Why does the inverter output power fail to reach the rated power of the components?

A friend of mine, Mr. Zhang, who is new to photovoltaics, installed a photovoltaic power station on his roof. He used 120 340W modules, with a total module power of 40.8kW and an inverter of 40kW. After running for a few months, he checked the power generation on his mobile phone and found that the daily power generation was between 20 and 240 degrees. When checking the real-time power generation, Mr. Zhang found that the output power of the photovoltaic power station in the past few months was only about 36kW at most, and did not reach 40kW. What happened?

The output power of a photovoltaic system is generally difficult to reach the nominal power of the component. This is because the power of the component is tested under very good weather conditions and the component temperature is relatively low. Normal weather cannot meet this condition. Not all photovoltaic components can receive solar energy because they are not always at the best angle. In addition, photovoltaic systems have losses: dust, obstruction, shadows, etc. on the components, line losses such as cables, switches, and connectors; losses of equipment such as inverters, transformers, and distribution cabinets. In general, for large power stations, the maximum output power of the system may be only about 85%-90% of the rated power of the component; for small distributed power stations, the maximum output power of the system may be only about 90%-95% of the rated power of the component.

1. Solar irradiance factor

We often say that a 340W photovoltaic module has a maximum power of Pmax/W, which means that its peak power is 340W in a standard test environment. Only under standard test conditions (irradiance of 1000W/m2, battery temperature of 25°C) is the output power of the photovoltaic module the "nominal power" (340W). When the irradiance and temperature change, the power will also change. Under non-standard conditions, the output power of the photovoltaic module is generally not the nominal power, as shown in the following figure:

STC (Standard test condition): irradiance 1000W/m2, battery temperature 25℃, spectrum AM1.5. NOC t (Normal Operating Cell Temperature): irradiance 800W/m2, ambient temperature 20℃, spectrum AM1.5, wind speed 1m/S. As can be seen from the table, when the conditions change from the ideal STC environment to the actual NOCT environment, the power of the component changes greatly, such as a 330W component becomes 249W.

2. System loss factors

The key factor affecting power generation is system efficiency. The main factors considered in system efficiency include: efficiency reduction caused by dust and shadows on components, power reduction caused by component temperature, impedance matching loss caused by DC cables, efficiency reduction caused by mismatch between component series voltage and inverter voltage, MPPT tracking loss of the inverter, power loss of the inverter itself, power loss of AC cables, power loss of transformers, and many other factors.

System loss is the loss of comprehensive power generation, and the value changes according to conditions. The factors that affect the maximum power output are components and DC cables. In order to increase the output power, various losses must be minimized. After the photovoltaic installation site is determined, factors such as the installation angle of the components, component temperature, the loss of the inverter itself, and the MPPT tracking efficiency are difficult to change. However, factors such as DC cable loss, DC cable impedance matching, and component and inverter voltage matching are controllable.

1) Minimize the length of the DC cable. A DC cable is used from the module to the inverter. The length of the cable has a great impact on the power generation of the system. On the one hand, it is the loss of the DC cable itself, and on the other hand, it is the impedance matching. The inverter should be as close to the module as possible, and the DC cable should be controlled within 20 meters. The DC cables of each MPPT should be as consistent as possible.

2) The operating voltage of the components after series connection should be as close as possible to the rated voltage of the inverter to reduce the loss of the inverter.

How do you judge whether the photovoltaic system is generating electricity normally? First, look at the output power of the inverter. When the weather is particularly good, if it can reach more than 90% of the component power, it means that there is no problem with the design of the system. Then calculate the power generation and check the local average daily power generation hours. Multiply the hours by 365 and then multiply by the system efficiency to get the average annual power generation. The general system efficiency is 0.8. For example, in Guangdong, the average daily power generation hours are 3.5. For a 40kW power plant, the average annual power generation is 40*3.5*365*0.8=40880 degrees. If the power generation is within this range, it means that there is no problem with the design and installation of the system.