125 kW / 261 kWh All-in-One C&I BESS: FX-ESS-125/261-YB

Published: October 30, 2026 | Author: Weltrus Energy Team

When the kW/kWh Pair Is Not Yet a Design

A commercial site often starts with two numbers: the inverter power the load can absorb, and the energy that should sit behind it. 125 kW and 261 kWh are those two numbers for this cabinet. They describe a nameplate. They do not yet describe the PV string plan, the generator interlock, the transfer time the process will accept, or the pad the crane has to land on.

FX-ESS-125/261-YB is the model printed on the Weltrus two-page PV-storage sheet for this pair. The product title on that sheet is a 125 kW / 261 kWh PV-storage all-in-one system. One enclosure is meant to hold the battery, the hybrid power stage, and the ports a C&I one-line usually draws as separate boxes. That is the procurement point of the model. A containerized multi-megawatt block answers a different site. A wall-mount residential hybrid answers a different site again.

The sheet has a cover page and a parameter page. The cover page states operating modes and a few performance lines in bullets. The parameter page states volts, amps, temperatures, and the cooling method in rows. This note follows the rows for any figure a buyer would stamp on a drawing. It records the cover bullets that the rows do not repeat, so a bid does not treat a slogan as a test result.

The cover diagram shows PV into the cabinet, a diesel generator into the cabinet, main loads on the cabinet output, and a grid connection. It also draws a second path with an external PV inverter. The cabinet therefore has its own DC input, and the diagram still allows an AC-coupled array. Write down which path the modules will use. A string count that assumes all 10 MPPTs are free is wrong if half the array already lands on a separate inverter.

Battery Stack Behind the 261 kWh Line

The battery block is LFP. The cell line prints 3.2 V and 314 Ah. The system configuration prints (1P52S) 5S. Fifty-two cells in series, times five series strings in series, is 260 cells in series. At the printed 3.2 V cell nominal, 260 times 3.2 V is 832 V, which is the nominal system voltage on the next row. The voltage window is 715 V to 936 V.

Rated system energy is 261 kWh. The arithmetic check is 314 Ah times 832 V, which is 261,248 Wh, and that rounds to the printed 261 kWh. Use the printed 261 kWh in the energy model. The cell-level multiplication is a consistency check, and it is not a second, larger battery.

Rated charge and discharge current is 157 A, and the row marks that current as 0.5C. Half of 314 Ah is 157 A, so the mark matches the cell size. At 832 V, 157 A is about 131 kW on the DC bus. The AC ports are rated 125 kW. The gap between those two figures is where conversion loss and the BMS limit live. The sheet does not print a round-trip efficiency, so leave that cell blank until the manual or a test report fills it.

Divide 261 kWh by 125 kW and the quotient is about 2.09 hours. That quotient is the nameplate duration if every rated kilowatt-hour were discharged at the full AC rating without a reserve. The sheet does not print a minimum state of charge, a usable-energy factor, or a warranty throughput. A two-hour backup story is a design assumption until those limits arrive. Peak shaving that only needs a short discharge can sit inside the nameplate. A process that must ride through a multi-hour outage needs more cabinets, a generator, or both. The generator port exists for that second case.

PV Input: 250 kW Into a 125 kW AC Cabinet

Maximum available PV input power is 250 kW. Maximum input voltage is 1000 V. Rated PV voltage is 600 V. Start-up voltage is 180 V. The MPPT window is 150 V to 950 V. Maximum input current is 10 times 42 A. Maximum input short-circuit current is 10 times 60 A. The tracker line is 10 MPPTs and 20 strings, so the even split is two strings on each tracker.

250 kW of PV against 125 kW of AC is the two-times oversizing the cover page names. The AC rating does not become 250 kW because the array is larger. Power above what the battery and the AC port can take is clipped. The extra PV is there to charge on a short day and to hold the AC port closer to its rating when irradiance is moderate. String voltage still has to stay inside 150 V to 950 V while the array is hot, and under the 1000 V ceiling when it is cold. Those two checks are a stringing exercise, and this page does not pick a module.

A 1000 V PV input is a different bus from the 832 V battery. Keep the DC disconnects, the fuses, and the labels on the correct bus. The short-circuit current of 60 A per tracker input is the figure for conductor and fuse sizing on that input, and the operating current of 42 A is the figure for continuous current. Confirm both against the module datasheet once the string count is real.

Grid, Generator, and Off-Grid Ports

The AC block is three different ports, and they do not share one current rating.

Grid input

Maximum grid input current is 250 A. Rated grid voltage is 3+N+PE, 380/400 V, 50/60 Hz. Maximum grid input power is 125 kW. The current ceiling and the power ceiling are both printed. A feeder sized only to 125 kW at 400 V will sit near 180 A before the 250 A row is considered. Ask which device, the cabinet breaker or the upstream breaker, is meant to see 250 A.

Generator input

The generator input is its own block. The rows in that block are rated input current 189.9 A / 180.4 A, rated voltage 3/N/PE at 220/380 V and 3/N/PE at 230/400 V, and rated frequency 50/60 Hz. That block does not print a kilowatt line. The two currents are what 125 kW draws at 380 V and at 400 V with unity power factor, so they are one power class at two voltages. A diesel set for this port is a 125 kW class machine at the site voltage, plus the step-load margin the manufacturer requires. This sheet does not print a minimum generator size or a start ramp.

Grid output and off-grid output

The grid-output block opens with rated output power of 125 kW, then rated current of 180 A. Rated voltage repeats the 220/380 V and 230/400 V pairs. Rated frequency is 50/60 Hz. Power factor is printed as greater than 0.99, with a range of 0.8 leading to 0.8 lagging. Total current harmonic distortion is below 3 percent.

The off-grid block has its own rated output power of 125 kW. The next row is maximum output apparent power: 1.4 times rated for 10 seconds, and 1.6 times rated for 200 milliseconds. Voltage, frequency, and total voltage harmonic distortion below 3 percent follow. Keep that overload line on the off-grid row. Motor starts and welders belong in a load list checked against those two durations.

The cover page says the cabinet supports on-grid, off-grid, and hybrid operation, and it names peak shaving, backup power, and PV plus battery plus diesel dispatch. The ports above are what make those sentences electrically possible. The economic result depends on the tariff, the load shape, and the control setpoint. The sheet does not print a savings figure.

Cooling, Fire, Size, and Environment

Cooling is printed as intelligent liquid cooling with a 50 percent water and 50 percent ethylene glycol solution. That is the thermal design to specify. An air-cooled cabinet with the same kilowatt-hour label is a different product. The glycol mix is the fluid the maintenance plan has to stock, and the sheet does not print a service interval or a leak-detection method. Ask for both with the O&M manual.

Fire suppression is printed as pack-level and cabinet-level aerosol plus water fire suppression. The cover page calls the same architecture a three-level scheme. The table is the parts list: aerosol at pack and cabinet, plus water. A site fire strategy still has to say where the water goes, what the aerosol does to occupied space, and which signal reaches the building alarm. Those three items are installation scope. They are not extra rows on this sheet.

Permissible ambient temperature is -25 degrees C to 55 degrees C, with derating above 45 degrees C. Permissible humidity is 0 to 95 percent relative humidity, non-condensing. Altitude is 2000 m, with derating above 2000 m. The sheet does not print the derating slope, in percent per degree or percent per meter. A site at 48 degrees C or at 2500 m needs that slope before the energy model keeps the full 125 kW.

Dimensions are 1485 mm by 1422 mm by 2343 mm, listed as width, depth, and height, including the inverter and the external cabinet accessories. The plan area of the first two figures is about 2.11 square meters. Weight is approximately 2810 kg on the same included basis. A quote that removes the side compartment, or that ships the inverter separately, is no longer this weight or this footprint. Enclosure protection is IP54. Noise is below 75 dB. The noise row does not print the distance or the operating point, so a boundary-noise study needs the test distance from the manual.

External communication is an RJ45 port. The protocol line prints Modbus TCP and MQTT. The cover page also names local, cloud, and app operation. The two protocols are what an integrator can address from this sheet. The app and the cloud host are a software scope. Ask which of the three terminals is included in the cabinet price.

The certification row prints IEC 62619:2022, EN 62477-1, EN 61000-6-2/-4, IEC 60730, UL 9540A:2025, and UN 38.3. UL 9540A on that row is the fire-propagation test method the sheet cites. It is not, by itself, a statement that a given authority having jurisdiction has accepted the installation. UN 38.3 is the transport line. The grid-connection row is longer. It includes G99, VDE-AR-N 4105, EN 50549-1, EN 50549-2, CEI 0-21, C10/11, NRS 097-2-1, IEC 62116, and IEC 61727, and it continues with other national documents. Copy the code that matches the point of interconnection from the PDF into the RFQ. A shortened list is not the approval.

What to Confirm Before the Bid

Four cover-page lines are commercially useful and electrically unconfirmed by the parameter table.

• Transfer time. The cover prints 20 ms seamless on/off-grid switching. The table has no transfer-time row, no test load, and no statement of whether the 20 ms figure is a break or a ride-through. A process load that drops out at 10 ms needs the test record, not the bullet.
• Parallel cabinets. The cover prints AC parallel operation up to 10 units. The table does not print a parallel-current rating, a communication wiring diagram, or a shared-generator rule. Ten times 125 kW is 1.25 MW only after that diagram exists. Ten times 261 kWh is the matching energy arithmetic, with the same caveat.
• Family span. The cover prints capacity coverage from 156 kWh to 5 MWh. That sentence describes a product line. This cabinet is the 261 kWh model. A 5 MWh site is a different equipment list.
• Enclosure rating. The cover bullet prints IP55. The parameter table prints IP54. Put IP54 on the drawing unless the nameplate and the revised sheet both print IP55.

Cycle life, calendar life, and warranty years are absent from both pages. An older brochure figure for this kilowatt-hour class does not fill the blank. Ask for the warranty document that names FX-ESS-125/261-YB, including throughput, operating-temperature assumptions, and what liquid-cooling service is required to keep the warranty.

Put this list in the request: site voltage and frequency, on-grid or off-grid or hybrid, whether a generator is in the one-line, the PV path (cabinet MPPTs, external inverter, or both), the ambient and altitude used for derating, the fire-alarm interface, and the grid code. Weltrus can match those items to the report and the manual for this model. This page does not assign a price, a lead time, or a report number.

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