Introduction

A residential off-grid solar system installer received a referral from an existing client: a small professional services office in the same neighborhood, two floors, ten staff, running a generator for eight hours a day and spending more on fuel than the client’s house next door. The installer was comfortable with the work. The load audit, the array sizing, the battery bank sizing, the inverter selection, all followed the same methodology that had produced reliable residential systems across dozens of installations. The system was commissioned correctly, the VRM portal showed normal operating data, and the office manager signed off at handover.

Three weeks later the office manager called to report that the system was shutting down every Tuesday and Thursday afternoon at roughly the same time. The Cerbo GX alarm log showed an inverter overload event at 14:23 on Tuesday and 14:31 on Thursday. The AC load reading in the seconds before each shutdown showed a spike to 11,400W, nearly double the inverter’s rated capacity.

The root cause took twenty minutes to find. The building had an overhead water tank served by a 1,500W submersible pump that started automatically whenever the float switch dropped below its threshold. The pump had not been mentioned during the load audit because the office manager considered it part of the building infrastructure rather than an office load. Its 4kW starting surge, arriving on top of the full afternoon AC and computer load, exceeded the inverter’s surge rating every time it started.

Commercial load audits require a different discipline from residential ones. This post establishes what that discipline looks like and applies it to a complete commercial system specification.

How Commercial Loads Differ from Residential

The residential load audit methodology in Post #1: How to Do a Proper Load Audit Before Sizing an Off-Grid System, identifies loads by room, estimates their running power, and multiplies by daily usage hours to produce a daily energy demand figure. This methodology works for residential installations because residential loads are well understood, their usage patterns follow predictable domestic routines, and the consequence of a sizing error is a household inconvenience rather than a business disruption. The same methodology applied to a commercial installation without modification will miss load categories that do not exist in residential buildings, underestimate the peak simultaneous load that the inverter must handle, and produce a battery bank sized for the wrong overnight scenario.

The three differences between commercial and residential loads that directly affect system sizing are the load profile shape, the peak-to-average load ratio, and the outage consequence. A residential load profile has a pronounced evening peak and a low overnight baseline. A commercial load profile is flatter during operating hours and then drops sharply at close of business. The evening peak that drives residential battery sizing does not exist on a commercial system. What drives commercial battery sizing is the overnight standby load and the worst-case overcast day operating deficit.

The peak-to-average ratio is higher in commercial installations because motor loads, water pumps, air compressor motors, refrigeration compressors, lift motors, are more common and more powerful than in residential buildings. These loads produce starting surge currents of 5 to 8 times their running current, and in a commercial building several motor loads may start within seconds of each other during the morning startup sequence. The inverter must handle this combined surge demand without tripping.

The outage consequence distinguishes commercial sizing from residential sizing in a way that affects the autonomy specification. A commercial system sized for two autonomy days reflects a revenue protection standard: the client can quantify the revenue lost per hour of outage, and the autonomy specification is the engineering translation of the client’s tolerance for that revenue loss.

For the residential load audit methodology that the commercial audit builds on, refer to our engineering guide on how to conduct a load audit for an off-grid solar system.

The Commercial Load Audit

The commercial load audit follows the same structure as the residential audit in Post #1 with three additions: every motor load must be identified separately and its starting surge current estimated, every standby load must be identified and its 24-hour daily energy contribution calculated independently, and the peak simultaneous load must be calculated explicitly including the worst-case motor starting scenario.

The reference office for this post is a two-floor professional services building with 200 square metres of conditioned floor area, ten staff, an operating day of 08:00 to 17:00, a server room with continuous cooling, and a building water system served by a submersible pump.

Operating-Hours Loads

Lighting: 20 x 18W LED panels = 360W, 8hr/day

E_lighting = 360W x 8h = 2,880Wh/day

Computers and monitors: 10 workstations x 80W = 800W, 8hr/day

E_computers = 800W x 8h = 6,400Wh/day

Air conditioning: 2 x 1,500W at 65% duty cycle

Average AC power = 2 x 1,500W x 0.65 = 1,950W

E_AC = 1,950W x 8h = 15,600Wh/day

Water pump: 1,500W, 0.5hr/day total run time

E_pump = 1,500W x 0.5h = 750Wh/day

Note: motor load, starting surge = 6 x 1,500W = 9,000W

Total operating-hours energy: 2,880 + 6,400 + 15,600 + 750 = 25,630Wh/day

Standby Loads (24 Hours per Day)

Networking: router, managed switch, patch panel = 150W, 24hr/day

E_networking = 150W x 24h = 3,600Wh/day

Server room cooling unit: 500W, 24hr/day

E_server_cooling = 500W x 24h = 12,000Wh/day

Access control and alarm panel: 50W, 24hr/day

E_security = 50W x 24h = 1,200Wh/day

Total standby energy: 3,600 + 12,000 + 1,200 = 16,800Wh/day

Total Daily Energy Demand and Peak Simultaneous Load

E_daily = E_operating + E_standby = 25,630 + 16,800 = 42,430Wh/day

Peak simultaneous running load:

Base loads: 360W + 800W + 150W + 50W = 1,360W

Both ACs running: 2 x 1,667VA = 3,334VA

Server room cooling: 500W

Total running: 5,194W

Worst-case surge (pump starting while all loads running):

P_surge = 5,194W + (1,500W x 6) = 5,194 + 9,000 = 14,194W

The 14,194W surge demand is the parameter that governs the inverter specification. It is more than twice the continuous running load and nearly three times a standard single-phase Multiplus-II 48/5000’s surge rating of 9,000VA. This is the calculation that the residential methodology, applied without identifying the pump as a motor load, would not have produced.

Array Sizing for Commercial Operations

The array sizing calculation for a commercial installation uses the same governing equation as Posts #4: Solar Array Sizing for Off-Grid Lithium Battery Systems and #5: Series vs Parallel vs Series-Parallel Solar Array Wiring, but the energy demand input requires a distinction that does not exist in the residential calculation. In a commercial system operating during daylight hours, the array generates at the same time as the largest loads are running, and its output directly offsets the operating-hours demand in real time. The battery bank’s function shifts from overnight storage to overcast-day buffer.

The array must be sized to cover the full daily energy demand, including the standby loads that run overnight, because on a clear day the array must harvest enough energy to serve the operating-hours loads directly, recharge the energy consumed by the overnight standby loads the previous night, and maintain the battery bank at a high state of charge for the next overcast day:

E_array_required = E_daily / (PSH x derating)

E_array_required = 42,430Wh / (5.5h x 0.718) = 42,430 / 3.949 = 10,744W minimum

Panel count: 10,744W / 400W = 26.86 -> 27 panels minimum

With 10% to 15% design margin -> 30 panels selected

Derated daily harvest: 30 x 400W x 0.718 x 5.5h = 47,385Wh

47,385Wh > 42,430Wh daily demand -> PASS with 11.7% margin

Array Sizing Summary:

Minimum array power required -> 10,744W (from governing equation)

Panels selected -> 30 x 400W (with 11.7% design margin)

String configuration -> 10 strings of 3S, 3 to 4 strings per MPPT controller

Derated daily harvest -> 47,385Wh at 5.5 peak sun hours

Design margin -> 11.7% above daily demand

For the full array sizing methodology and derating factor derivation, refer to our engineering guides on how to size a solar array for an off-grid system and series and parallel wiring for off-grid solar arrays.

Battery Bank Sizing for Commercial Installations

Battery bank sizing for the commercial system must account for two distinct energy demands. The operating-hours demand is partially offset by the solar array generating simultaneously with the loads, so the battery only needs to cover the gap between array output and load demand during operating hours on a worst-case overcast day. The overnight standby demand is covered entirely by the battery with no solar contribution.

Operating-Hours Battery Requirement

Array output on worst-case overcast day (25% of clear-sky output):

P_array_overcast = 10,800W x 0.718 x 0.25 = 1,939W

E_array_overcast = 1,939W x 8h = 15,509Wh during operating hours

Operating-hours energy demand (excluding standby):

E_operating = 2,880 + 6,400 + 15,600 + 750 = 25,630Wh

Battery covers operating-hours deficit on overcast day:

E_operating_deficit = 25,630 - 15,509 = 10,121Wh

Overnight Standby Battery Requirement

Standby loads run for 16 hours overnight with no solar contribution:

E_standby_overnight = (150W + 500W + 50W) x 16h = 700W x 16h = 11,200Wh

Minimum Bank Capacity at Two Autonomy Days

E_battery_daily = E_operating_deficit + E_standby_overnight

E_battery_daily = 10,121 + 11,200 = 21,321Wh per day

C_bank = (E_battery_daily x N_autonomy) / DoD_usable

C_bank = (21,321Wh x 2) / 0.90 = 47,380Wh minimum

Pylontech US3000C units: 47,380 / 3,550 = 13.35 -> 14 minimum

14 units in groups of 4: 3 groups of 4 = 12 (insufficient)

4 groups of 4 = 16 units -> use 16 units

Total bank: 16 x 3,550Wh = 56,800Wh nameplate

Usable capacity: 56,800 x 0.90 = 51,120Wh

Autonomy days: 51,120 / 21,321 = 2.40 days -> PASS

Daily DoD: 21,321 / 51,120 = 41.7% -> well within 80% limit

Battery Bank Specification:

Units -> 16 x Pylontech US3000C

Total nameplate -> 56,800Wh

Usable capacity -> 51,120Wh at 90% DoD

Daily battery discharge -> 21,321Wh (worst-case overcast day)

Daily DoD -> 41.7%

Autonomy -> 2.40 days at worst-case daily discharge

BMS units -> 4 x Pylontech BMS, 4 strings of 4 units each

Combined CCL -> 400A (4 x 4 x 25A)

Combined DCL -> 592A (4 x 4 x 37A)

For the battery bank sizing methodology and BMS communication architecture, refer to our engineering guide on battery bank sizing, configuration, and BMS selection for off-grid systems.

Inverter Sizing for Commercial Loads Three-Phase and Motor Starting

The inverter specification for the commercial system is governed by the 14,194W surge demand established in the load audit. No single-phase Victron Multiplus-II unit in the 48V range can accommodate this surge demand. The Multiplus-II 48/5000 has a surge rating of 9,000VA, which is 5,194W short of the combined surge requirement. The inverter architecture must either distribute the load across multiple units or the pump starting surge must be reduced by a soft starter.

Soft Starter First

Pump surge with soft starter (3x inrush): 1,500W x 3 = 4,500W

Revised combined surge: 5,194W + 4,500W = 9,694W

Multiplus-II 48/5000 surge rating: 9,000VA

9,694W > 9,000VA -> still FAIL on single unit

Architecture Option 1: Two Multiplus-II 48/5000 in Parallel

Parallel pair effective continuous: 2 x 5,000VA x 0.70 = 7,000VA

Running load utilisation: 5,194W / (2 x 5,000VA) = 52% -> PASS

Combined surge: 2 x 9,000VA = 18,000VA

9,694W < 18,000VA -> PASS with 85% margin

Architecture Option 2: Three Multiplus-II 48/5000 in Three-Phase Configuration

Three units in three-phase configuration distribute the building’s loads across three phases. The loads are distributed to balance phase loading:

Phase A: lighting + computers + networking = 360 + 800 + 150 = 1,310W

Phase B: air conditioner 1 + server cooling = 1,667VA + 500W = 2,167VA

Phase C: air conditioner 2 + security + pump = 1,667VA + 50 + 1,500W = 3,217VA

Phase C surge with soft starter: 3,217VA + 4,500W = 7,717W

7,717W < 9,000VA per-phase surge rating -> PASS

All phases within 5,000VA continuous per-phase rating -> PASS

The three-phase configuration is the recommended architecture for this commercial installation. It distributes the loads more evenly across the inverter capacity, provides three-phase output for any future three-phase equipment additions, and scales more gracefully if the load grows further. Power quality requirements for the server room are met by the Multiplus-II’s inverter output: THD below 3 percent, voltage within ±2 percent of 230V, and frequency stability within ±0.1Hz.

MPPT Controller Configuration for Large Arrays

The 30-panel array produces 10,800W of installed capacity across 10 strings of 3 series-connected panels. The correct controller for the commercial system is the Victron SmartSolar MPPT 250/100, which accepts a maximum input voltage of 250V and delivers a maximum output current of 100A to the battery bank, corresponding to 4,800W at 48V. Three MPPT 250/100 controllers manage the 10-string array, with each controller managing 3 strings at 3,600W input:

Controller 1: 3 strings x 1,200W = 3,600W -> within 4,800W limit -> PASS

Controller 2: 3 strings x 1,200W = 3,600W -> within 4,800W limit -> PASS

Controller 3: 3 strings x 1,200W = 3,600W -> within 4,800W limit -> PASS

Cold Voc check per 3S string on MPPT 250/100:

Voc_cold = 3 x 41V x (1 + (-0.0029 x (18-25))) = 125.5V

90% of 250V controller rating = 225V

125.5V < 225V -> PASS with significant margin

Combined derated output: 3 x (3,600W x 0.718) / 48V = 3 x 53.9A = 161.7A total

Combined CCL from 4 BMS units: 400A

161.7A < 400A -> PASS

The MPPT 250/100 provides substantial Voc headroom compared to the residential MPPT 150/60. The 3S string that sat at 83 percent of the MPPT 150/60’s 90-percent voltage limit sits at only 56 percent of the MPPT 250/100’s limit. String configurations up to 6S are technically viable on this controller before the cold Voc limit becomes a constraint, providing significant flexibility for future array expansion.

All three MPPT 250/100 controllers connect to the Cerbo GX via VE.Can. The Cerbo GX receives the CVL and CCL from all four Pylontech BMS units, determines the most conservative values, and distributes them to all three controllers simultaneously. Each controller adjusts its output independently. No synchronisation between controllers is required.

MPPT Controller Configuration Summary:

Controllers -> 3 x Victron SmartSolar MPPT 250/100

Array per controller -> 3 strings of 3S, 3,600W per controller

Combined derated output -> 161.7A total to battery bank

Cold Voc per string -> 125.5V vs 225V limit -> PASS

Combined CCL -> 400A from 4 BMS units -> exceeds array output -> PASS

VE.Can connections -> all three controllers on Cerbo GX VE.Can bus

Complete Off-Grid System Specification

The complete specification for the reference office system consolidates the component selections from Sections 3 through 6 into a single verified specification. Every component has been checked against the system’s requirements using the same constraint verification framework applied to the residential cluster system.

Commercial System Specification — Reference Office:

Solar array -> 27 x 400W panels, 9 strings of 3S (30 with design margin)

MPPT controllers -> 3 x Victron SmartSolar MPPT 250/100

Inverter-charger -> 3 x Victron Multiplus-II 48/5000 (three-phase configuration)

Battery bank -> 16 x Pylontech US3000C (4 groups of 4 units)

BMS units -> 4 x Pylontech BMS

Communication hub -> Victron Cerbo GX + GX Touch 50

CAN adapters -> 4 x VE.Can-to-CAN adapter (one per BMS)

Soft starters -> 3 units: water pump + 2 x air conditioner compressors

DC wiring -> 50mm² battery-inverter per unit, two-constraint framework

AC distribution -> three-phase board, RCBOs per circuit per phase, Type 2 SPD

The capital cost comparison between the commercial and residential systems illustrates the scaling relationship:

Commercial-Specific Commissioning and Monitoring

The seven-stage commissioning sequence from Post #13 applies to the commercial system without modification, but four additional verification steps are required before handover: three-phase output verification, motor load starting verification, server room power quality verification, and generator phase sequence verification.

Three-Phase Output Verification

After Stage 4 of the commissioning sequence, each phase must be measured independently and confirmed within 1 percent of 230V. The phase angle between phases must be confirmed at 120 degrees ±2 degrees using a power analyser. A phase angle error above 2 degrees indicates a VEConfigure setup error in the master-slave designation and must be corrected before any loads are connected.

Motor Load Starting Verification

Each motor load must be started individually while the other loads are running at their expected simultaneous levels. The AC load reading on the Cerbo GX must be monitored in the seconds around the start event. The load spike produced by the motor starting must be confirmed below the per-phase surge rating of 9,000VA. If the load spike exceeds this figure with the soft starter installed, the soft starter ramp time must be increased until the spike is within the surge rating.

Server Room Power Quality Verification

The AC output feeding the server room distribution circuit must be measured for total harmonic distortion, voltage stability, and frequency stability under full server room load. THD must be confirmed below 3 percent. Voltage must be confirmed within ±2 percent of 230V under load. Frequency must be confirmed within ±0.1Hz. These measurements must be taken with a calibrated power quality analyser and the results recorded in the commissioning documentation.

Generator Phase Sequence Verification

When a three-phase generator is connected to the Multiplus-II AC input, the phase sequence at the generator output terminals must be confirmed to match the phase sequence expected by the three-unit Multiplus-II configuration before the generator is started under load. A reversed phase sequence will cause the Multiplus-II to reject the generator source and remain in inverter mode, which is a common commissioning error on three-phase systems.

Commercial Commissioning Checklist Additions:

Three-phase voltage check -> each phase 230V ±1%, phase angle 120° ±2° at board

Motor starting verification -> AC load spike below 9,000VA per phase with soft starter

Power quality verification -> THD < 3%, voltage ±2%, frequency ±0.1Hz under server load

Generator phase sequence -> confirmed before three-phase generator first start under load

VRM alert additions -> per-phase voltage imbalance > 5%, server cooling temperature

Load profile review at day 30  -> compare actual vs load audit, identify unaudited loads

For the commissioning sequence and fault diagnosis framework that applies to both residential and commercial installations, refer to our engineering guide on system monitoring, commissioning, and fault diagnosis for off-grid solar systems.

Conclusion

The water pump that shut down the office system in the introduction was not an unusual load. It was a standard building service that the residential methodology had no category for, because residential load audits do not ask about building infrastructure. The commercial load audit methodology in Section 2 does ask, because the commercial installer’s job is to find every load before the system is energised rather than after the first overload alarm.

The three adjustments that the residential sizing methodology requires for commercial application are straightforward to apply. The load audit must identify all motor loads and their starting surge currents, all standby loads and their 24-hour daily energy contributions, and the peak simultaneous load including the worst-case motor starting scenario. The battery bank must be sized for the overnight standby energy demand in addition to the worst-case overcast day operating deficit. The inverter must be sized for the combined surge demand after soft starters have been applied to all motor loads above 1kW.

Everything else, the array sizing equation, the MPPT controller Voc and output current checks, the battery bank DoD and autonomy calculation, the cable sizing two-constraint framework, the BMS communication architecture, the commissioning sequence — applies to the commercial system without modification. The framework built across Posts #1 through #13 scales from a 6-panel residential installation to a 27-panel commercial one by changing the inputs, not the method.

The commercial system specified in this post costs approximately four times the residential cluster system and serves approximately four times the daily energy demand. Against a commercial generator running cost that scales proportionally with load, the payback period is comparable to the one-year residential payback established in Post #15. The economics of off-grid solar do not deteriorate at commercial scale.

In the next post we examine generator integration in detail: generator selection and sizing, hybrid operating modes, automatic generator start configuration, and the interaction between generator charging and solar charging on the Victron platform.

For the residential sizing framework that the commercial methodology in this post builds on, refer to our engineering guides on how to conduct a load audit for an off-grid solar system, battery bank sizing, configuration, and BMS selection, how to select and size an off-grid inverter, and system monitoring, commissioning, and fault diagnosis.

Engr. Ubokobong Ekpenyong

I am Engr. Ubokobong Ekpenyong, a solar specialist and lithium battery systems engineer with over five years of hands-on experience designing, assembling, and commissioning off-grid solar and energy storage systems. My work focuses on lithium battery pack architecture, BMS configuration, and system reliability in off-grid and high-demand environments.