18, and per EG4 it accepts up to 5 strings at up to 3 orientations — useful for complex roofs. The stackable ETHOS tower lets you start at 10.24 kWh and grow in 5.12 kWh steps as your storage needs increase.

Key Features

  • 21 kW of usable PV (DC) input, per EG4, across three MPPTs (26A/26A/15A) supporting up to 5 strings and up to 3 orientations
  • 12 kW AC from battery at night; up to 16 kW AC with solar PV contribution, per EG4 — split-phase 120/240V
  • 90A utility feed-through capability, per EG4
  • UL 3141 and UL 1741 listed inverter, per EG4; ETHOS battery carries UL9540, UL9540A, UL1973, and UN38.3, per BigBattery
  • Closed-loop communication between ETHOS and the inverter, per BigBattery
  • GridBOSS-ready: per EG4, pairing with GridBOSS adds AC coupling, smart loads, and generator support
  • Expandable: per BigBattery's kit page, a single FlexBOSS21 supports up to nine ETHOS modules (46.1 kWh) — this listing covers up to 30.72 kWh; call for larger configurations

Why BigBattery

BigBattery is a US-based energy storage company that has been building lithium battery systems since 2011 and, per BigBattery, is trusted by more than 50,000 Americans. BigBattery states its products are designed and engineered in Texas, includes free lifetime tech support with every product, and backs its batteries with a 12-Year Limited Warranty — described by BigBattery as "the longest warranty in the battery industry." OffGrid Horizon is a family-owned authorized BigBattery partner.

What's In the Kit

  • 1 × EG4 FlexBOSS21 hybrid solar inverter
  • 2 × BigBattery ETHOS 48V 5.12 kWh LiFePO4 modules — 10.24 kWh base configuration, per BigBattery's kit page for this SKU family

Storage is expandable to 30.72 kWh within this listing (and to 46.1 kWh on one inverter, per BigBattery) — call (302) 496-3554 to confirm your configuration before ordering. BigBattery lists ETHOS inverter connectors and battery-to-battery connectors as separate accessories, so confirm cabling with your order. Solar panels, racking, and array wiring are sold separately. For whole-home management, ask about the EG4 GridBOSS MID.

Specifications

Battery BigBattery ETHOS, stackable LiFePO4 — see the ETHOS battery system
Module capacity 5.12 kWh / 100Ah per module, 51.2V nominal
Base capacity (this listing) 10.24 kWh (2 modules); expandable to 30.72 kWh
Max continuous discharge 200A (2-module stack) / 250A (3+ modules), per BigBattery
Battery communications CAN bus / RS485; closed-loop with EG4, per BigBattery
Battery certifications UL9540, UL9540A, UL1973, UN38.3, per BigBattery
Cycle life 3,500–5,000 cycles (80% DoD, 0.5C), per BigBattery's spec sheet
Battery warranty BigBattery 12-Year Limited Warranty
Inverter EG4 FlexBOSS21 hybrid (per EG4)
PV input 21 kW usable; 3 MPPTs (26A/26A/15A); up to 5 strings, 3 orientations
AC output 12 kW from battery; up to 16 kW with PV contribution; split-phase 120/240V
Grid feed-through 90A utility feed-through, per EG4
Operating modes Grid-tied, hybrid, off-grid (grid export subject to utility approval)
Inverter certifications UL 3141, UL 1741, per EG4
Inverter warranty 10-year, per EG4 and BigBattery (registration required — see below)

Grid-Tied, Off-Grid, or Backup?

Per BigBattery, ETHOS + FlexBOSS21 configurations support grid-tied, hybrid, and off-grid installations, and grid-tied ETHOS/FlexBOSS systems are UL9540 configurations. Grid export requires an approved interconnection agreement with your utility. For backup, note the two output figures per EG4: up to 16 kW of AC with solar contribution during the day, and 12 kW from battery alone at night — whole-home backup is realistic for many homes with proper design, but coverage is load-dependent and should be confirmed against your panel schedule.

Installation

All AC and DC wiring — panel interconnection, grounding, bonding, and overcurrent protection — must be performed by a licensed electrician in accordance with the National Electrical Code (NEC) and local amendments. Residential energy storage installations typically require a permit and inspection, and grid-tied operation requires an approved interconnection agreement with your utility before the system may export power. Your installer should verify conductor sizing, disconnects, and clearances against current NEC requirements for energy storage systems.

Warranty & Support

Battery — BigBattery 12-Year Limited Warranty. Per BigBattery: "If any covered component within your battery fails due to a defect in materials or workmanship within the 12-year warranty period, we will repair or replace the battery at our cost." Included accessories carry a 1-year warranty per BigBattery, and BigBattery provides free lifetime tech support for all its products.

Inverter — 10-year warranty, per EG4 and BigBattery. Per EG4, inverter warranties must be registered within the first year of purchase (or proof of purchase from an EG4 authorized distributor provided) to remain valid.

Returns and warranty claims require an RMA. Contact us before shipping anything back — all returns must be authorized with a Return Merchandise Authorization first. OffGrid Horizon is a family-owned authorized BigBattery partner: call (302) 496-3554 and we will help open the claim and coordinate service.

Shipping

This kit ships as palletized LTL freight within the continental United States; curbside liftgate delivery is standard for residential addresses. The two-module ETHOS tower weighs 273 lb per BigBattery, plus the inverter pallet — plan for help on delivery day. Before signing the delivery receipt, count the pieces and inspect the packaging; note any damage or shortage on the receipt with the driver and contact us immediately so we can open a freight claim. Per BigBattery, standard domestic orders are projected to take 7–12 days to deliver.

Frequently Asked Questions

What exactly ships at the listed price?

The base configuration per BigBattery's kit page: one FlexBOSS21 inverter and two ETHOS modules (10.24 kWh). Call (302) 496-3554 to confirm included accessories or to configure a larger battery bank.

Why do I see both 12 kW and 16 kW output figures?

Per EG4: the FlexBOSS21 delivers 12 kW of AC power from battery during non-sunlight conditions and reaches the full 16 kW of AC output with solar PV contribution during the day.

How many solar strings can I connect?

Per EG4, three MPPTs configured 26A/26A/15A allow up to 5 strings, panels up to 13A, at up to 3 orientations — good flexibility for complex roofs.

Will this back up my whole home?

With 16 kW available during solar hours and 12 kW from battery, many homes can approach whole-home coverage — but it is load-dependent. GridBOSS adds smart-load and generator management to shed or shift heavy loads.

Can I add more battery later?

Yes. ETHOS stacks in 5.12 kWh steps; per BigBattery a single FlexBOSS21 supports up to nine modules (46.1 kWh), and multi-inverter configurations go larger still — call us for options.

Does it export to the grid?

Grid-tied operation with export is supported in UL9540 configurations per BigBattery, subject to your utility's interconnection approval.

Deciding between FlexBOSS18 and FlexBOSS21, or sizing a larger bank? Call OffGrid Horizon at (302) 496-3554 — family-owned, authorized BigBattery partner.

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