EG4 FlexBoss21

FlexBoss21 is the 18KPV story with more PV headroom and integrated load management. Everything about battery pairing carries across from the 18KPV, including the 250 amp charge figure that quietly demands a serious bank. It is the wrong inverter for a two-battery starter system and the right one for a build you intend to grow into.

Published figures

Continuous output

12,000 W

Surge output

18,000 W

Battery voltage window

40 to 60 V

Max charge current

250 A

Battery comms

CAN, RS485

Output

120/240 V split phase

Grid interactive

Yes

PV input

21,000 W, 600 Voc max

What this inverter will and will not do

• It works alongside the utility, which means an interconnection agreement, a permit, and an inspection in most jurisdictions. Budget months, not weeks, for that process.
• Output is 120/240 V split phase from one box, so standard household 240 V loads run directly.
• Battery communication runs over CAN, RS485, and closed loop still depends on both sides agreeing on a message format rather than only on a bus standard.

Sizing a battery bank for it

This inverter delivers up to 250 amps of charge current, and that figure, not your daily energy use, sets the minimum pack count. Every battery below is shown at the smallest bank that can absorb the full charge rate.

Fuse and cable for the battery run

Running flat out this inverter pulls about 250 amps at 48 volts, and a surge takes it to roughly 375 amps. Size the protection for the peak, not the average: a 350 amp Class T fuse at the battery positive terminal and two parallel runs of 2/0 cable on the battery run. Class T rather than a standard fuse because a 48 volt lithium bank can deliver enough short circuit current to arc across a fuse that lacks the interrupt rating. Confirm the sizing against your installation manual, which accounts for run length and ambient temperature.

Communication

Manufacturer documentation names EG4 PowerPro WallMount 48V 280Ah, EG4 WallMount Indoor 48V 280Ah for closed-loop operation. Any battery not on that list runs open loop until a document says otherwise.

Where the figures come from

Manufacturer documents: EG4 product and document library. Figures here should be read against the revision that covers your serial number, because manufacturers revise specifications without changing the model name.

Questions people actually ask

Worth reading before you install

• How many batteries this inverter actually needs
• Split phase or single phase, and what it costs to get wrong
• Off grid or hybrid, and what each one will not do
• Cable sizing for the battery run