Product Description

This kit pairs the EG4 FlexBOSS21 hybrid inverter — up to 21 kW of usable PV input and 16 kW of AC output with solar contribution per EG4 — with the EG4 WallMount 314Ah Indoor battery, a self-heating 16 kWh unit built for garages, basements, and utility rooms. Per EG4, this battery-and-inverter pairing carries the UL9540 energy storage system designation. Configure one to three batteries for 16, 32, or 48 kWh. OffGrid Horizon is a family-owned authorized EG4 dealer, and every kit ships with full manufacturer warranty support.

Who This Is For

Larger homes with indoor battery placement: heavier simultaneous 240V loads (central air, well pump, range, shop equipment) or bigger solar arrays that would outgrow the FlexBOSS18, combined with garage or basement battery mounting where the Indoor model's lighter weight, self-heating, and published UL9540 pairing fit best. Per EG4, the FlexBOSS21 accepts up to 5 PV strings and passes through 90A of utility power at 120/240V.

Key Features

  • UL9540 system designation with the FlexBOSS21, plus UL1973 certification and UL9540A test data on the battery — exactly as EG4 publishes
  • Up to 21 kW of usable PV input across up to 5 strings per EG4
  • 16 kW AC output with solar contribution; 12 kW continuous from battery alone per EG4
  • Self-heating batteries — a 448W internal heater activates at or below 32°F per EG4, so winter does not stop charging in unheated spaces
  • 16 kWh per battery, 10,000-cycle rating at 80% depth of discharge per EG4
  • 90A grid feed-through at 120/240V per EG4 for whole-home pass-through wiring
  • Closed-loop communication over CAN/RS485 with app-based remote monitoring

Why EG4

EG4 Electronics is a Texas-based manufacturer that builds both halves of this system, publishes full spec sheets and fire-safety test data, and backs the inverter and batteries with 10-year warranties. Because one company engineers the pairing, the UL9540 system documentation covers the exact hardware in this kit.

What's In the Kit

  • 1 x EG4 FlexBOSS21 hybrid inverter — split-phase 120/240V, UL3141 and UL1741 certified per EG4
  • 1, 2, or 3 x EG4 WallMount 314Ah Indoor batteries (16 kWh each) — the listed configuration includes one battery; select your capacity from the product options

Solar panels, racking, and array wiring are site-specific and sold separately. The system also operates as battery backup without solar.

Specifications

Inverter EG4 FlexBOSS21 hybrid, split-phase 120/240V
PV input Up to 21 kW usable, up to 5 strings, per EG4
AC output 16 kW with solar contribution; 12 kW from battery alone, per EG4
Grid feed-through 90A at 120/240V, per EG4
Inverter certifications UL3141; UL1741, per EG4
Battery EG4 WallMount 314Ah Indoor, 51.2V, 16 kWh nominal per unit
Capacity options 16 / 32 / 48 kWh (1, 2, or 3 batteries)
Battery output 200A max continuous discharge per unit, per EG4
Battery enclosure IP20 — indoor installation
Battery certifications UL1973; UL9540A; UL9540 with FlexBOSS21, per EG4
Self-heating 448W heater, on at 32°F or below, per EG4
Battery cycle life 10,000 cycles at 80% DoD, per EG4
Battery dimensions / weight 34.1 x 18.1 x 9.6 in / 293.2 lb per unit
Communication Closed-loop CAN/RS485; app-based remote monitoring
Warranty Inverter 10 years; batteries 10 years or 10,000 cycles, per EG4
Base SKU K0210

Grid-Tied, Backup, or Off-Grid

Grid-tied: the FlexBOSS21 manages solar production, battery charging, and export under UL1741 interconnection rules — your utility and AHJ govern export permissions. Backup: when the grid drops, the system transfers to battery power; if you chose the 21 for its output headroom, plan storage to match — heavier loads draw a bank down faster, so 32 or 48 kWh suits whole-home coverage. Off-grid: the same hardware runs fully off-grid, sized by your daily load and sun hours. For whole-home microgrid control — AC coupling, smart load management, and generator integration per EG4 — add the EG4 GridBoss 200A MID.

Inverter Compatibility

The batteries in this kit run closed-loop with the included EG4 FlexBOSS21 hybrid inverter, so state-of-charge and charge parameters stay coordinated out of the box — and per EG4 the pairing carries the UL9540 system designation. Individual WallMount 314Ah Indoor batteries are available separately when you are ready to expand.

Installation

Installation must be performed by a licensed electrician — the inverter interconnects with your main panel and, in grid-tied modes, with the utility, and energy storage installations require local permits and inspection. Each battery weighs 293.2 lb and mounts to the wall or floor per EG4's manual; anchor wall-mounted units into structural framing and plan for lifting equipment. The batteries are IP20 rated and must be installed indoors; the NEMA 4X inverter can mount outside nearby if preferred. Follow EG4's installation manuals for clearances and torque values, and confirm indoor energy-storage placement rules with your building department.

Warranty & Support

Per EG4, the FlexBOSS21 carries a 10-year warranty and each WallMount Indoor battery is covered for 10 years or 10,000 cycles, whichever comes first. Register your equipment with EG4 after purchase and keep your OffGrid Horizon invoice — as an authorized dealer, our records document your coverage. For warranty or configuration help, call us at (302) 496-3554.

Shipping

Kits ship by LTL freight on pallets within the United States, curbside with a liftgate. Count every piece against the packing list and inspect for damage before signing the delivery receipt — note any issues with the driver to preserve your freight claim.

Frequently Asked Questions

How many batteries are included?

The base configuration includes one 16 kWh battery. Select the 2-battery (32 kWh) or 3-battery (48 kWh) option from the product configuration menu before checkout.

How much power do I get from batteries at night or in an outage?

Per EG4, 12 kW continuous from battery alone. One battery sustains about 10.2 kW of discharge (200A), so plan on two or more batteries to use the inverter's full battery-mode output.

What does the UL9540 system designation mean for permitting?

UL1973 covers the battery, UL9540A is a fire-safety test method, and UL9540 covers the complete energy storage system. Per EG4, this battery achieves UL9540 when paired with the FlexBOSS21 — documentation many building departments request for indoor installations.

FlexBOSS21 or FlexBOSS18?

Per EG4, the 21 offers more PV input (21 kW vs 18 kW), more strings (5 vs 4), and more output (16 kW with solar, 12 kW battery-only, vs 13 kW and 10 kW). Choose the 21 for larger homes, heavy 240V loads, or bigger arrays; the FlexBOSS18 Indoor kit covers moderate loads with the same batteries.

Can I add batteries later?

Yes — per EG4 the WallMount 314Ah Indoor parallels up to 32 units. Most owners start with one or two and expand.

Are solar panels included?

No. Panels, racking, and array wiring are sized to your site and sold separately. The kit operates as grid-charged battery backup without any solar attached.

Matching battery count to the FlexBOSS21's output for your home? Call OffGrid Horizon at (302) 496-3554 — our family-owned team will run your loads before you order.

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