Core Performance Parameters of an Off-Grid 20kW Solar Battery System

　　The off-grid 20kW solar battery system is primarily designed for use in areas without grid coverage (such as remote residences, field operations, and emergency power supply scenarios). Its performance parameters focus on the independence and stability of the entire "generation - energy storage - power supply" chain. These core parameters can be categorized into five key modules:

　　I. PV Module Performance Parameters (Core of Off-Grid Power Generation)

　　As the "energy source" of an off-grid system, PV module parameters directly determine the upper limit of daily power generation. Prioritizing conversion efficiency and environmental adaptability is key:

　　Peak Power (Pmax): The power of a single module is typically between 330W and 400W. The system requires 50-60 modules connected in series or parallel, with a total peak power requirement stable within 20kW ±5%. Excessive deviations in total peak power will result in the system's actual power generation capacity failing to meet design standards, impacting daily power supply needs.

　　Conversion Efficiency (η): Monocrystalline silicon modules typically have a conversion efficiency of 18%-23%, while polycrystalline silicon modules have a conversion efficiency of 16%-19%. Since off-grid systems lack grid power, every 1% increase in conversion efficiency can increase the system's average daily power generation by approximately 0.5-1 kWh under standard sunlight conditions. This can significantly impact power supply stability over time.

　　Power Temperature Coefficient (Pmax/°C): The typical range is -0.38% to -0.45%/°C, meaning that for every 1°C increase in temperature, the module's peak power decreases by 0.38%-0.45%. In high-temperature environments, modules with a lower coefficient (e.g., -0.38%/°C) should be preferred to minimize the negative impact of temperature on power generation efficiency.

　　Open-Circuit Voltage (Voc): The open-circuit voltage of a single module is typically 38V-45V. When multiple modules are connected in series, the total open-circuit voltage can reach 1900V-2700V. It's important to note that the total open-circuit voltage must match the maximum DC input voltage of the off-grid inverter (most inverters support 1000V-1500V input). Therefore, the module series grouping arrangement should be carefully planned based on the inverter's specifications.

　　Short-circuit current (Isc): The short-circuit current of a single module is generally 8.5A-10A. This parameter affects the stability of the string current. Excessive current can easily cause cable heating. In actual configurations, PV cables with appropriate wire diameters are required; 6mm²-10mm² is recommended to ensure safe current transmission.

　　Operating temperature range: Typically -40°C to +85°C, which covers most extreme environments (such as low temperatures in northern winters and high temperatures in southern summers). In low-temperature environments, care must be taken to prevent the module glass from freezing and cracking due to large temperature differences or external impact.

　　Degradation rate: The first-year degradation rate must be ≤2.5%, followed by ≤0.7% per year thereafter, for a cumulative degradation rate of ≤20% over 25 years. Since off-grid systems lack grid backup, their low degradation rate ensures they maintain over 80% of their power generation capacity over their 25-year lifecycle, guaranteeing long-term power supply stability.

　　II. Energy Storage Battery Performance Parameters (Core to Off-Grid Power Supply Guarantee)

　　Off-grid systems rely on energy storage batteries to store daytime electricity and provide power at night or during periods of low light. Therefore, the battery's capacity, cycle life, and charge-discharge characteristics are crucial (using mainstream lithium iron phosphate batteries as an example):

　　Rated Capacity (Cn): Systems typically utilize a 50kWh-100kWh battery pack to support a 20kW load. When selecting a battery, consider the following: "Daily Power Consumption × 1.2-1.5" (e.g., if daily power consumption is 40kWh, a 60kWh battery pack is recommended) to minimize power outages during cloudy or low-light conditions.

　　Nominal Voltage: The nominal voltage of a single battery is 3.2V. Multiple batteries connected in series form battery packs of varying specifications, such as 48V, 96V, or 192V. This voltage must be fully consistent with the DC input voltage range of the off-grid inverter (e.g., a 48V battery pack must be used with an inverter that supports 48V DC input). Failure to do so may cause the device to malfunction or be damaged.

　　Cycle Life (80% Capacity Fade): At a 0.5C charge/discharge rate, the cycle life must be ≥2000 cycles. If the system is charged and discharged once per day, 2000 cycles can meet approximately 5.5 years of use. Selecting a battery pack with a cycle life of ≥3000 cycles can extend the service life to over 8 years, reducing future replacement costs.

　　Charge/Discharge Rate (C-rate): Typical charge/discharge rates are 0.2C-0.5C, with some batteries supporting 1C short-term fast charging. Taking a 0.5C charge rate as an example, a 50kWh battery pack can be fully charged or fully discharged in 2 hours. A short-term 1C charge and discharge rate can handle sudden high loads (such as starting a high-power motor), improving the system's emergency power supply capability.

　　Deep Discharge (DOD): The recommended DOD in actual use is ≤80%, allowing the battery to discharge to 20% of its remaining capacity. Prolonged DOD exceeding 80% (e.g., discharging to 10% of its remaining capacity) will accelerate battery aging and shorten its cycle life by 30%-50%. Inverter parameter settings should be used to limit the DOD.

　　Self-Discharge Rate: At a normal temperature of 25°C, the monthly average self-discharge rate should be ≤3%. If the system is idle for an extended period (e.g., monthly non-use), the self-discharge loss of a 50kWh battery pack should be ≤1.5kWh. This reduces capacity loss during idle periods and prevents low battery levels upon subsequent use.

　　Operating Temperature Range: Charging temperature range is 0°C to 50°C, and discharging temperature range is -20°C to 60°C. In low-temperature environments (below -10°C), the battery discharge capacity will decrease by 10%-30%. When configuring the system in cold northern regions, it is recommended to install an additional battery heating module to ensure power supply in low temperatures.

　　III. Off-grid Inverter Performance Parameters (Energy Conversion and Power Supply Control Core)

　　Off-grid inverters must convert the DC power output of PV panels or energy storage batteries into AC power while ensuring stable power supply to the load. Core parameters are as follows:

　　Rated output power: Must match the installed system capacity, i.e., 20kW, to ensure it can support a continuous load output of 20kW. Considering the inrush current generated by inductive loads such as motors and air conditioners during startup, the inverter peak power must be ≥25kW to cope with the high load demand during startup and avoid frequent inverter shutdowns.

　　DC input voltage range: Must match the battery pack voltage specifications (e.g., 48V, 96V, 192V). During actual operation, DC input voltage fluctuations must be controlled within ±10%. Excessive voltage may damage the inverter's internal components; excessively low voltage may cause power outages, impacting normal load operation.

　　AC output parameters: Output voltage can be selected as single-phase 220V or three-phase 380V, with a stable frequency of 50Hz±0.5Hz. To ensure proper operation of sensitive loads such as refrigerators and computers, the output voltage deviation must be ≤±2% and the frequency deviation must be ≤±0.2Hz to prevent damage to equipment caused by voltage or frequency fluctuations.

　　Conversion efficiency: Under rated load conditions, the conversion efficiency must be ≥92%; under light load conditions (20% of rated load), the conversion efficiency must be ≥88%. Every 1% increase in conversion efficiency, calculated at 3,000 hours of annual system operation, can reduce energy loss by approximately 150-200 kWh per year, significantly reducing energy storage battery consumption over the long term.

　　Overload Capacity: The inverter must have a certain level of overload tolerance, typically capable of 10 minutes of continuous operation at 120% of the rated load and 1 minute at 150% of the rated load. This performance can handle sudden high loads (such as starting two 5kW motors simultaneously), preventing inverter shutdown due to short-term overloads and ensuring power supply continuity.

　　Protection Functions: The inverter must integrate comprehensive protection mechanisms, including overvoltage protection, undervoltage protection, overcurrent protection, short-circuit protection, overtemperature protection, and reverse polarity protection. Since off-grid systems lack grid-side protection, the inverter's protection functions effectively prevent equipment damage or fire risks and are critical to safe system operation.

　　Standby Power Consumption: The power consumption in standby mode must be ≤50W. Off-grid systems rely on energy storage batteries for power. Low standby power consumption reduces inefficient battery drain. Based on a one-day standby period, power consumption is ≤1.2 kWh, preventing premature battery depletion due to standby power loss.

　　IV. Overall System Performance Parameters

　　From the perspective of the entire "power generation - energy storage - power supply" chain, system-level parameters directly reflect actual performance:

　　Daily Average Power Generation (Standard Sunlight): Under standard conditions of 1000W/㎡ and 4 hours of sunlight per day, the system typically generates 80-120 kWh of power per day. In low-light conditions (such as a cloudy day with an irradiance of 300W/㎡), the average daily power generation drops to 25-40 kWh. Therefore, the power generation capacity should be reasonably estimated based on the installation area's lighting conditions.

　　Power Supply Guarantee Time: Under full load (20kW load), the power supply guarantee time is 2.5-5 hours; under half load (10kW load), it is 5-10 hours. The specific duration depends on the battery pack capacity. For example, a 50kWh battery pack supports full load power for 2.5 hours, while a 100kWh pack can provide power for 5 hours.

　　Charge and discharge control accuracy: The charge cut-off voltage deviation must be ≤±1%, and the discharge cut-off voltage deviation must be ≤±1%. Accurate charge and discharge control can extend battery life. For a 48V battery pack, for example, the charge cut-off voltage must be controlled within 49.2V±0.5V, and the discharge cut-off voltage must be controlled within 40V±0.4V to prevent damage to the battery from overcharging or over-discharging.

　　Load adaptability: The inverter must support a variety of load types, including resistive loads (such as light bulbs and water heaters), inductive loads (such as motors and air conditioners), and capacitive loads (such as capacitor compensation cabinets). The starting current of inductive loads can reach 3-5 times the rated current. Ensure that the inverter's overload capacity matches the load's starting requirements to avoid startup failures.

　　Backup power supply switchover time: If the system is equipped with a backup power source (such as a diesel generator), the switchover time must be ≤10ms. This parameter can be used to quickly connect to a backup power source in extremely cloudy conditions or when the battery is low, preventing data loss or malfunctions caused by power outages in sensitive loads such as servers and medical equipment.

　　V. Environmental Adaptability Parameters (Core Requirements for Off-Grid Scenario)

　　Off-grid systems are often installed in remote areas and must withstand complex environments. Environmental parameters determine system reliability:

　　Operating Temperature: The operating temperature range for PV modules is -40°C to +85°C, for batteries -20°C to +60°C, and for inverters -25°C to +55°C. In high-temperature environments (≥40°C), ensure that the inverter cooling fan is operating properly to prevent component overheating. In low-temperature environments (≤-10°C), activate the battery heating module to prevent a significant drop in battery capacity.

　　Protection Rating (IP Rating): PV modules must meet IP67 protection, protecting them from heavy rain and internal short circuits. Inverters must meet IP65 protection, protecting them from rain splashes. Battery cabinets must meet IP54 protection, effectively preventing dust and moisture accumulation and potentially affecting performance.

　　Wind Resistance: PV mounting brackets must be able to withstand winds up to force 12 (corresponding to wind speeds ≥ 32.7 m/s). For rooftop installations, brackets must be reinforced. For ground installations, brackets must be secured with counterweights or piles to prevent strong winds from causing module dislodgment and ensure system structural safety.

　　Weather Resistance: PV modules must be resistant to UV aging, ensuring no cracking or delamination for 25 years. Brackets must be galvanized or plastic-sprayed, with a salt spray resistance of at least 5,000 hours. In coastal areas, salt spray resistance is particularly important to prevent corrosion from sea breezes. In plateau areas, the module's UV resistance must be enhanced to prevent accelerated aging from strong radiation.

　　Key Parameter Matching Recommendations

　　Load Matching: If the system has inductive loads ≥5kW (such as pumps and compressors), ensure the inverter peak power is ≥30kW to handle the high current surge during startup and avoid overload shutdowns.

　　Climate Adaptability: When configuring in low-temperature northern regions (below -15°C), select a battery pack with heating function to minimize the impact of low temperatures on capacity. In hot southern regions, prioritize photovoltaic modules with a power temperature coefficient ≤ -0.4%/°C to reduce temperature-related losses in power generation efficiency.

　　Battery Life: In rainy and low-light regions (<1200 hours of sunshine per year), increase the battery capacity to 100-120kWh to ensure the system can still provide power even under 3-5 consecutive days of cloudy weather to avoid insufficient battery life.

　　The performance parameters of an off-grid 20kW solar cell system must be designed around "independent power supply reliability." When selecting a system, consider the actual load type, installation environment, and battery life requirements. Avoid simply pursuing numerical parameters and instead focus on the compatibility and stability of the entire link parameters.

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