Foreword

This project is being implemented on the roof of a hotel in Sanya, Hainan. The usable roof area measures 150 square metres, and the hotel’s average daily electricity consumption is 200 kilowatt-hours. The site houses 380V three-phase power equipment, which places high demands on the stability of the power supply and the balance of the three-phase output. The Hainan region enjoys abundant sunlight, but is characterised by high temperatures and humidity in summer, as well as frequent typhoons. The photovoltaic energy storage system must therefore be adapted to local climatic conditions, whilst balancing power generation efficiency, equipment protection ratings and operational reliability.

This system adopts a hybrid architecture combining daytime solar power generation with night-time peak shaving and load balancing via energy storage. It utilises solar photovoltaic power generation, with surplus electricity stored in the energy storage batteries. During periods when solar generation is insufficient, the batteries discharge energy to supply the hotel’s loads. During night-time peak periods on the mains grid, the batteries discharge energy to shave peaks and balance loads, thereby increasing the proportion of self-generated and self-consumed solar power, reducing the hotel’s electricity costs, whilst also providing short-term backup power supply capability.

Taking into account the site’s electricity consumption characteristics, this solution specifies a 25 kW inverter, 630 W solar panels and a 64 kWh low-voltage lithium-ion battery pack as core equipment. Electrical wiring, power matching and string-level electrical verification have been completed, whilst analyses of the system’s operational performance and load-carrying capacity have been conducted to provide a comprehensive technical reference for project implementation.

System Configuration Diagram

I. Introduction to Core Equipment

25 kW inverter

25 kW Three-Phase Hybrid Grid-Tied Inverter

The 25 kW three-phase hybrid energy storage inverter supports PV DC input, battery charging and discharging, and grid interaction. It features dual MPPT PV input interfaces with a maximum PV input power of 50 kW, making it suitable for high-power PV arrays. The AC side supports 3P+N+PE three-phase four-wire output, enabling 100% three-phase unbalanced output, making it suitable for mixed loads comprising three-phase 380V power equipment and single-phase lighting in hotels; it features a 10-second 200% overload capacity and an off-grid switching time of less than 10 ms, ensuring uninterrupted power supply to the load. The battery side has a maximum charge and discharge current of 420 A, making it suitable for low-voltage lithium-ion batteries. With an IP66 protection rating, it is capable of withstanding the high-temperature and high-humidity outdoor conditions in Sanya, and features comprehensive AC and DC protection functions to ensure the safe operation of the system.

630 W solar panel

Diagram of solar panels installed on a roof

630W high-power monocrystalline silicon solar panels with high conversion efficiency, suitable for distributed photovoltaic systems on commercial and industrial roofs. The modules feature excellent high-temperature output performance, making them suitable for the high-temperature operating conditions of Sanya’s summers; their electrical parameters are compatible with the MPPT voltage operating range of the inverter. This project comprises a total of 48 panels, configured with 12 panels in series and 2 strings in parallel connected to a single MPPT channel. The two arrays are connected to the inverter’s two MPPT interfaces respectively, with a total installed capacity of 30.24 kW. Taking into account roof area, light attenuation and temperature-related losses, the project is designed to generate an average of 151 kWh per day. The modules possess excellent resistance to wind and salt spray, making them suitable for use on the roofs of seaside hotels.

64 kWh low-voltage lithium-ion battery pack

64 kWh Stacked Lithium-Ion Energy Storage Battery Pack

The 64 kWh low-voltage lithium-ion battery pack consists of four 16 kWh battery modules connected in parallel, forming a nominal 51.2 V low-voltage system. It is equipped with a Battery Management System (BMS) that supports CAN/RS485 communication and interfaces with the inverter to provide battery charge/discharge protection and balance management. The battery operates within a voltage range of 40–60 V, matching the electrical specifications of the inverter’s battery side. The system is designed to store 64 kWh per day, prioritising the storage of surplus photovoltaic energy. The battery features protection against overcharging, over-discharging, overcurrent and temperature, and is suitable for high-humidity coastal environments. It works reliably with the inverter to facilitate peak-period discharge and the storage of surplus photovoltaic energy, whilst meeting the hotel’s power supply requirements during morning and evening peak periods.

II. System Configuration and Electrical Design
The project roof area is 150 m², with 48 x 630 W solar panels installed, giving a total installed capacity of 30.24 kW. Wiring scheme: Every 12 modules are connected in series to form one string; two strings are connected in parallel to a single MPPT channel; the 48 modules are divided into two arrays (each MPPT channel connects to 2 strings × 12 modules, totalling 24 modules), which are connected to the two MPPT interfaces of the inverter respectively. The inverter has a maximum PV input of 50 kW; the PV system for this project is reasonably oversized. The project is designed to generate an average of 151 kWh per day, with a daily energy storage capacity of 64 kWh.

System Topology Diagram

Key limits on the PV side of the inverter: MPPT operating voltage 150–850 V, maximum input current per MPPT channel 40 A, maximum DC input voltage for the entire unit 1,000 V. STC parameters for a mainstream 630 W module: Vmp = 40.7 V, Imp = 15.48 A, Voc = 47.98 V, Isc = 16.49 A, Voc temperature coefficient −0.25%/°C. In Sanya, extreme low temperatures of approximately 5 °C cause the open-circuit voltage to rise; in summer, the operating temperature of the modules can reach 65 °C, which lowers the operating voltage.

Single-string (12 cells in series) calculation:

STC standard test conditions: Maximum power voltage per string (Vmp‑string) = 12 × 40.7 = 488.4 V; open-circuit voltage per string (Voc‑string) = 12 × 47.98 = 575.76 V; operating current per string (Imp) = 15.48 A; short-circuit current per string (Isc) = 16.49 A

Low-temperature correction at 5 °C for open-circuit voltage (verified not to exceed 1000 V): ΔT = 5 – 25 = –20 °C; after low-temperature correction, Voc‑cold = 575.76 × [1 – 0.0025 × (–20)] = 604.55 V, which is well below the inverter’s maximum DC input of 1000 V and therefore safe.

Maximum power voltage correction at high temperature (65 °C) (verified to be above the MPPT lower limit of 150 V): ΔT = 65 – 25 = +40 °C. At high temperature, Vmp‑hot = 488.4 × [1 – 0.0025 × 40] = 439.56 V, which is higher than the MPPT lower limit of 150 V and falls within the 150–850 V tracking window; the inverter can track the maximum power point normally.

Single MPPT channel (2 strings in parallel):

The voltage remains constant for the string; the total operating current Imp_mppt = 2 × 15.48 = 30.96 A; the short-circuit current Isc_mppt = 2 × 16.49 = 32.98 A. The inverter’s maximum input current for a single MPPT channel is 40 A; as 32.98 A < 40 A, the current falls within the equipment’s permissible range, with a safety margin. The electrical parameters for both MPPT channels are identical.

The energy storage configuration comprises a 64 kWh low-voltage lithium-ion battery bank, consisting of four 16 kWh battery packs connected in parallel and directly linked to the inverter’s battery port; the system’s daily energy storage capacity is 64 kWh. Electricity generated by the PV system is prioritised for supplying the hotel’s loads, with any surplus stored in the batteries; during morning and evening peak consumption periods, when sunlight is weak, the batteries discharge to bridge the power shortfall. The hotel’s total daily electricity consumption is 200 kWh. As there are 380 V three-phase electrical appliances, the inverter supports unbalanced three-phase output and can simultaneously power three-phase power equipment and single-phase lighting loads in guest rooms. The system is capable of grid-connected operation and can be connected to the mains grid; when photovoltaic and energy storage capacity is insufficient, the mains supply provides supplementary power.

III. Table showing the power and runtime of electrical appliances supported by the system

Note: Calculated based on a battery capacity of 51.2 kWh (80% depth of discharge) and a system overall efficiency of 0.9; this represents the theoretical runtime when powered solely by the battery, excluding supplementary power from photovoltaic generation.

Table

When the system is supplemented by photovoltaic power generation during the day, the actual operating time will be significantly longer than the figures shown in the table above; the inverter has a 10-second 2x overload capacity, making it compatible with the inrush current of motor-driven equipment.

IV. Explanation of Operational Strategies

The system operates in ‘self-generation for self-consumption, with surplus electricity stored’ mode.

Video: Schematic illustration of how the system works

During the day, the hotel’s electricity is primarily supplied by the photovoltaic system. When the amount of electricity generated by the photovoltaic system exceeds the current load, the surplus energy is used to charge the 64 kWh battery, with a daily storage capacity of 64 kWh;

During morning and evening peak consumption periods, when sunlight is weak, the battery discharges to meet load requirements. The hotel’s average daily electricity consumption is 200 kWh, whilst average daily PV generation is 151 kWh. The stored energy is released during peak consumption periods when electricity tariffs are high, utilising the 64 kWh of stored energy; the remaining electricity is supplied by the mains grid. This increases the proportion of self-generated and self-consumed PV electricity, thereby reducing the hotel’s electricity costs.

Given Sanya’s high ambient temperatures, the inverter automatically derates when temperatures exceed 45°C. The equipment is rated to IP66 protection and is salt-fog resistant. The system is configured with PV reverse-connection protection, AC/DC surge protection, insulation monitoring and islanding protection, meeting the safety standards for coastal commercial and industrial projects. The inverter supports a monitoring backend, allowing remote viewing of operational data such as power generation, battery SOC and load power, facilitating management by the hotel’s operations and maintenance staff.

Summary 

This rooftop PV-storage project for a hotel in Sanya utilises a 150 m² roof area and comprises 48 x 630 W PV panels, a 25 kW hybrid storage inverter and a 64 kWh low-voltage lithium-ion battery bank. The overall solution is tailored to the electricity consumption patterns of hotels in Sanya, Hainan, whilst also accommodating the requirements of 380 V three-phase power equipment. Following calculations of string voltage and current, the wiring configuration—comprising 12 panels in series and 2 strings in parallel—ensures that, under Sanya’s high and low temperature conditions, both voltage and current remain within the inverter’s MPPT operating window and current limit, indicating appropriate electrical matching.

The system is designed to generate an average of 151 kWh per day and store 64 kWh per day. It prioritises supplying the hotel’s loads with electricity converted from solar energy, whilst using the energy storage to smooth out morning and evening peak demand, thereby reducing the hotel’s reliance on the mains grid and lowering electricity costs. The equipment has been fully engineered to withstand Sanya’s coastal climate, characterised by high temperatures, high humidity and salt spray, and is equipped with comprehensive protective and electrical safety features. During subsequent operation and maintenance, dust should be regularly removed from the photovoltaic modules and the battery SOC monitored to ensure the system continues to deliver stable performance over the long term.