Foreword

The European energy market has long faced practical challenges such as fluctuating electricity prices and an unstable energy supply; electricity prices in Switzerland have remained at a relatively high level for many years, and demand for integrated photovoltaic and energy storage systems among private residential customers continues to rise.

This project involves the bespoke design of a photovoltaic energy storage system for a large private villa in Switzerland. The client has commissioned a locally qualified electrician to carry out a comprehensive survey and assessment of the property’s domestic electrical loads, and has explicitly requested the installation of a complete photovoltaic energy storage system capable of operating both on-grid and off-grid.

 With the core objectives of self-consumption, grid backup and emergency power supply, the project takes into account the characteristics of the local temperate mountainous climate, giving full consideration to environmental factors such as short daylight hours in winter, significant diurnal temperature variations, frequent cloud and fog, and winter snow accumulation. It incorporates targeted designs for the modules’ low-temperature adaptability and low-light power generation capacity, as well as the duration of emergency power supply provided by the energy storage system and the potential for future expansion.

Configuration diagram for a 30 kW, 60 kWh solar-storage system

The complete system comprises 40 x 620W photovoltaic modules and 2 x 15kW hybrid inverter-controller units, paired with 4 x 15kWh energy storage batteries, giving a total storage capacity of 60kWh. The system’s average daily power generation is 124kWh, with a daily storage capacity of 60kWh. Whilst meeting the villa’s daily electricity requirements, the system provides a minimum of 8 hours of emergency backup power and allows for future expansion of the energy storage capacity to accommodate the homeowner’s growing electricity needs. This proposal provides a comprehensive explanation covering equipment parameter verification, electrical compatibility, load adaptation, operating modes, and key points for installation, construction and maintenance, thereby providing a technical basis for the project’s implementation.

📌 Project Overview

This project is being implemented at a large private villa in Switzerland, where a hybrid grid-connected and off-grid system comprising photovoltaics, energy storage and a grid backup is being installed. A local electrician has already carried out a survey of the household’s electricity consumption. Switzerland has a mountainous temperate climate, characterised by short daylight hours in winter, significant diurnal temperature variations, frequent cloud and fog, and numerous low-light conditions. Snow accumulation in winter places high demands on the low-light power generation performance of the photovoltaic modules, the backup duration of the energy storage system, and the system’s ability to operate at low temperatures.

System operation logic: During the day, solar power is prioritised to supply the villa’s loads, with surplus electricity stored in the energy storage batteries; at night, power is supplied by the batteries, with the mains serving as a backup power source; in the event of a mains power cut, the system can switch to off-grid emergency power supply, providing a minimum of 8 hours of backup power, and the energy storage system supports future expansion.

Specified system configuration:

1) Photovoltaic array: 40 x 620W modules, total installed capacity 24.8 kWp;

2) Inverter units: 2 x 15 kW hybrid inverter-controller units, with a combined rated output of 30 kW; these support grid-connected and off-grid switching, with each unit featuring 2 built-in MPPT interfaces;

3) Energy storage units: 15 kWh lithium-ion batteries, voltage 51.2 V, capacity 300 Ah, cycle life ≥ 8,000 cycles; 4 units provide a total storage capacity of 60 kWh.

Topology diagram of a 30 kW, 60 kWh photovoltaic-storage system

Verification of Open-Circuit Voltage in Photovoltaic Strings

Core equipment selection and connection scheme: Each 620W TOPCon module has an STC open-circuit voltage (Voc) of 48.4V; ten modules are connected in series to form a single string, with a standard test condition (STC) open-circuit voltage of 484V.

The low temperatures typical of Swiss winters increase the modules’ open-circuit voltage; at the extreme low temperature of –20 °C, the maximum open-circuit voltage per string is approximately 517 V. The 15 kW hybrid inverter has an MPPT operating voltage range of 125–850 V and a maximum DC input of 1,000 V. Across the entire temperature range, the string voltages remain within the inverter’s MPPT operating range and do not exceed the DC withstand voltage, ensuring safe electrical compatibility.

The four strings of modules are connected to two inverters, utilising a total of four MPPT channels, with each MPPT channel connected to one string comprising 10 modules. The string voltages are designed with a sufficient safety margin to accommodate voltage fluctuations caused by extreme winter cold and module temperature variations, thereby mitigating the risk of overvoltage damage to the inverter’s DC-side components.

⚡ Selection of Core Equipment and Connection Solutions

Specifications for the 15 kW Hybrid Inverter

Two 15 kW three-phase hybrid inverter-controller units have been selected, each with a rated AC output power of 15 kW; Each unit is equipped with two independent MPPT photovoltaic input interfaces, with an MPPT DC operating voltage of 125–850 V and a maximum DC input voltage of 1,000 V; it supports seamless switching between grid-connected and off-grid modes, and integrates battery charge/discharge management, mains backup and intelligent load control functions, whilst featuring overvoltage, overcurrent, overload, and high/low temperature protection, and is compliant with European low-voltage grid standards; When two inverters operate in parallel, the total system output capacity can reach 30 kW.

PV wiring configuration: 40 x 620 W modules are divided into 4 strings, with 10 modules per string; Strings 1 and 2 are connected to the two MPPT channels of the first 15 kW inverter; strings 3 and 4 are connected to the two MPPT channels of the second 15 kW inverter. With each inverter managing two sets of PV strings, this configuration reduces mutual interference caused by module shading and snow accumulation, thereby enhancing the system’s overall power generation output.

Capability to connect and expand with 4 × 15kWh batteries

Four 15kWh batteries with a total capacity of 60kWh, utilising high-voltage lithium iron phosphate energy storage batteries; Each inverter is connected on the DC side to two 15kWh batteries, meaning a single inverter is paired with a 30kWh battery bank. The two inverters manage their respective battery banks independently; the two battery banks are not connected in series and are each managed by the inverter’s BMS, thereby reducing the risk of the entire energy storage system failing due to a fault in a single battery bank. Feasibility of future expansion: The inverters are equipped with reserved communication interfaces and power headroom, allowing 15 kWh battery units of the same specification to be connected in parallel directly, without the need to replace the inverter hardware, thereby enabling upward expansion of the energy storage capacity. Should additional electrical appliances be installed in the villa at a later date, or should the frequency of electric vehicle charging increase, energy storage units can be added directly to accommodate the growth in electrical load.

Confirmation of System Compatibility

1) PV-to-inverter matching verification: The total installed PV capacity is 24.8 kWp, whilst the combined PV input capacity of the two inverters is 30 kW; the installed PV capacity accounts for approximately 82.7 per cent of the inverters’ total PV input capacity. As solar irradiance levels in Switzerland are relatively low during winter, this ratio helps to maximise power generation under low-light conditions without overloading the inverters; all string voltages and currents meet the MPPT parameters, indicating a reasonable overall match.

This capacity-to-output ratio ensures that the system does not overload during high-irradiance summer conditions whilst fully utilising the inverters’ MPPT tracking capabilities under low-irradiance winter conditions, in line with standard design practices for residential energy storage projects in Europe.

2) Verification of the 60 kWh battery’s 8-hour backup capacity: The total power of 24-hour uninterrupted loads—including refrigerators, freezers, surveillance systems, security fencing and Starlink—is approximately 850 W; Under emergency backup conditions, with three air conditioners, three televisions and basic lighting operating simultaneously, the system’s combined average load is approximately 6.8 kW. Calculated using a usable depth of discharge of 0.9: 60 kWh × 0.9 ÷ 6.8 kW ≈ 7.9 hours.

If some non-essential high-power equipment is switched off during the backup period, the system can reliably meet the design requirement of a minimum of 8 hours of backup power. In the event of a mains power failure, high-power loads such as electric hobs, high-power cooking appliances and electric vehicle charging must be actively avoided to ensure sufficient power for basic living needs.

📊Classification of Villa Loads and Reference Table for Standby Power Operation

Note: Based on on-site load statistics, a maximum of three air conditioning units operate simultaneously, and a maximum of three televisions are switched on at the same time; high-power appliances such as rice cookers, microwave ovens and air fryers are not used simultaneously; refrigerators, freezers, security surveillance systems and Starlink constitute uninterrupted, continuous loads.

👷 Explanation of System Operating Modes

Video: Schematic illustration of how a 30 kW, 60 kWh solar-storage system works

1. Grid-connected self-consumption model (during daylight hours when there is ample sunlight)🌤
   Electricity generated by the photovoltaic modules is prioritised for powering the villa’s various electrical loads; surplus electricity is used to charge the energy storage batteries, with an average daily storage capacity of approximately 60 kWh; when photovoltaic generation is insufficient, the grid supply provides supplementary power, thereby minimising the amount of electricity purchased from the grid.
2. Night-time energy storage power supply mode
   In the absence of sunlight, the battery supplies power to all loads in the house; when the battery’s State of Charge (SOC) falls to a preset lower limit, the system automatically switches to mains power to prevent deep discharge, which would damage the battery’s lifespan.
3. Off-grid backup power mode (mains power failure)
   The system automatically disconnects from the mains supply and switches to off-grid operation, with the photovoltaic system and batteries jointly supplying power to the loads; priority is given to ensuring uninterrupted power for essential loads, lighting and a limited number of air conditioning units, whilst intermittent high-power equipment and electric vehicle charging should be operated during off-peak hours to ensure a minimum of 8 hours of emergency power supply.
4. Key Points for Adapting to the Swiss Environment
   System considerations: The system utilises N-type double-glass 620W photovoltaic modules, which offer excellent power generation performance in low-light conditions and are resistant to PID, salt spray and low temperatures, making them well-suited to the local climate characterised by frequent cloud cover and significant temperature fluctuations. In winter, when snow accumulation on the modules obstructs sunlight, the inverter’s MPPT low-voltage start-up feature enables rapid resumption of power generation once the snow has melted. The double-glass structure also provides protection against rain, snow and hail, thereby extending the outdoor service life of the modules.

📢 System Notes

1. On-site photovoltaic installation strictly adheres to a string configuration of 10 modules per string. The open-circuit voltage in low-temperature conditions is verified on site, and it is strictly prohibited to exceed the inverter’s DC withstand voltage limit; the installation tilt angle of the modules is optimised in accordance with the local latitude in Switzerland, balancing the need for snow to slide off in winter with solar irradiance efficiency.

2. Battery installation locations must ensure adequate ventilation and heat dissipation. Given the low temperatures typical of Swiss winters, the battery compartment should be maintained within an operating range of 0–35°C as far as possible to safeguard battery charging and discharging efficiency and cycle life, whilst preventing significant degradation of usable capacity in cold conditions.

3. Under off-grid emergency power supply conditions, load management must be implemented effectively. High-power-consumption equipment, such as electric vehicle chargers and electric cookers, should be used outside the battery’s emergency power supply periods wherever possible to ensure the 8-hour backup power target is met. On a day-to-day basis, power generation and battery State of Charge (SOC) can be monitored via the inverter monitoring platform.

4. When adding new batteries for future capacity expansion, battery cells of the same specification (15 kWh) must be selected. Wiring, commissioning and BMS communication configuration must be carried out by a locally licensed electrician. The parameters of new and existing batteries must be consistent to avoid safety hazards caused by capacity imbalance.

📝Summary of the article

This technical solution involves the design of a 30kW grid-connected and off-grid photovoltaic energy storage system for a large private villa in Switzerland. The photovoltaic array comprises 40 x 620W modules, with a total installed capacity of 24.8kWp. Electrical verification confirmed that the open-circuit voltage of the 10 series strings falls within the MPPT operating range of the two 15kW hybrid inverters, indicating that the equipment is appropriately matched. Energy storage comprises four 15kWh batteries, with a total capacity of 60kWh; each of the two inverters manages two batteries, and the hardware supports future parallel expansion.

Based on calculations of the villa’s actual load, the system can provide a minimum of 8 hours of emergency backup power when non-essential high-power equipment is switched off. The system is configured with multiple operating modes, including grid-connected self-consumption, night-time energy storage power supply, and off-grid emergency backup. N-type double-glass modules, suited to the Swiss mountain climate, have been selected to cope with winter conditions such as low temperatures, snow accumulation and low light levels. Key implementation points—including installation, battery room environment, backup power load management, and capacity expansion and commissioning—have been clearly defined. The complete system addresses three core requirements: cost reduction through self-generation and self-consumption, mains power backup, and emergency power supply, thereby meeting all the daily electricity needs of a villa household.