Foreword

The Oman region enjoys excellent solar irradiation conditions; however, some small and medium-sized retail premises face high costs associated with connecting to the mains supply and experience an unstable power supply. By using hybrid inverters operating in pure off-grid mode, these premises can achieve independent power supply entirely through solar energy, without relying on the external grid.

This project involves a purely off-grid distributed photovoltaic-storage system installed on the roof of a commercial building. The electrical design has been developed in accordance with the local high-temperature tropical climate, and calculations have been carried out to ensure the compatibility of the photovoltaic array, energy storage batteries and hybrid inverter-controller unit; A comprehensive analysis of typical electrical equipment and power ratings in the retail premises has been carried out, with separate calculations made for the duration of power supply supported by combined PV-storage operation and by battery-only storage. Whilst ensuring the current power supply requirements of the retail premises are met, provision has been made for future PV capacity expansion, whilst taking into account system reliability, safety and the capacity to accommodate future load growth.

阿曼商铺屋顶太阳能板安装完成图

一、系统总体配置

整套纯离网系统由76片630W光伏板、40kW 混合逆控一体机、32个铅酸蓄电池组、交直流配电单元、系统监控单元组成，逆变器虽具备并网硬件能力，本项目设置为纯离网运行模式，不接入市电电网，全部电能来自光伏发电。

40kw96kwh离网光伏储能系统配置图

 1）光伏组件：76 块 630W 光伏组件；单块组件参数：最大功率 630W，工作电压 42.0V，开路电压 50.3V，工作电流 15.01A，短路电流 15.94A；光伏总装机功率：76×630W=47.88kWp。

 2）混合逆控一体机：40kW 混合逆控一体机，4路独立 MPPT 光伏输入通道，MPPT 工作电压区间 500‑850V，电池工作电压区间 250‑800V；本机本项目工作在纯离网模式，实现光伏充电、电池充放电管理以及交流逆变输出。 

40 kW grid-connected inverter

3) Energy storage batteries: 32 × 12V 250Ah lead-acid batteries connected in series; the battery pack has a nominal total voltage of 384V and a capacity of 250Ah, giving a total energy storage capacity of 96kWh, which meets the project’s revised design specification for daily energy storage of 96kWh.

12V 250Ah lead-acid battery

 4) Design parameters: Average daily electricity generation of 240 kWh; average daily energy storage of 96 kWh; photovoltaic power is prioritised to supply the shop’s AC loads, with surplus energy stored in the battery; at night, the shop’s loads are powered entirely by the battery.

二、商铺用电设备、功率统计

本中小型商铺为零售经营场所，主要用电设备为照明、制冷空调、展示冰柜、风扇、收银设备及小型办公设备，设备功率明细如下：

表格

商铺同时最大负载合计：13.5kW；逆变器 40kW 逆变输出功率远大于商铺最大同时负载，逆变功率裕量充足。日均综合用电约 192kWh。

铅酸蓄电池考虑使用寿命，设置最大放电深度 DOD=80%，电池实际可用容量 = 96kWh ×0.8 =76.8kWh。 

1）光伏 + 储能联合运行（白天有光照工况）：白天光伏发电 240kWh，优先供给 13.5kW 商铺负载，剩余电量对电池充电。在有效日照时段内，光伏持续输出，负载由光伏直接供电，电池做功率缓冲，只要存在有效光照，系统可不间断持续带载运行；光照不足时段，切换由电池补充供电。

2）纯储能工况（无光照，夜间 / 阴雨天，完全不依靠光伏发电）：全部负载 13.5kW 由蓄电池单独供电； 理论可带载时长 = 可用电池容量 ÷ 同时最大负载 =76.8kWh ÷13.5kW≈5.69h。

实际工程场景，商铺夜间不会满负荷全开，夜间仅开启照明、监控、冰柜（合计约 3.0kW），纯储能模式下可连续运行：76.8÷3.0=25.6h，可以完整覆盖商铺夜间低负荷用电，同时具备应对短期阴雨天的供电能力。

说明：遇到连续多日阴雨天，纯储能供电时长仍会受限，本系统配置预留一路 MPPT，后期可追加光伏组件，提升电池充电能力，进一步延长无光照条件下供电时间。

三、光伏阵列组串电气设计

本项目为阿曼高温工况，不核算低温下开路电压升高工况。76 块组件组成 6 串光伏组串，接入混合逆控一体机三路 MPPT 通道，第四路 MPPT 通道闲置，作为后期光伏扩容备用接口；同一 MPPT 接口内部采用两串一并接线，同一 MPPT 下两组串组件串联数量保持完全一致。 

String allocation:

Total: 26 + 26 + 24 = 76

MPPT-1: 2 strings, 13 modules per string, totalling 26 modules

MPPT-2: 2 strings, 13 modules per string, totalling 26 modules

MPPT-3: 2 strings, 12 modules per string, totalling 24 modules;

The DC terminals of unused MPPT channels are sealed with insulated protective caps, allowing for future expansion.

Voltage parameter verification (STC – Standard Test Conditions):

• String of 3 cells in series: Open-circuit voltage Voc = 13 × 50.3 V = 653.9 V; Maximum power point voltage Vmpp = 13 × 42.0 V = 546.0 V

  String of 12 cells in series: Open-circuit voltage Voc = 12 × 50.3 V = 603.6 V; Maximum power point voltage (Vmpp) = 12 × 42.0 V = 504.0 V

  The open-circuit voltages and maximum power operating voltages for both string configurations fall within the inverter’s MPPT operating voltage range of 500–850 V. During the summer in Oman, the junction temperature of the modules rises and the open-circuit voltage decreases with temperature; however, this will not exceed the equipment’s DC withstand voltage limit, and the voltage matching meets the operating conditions for off-grid charging.

Current calibration:

The operating current per string is 15.01 A. With two strings connected in parallel via a single MPPT channel, the total operating current is 30.02 A, which is below the maximum input current limit for a single MPPT channel; there is therefore no risk of overcurrent. The total PV DC power is 47.88 kW, which exceeds the hybrid inverter’s 40 kW AC output capacity; this constitutes a reasonable over-specification. During periods of strong sunlight, the inverter will output at the 40 kW upper limit, and any excess PV power will be subject to power limiting.

IV. Design of Energy Storage Battery Systems

32 12V 250Ah lead-acid batteries are connected in series to form a 384V energy storage battery pack, which falls within the inverter’s battery operating voltage range of 250–800V. The total energy storage capacity is 96kWh, meeting the daily energy storage requirement of 96kWh for a medium-sized retail outlet in Oman.

This project utilises a hybrid inverter operating in pure off-grid mode, with the batteries serving as the sole power source during the night and in low-light conditions. The depth of discharge must be strictly controlled at ≤80 per cent; deep discharge is prohibited to extend the overall service life of the lead-acid batteries.

The hybrid inverter-controller unit features a built-in charge management unit, which charges the battery pack via the photovoltaic array during the day; at night, the battery pack supplies power to the shop’s loads via the inverter. The battery side is equipped with DC fuses and DC circuit breakers, taking full account of safety precautions for 384V high-voltage DC to provide overcurrent and short-circuit protection, thereby ensuring the safe operation of the batteries in off-grid conditions.

V. System Operation Strategy (Pure Off-Grid Mode)

The system sets the hybrid inverter to pure off-grid mode, without connection to the external mains supply.

Video: How a 40 kW, 96 kWh solar-storage system works

During the day, electricity generated by the photovoltaic system is prioritised for supplying the AC loads of the shop; when the photovoltaic output exceeds the real-time load, the surplus electricity is used to charge the battery, achieving a daily energy storage capacity of 96 kWh; when photovoltaic generation is less than the load consumption, the battery supplements the power output; when the battery charge level drops to the lower protection limit, the integrated inverter-controller automatically cuts off the AC output to protect the battery and prevent damage from over-discharge. The system is equipped with local monitoring, allowing real-time viewing of key operational data such as photovoltaic power generation, battery voltage, State of Charge (SOC) and load consumption.

VI. Key Points on Construction and Safety

1. Insulation caps have been fitted to all DC connectors on idle MPPT channels to prevent electrical leakage and short circuits caused by dampness and dust; it is normal for the monitoring system to display a voltage of 0V for these channels.
2. DC and AC cables have been selected in strict accordance with local electrical regulations; as the battery side operates at 384V high-voltage DC, the cables and insulating components must meet the high-voltage rating; The photovoltaic mounting structures have been treated to resist corrosion and high temperatures, making them suitable for Oman’s outdoor environment.
3. Lead-acid batteries are housed in a well-ventilated and heat-dissipating space. As there are 32 battery cells connected in series, regular inspections of individual cell voltages are required to mitigate imbalance issues within the series battery bank; battery inspection and maintenance are particularly critical for off-grid systems.
   
   Layout plan for the battery room
4. The system is equipped with AC and DC circuit breakers, surge protectors and a comprehensive earthing system to meet the electrical safety requirements for commercial premises.

VII. Summary

This fully off-grid photovoltaic-storage solution utilises all 76 630W modules, with string voltage and current fully calibrated and matched; one MPPT channel has been reserved for future expansion. The 96kWh lead-acid storage system meets the daily storage requirement of 96 kWh, whilst the average daily photovoltaic generation is 240 kWh.

The system’s inverter capacity can cover the shop’s maximum simultaneous load of 13.5 kW; under sunlight, the PV system combined with the storage system can provide a continuous power supply; in the absence of sunlight, relying solely on the storage system, it can supply power at full load for approximately 5.69 hours, and in low-load mode at night for approximately 25.6 hours, providing a degree of resilience against short periods of overcast or rainy weather. The entire system utilises a hybrid inverter operating in pure off-grid mode, providing power to the shop entirely independently of the mains grid. It is tailored to local conditions in Oman, with a well-balanced configuration that accommodates both current power requirements and future expansion and upgrades.