The electric meter records the power consumption of the load in the building connected to the electric meter, and the unit is kWh. The electricity meter records the number of electricity consumption units, the typical unit is 1kWh. Then, the electricity user pays the electricity fee according to the electricity price set for it. Electricity distributors usually set different rates for residential, industrial and/or commercial users.
The simplest electric meter is a mechanical device with a calibration turntable that rotates when electricity is used, as shown in Figure 1. The digital electric meter in Figure 2 is more complicated and can record the time of electricity consumption in the day. This type of electricity meter can be used when the price of electricity in a day changes over time. According to the power supplier and the region, determine the type of electricity meter installed in the grid-connected photovoltaic system.
Net energy metering
Net energy metering is a method used by a power company to measure the difference between the power consumption of a certain site and the amount of power generated on the site. In a typical residential system, during the peak illumination period (usually from 10 am to 3 or 4 pm), the power generated by the system is output to the grid; in the evening hours, users use the power of the grid. If the power generation is less than the power consumption, there is no net power output, and the user pays the electricity bill for the difference to the grid. If the electricity generation is more than the electricity consumption, there will be a net surplus generation (NEG), and the grid will pay the user the NEG part of the electricity fee, or it will be converted to the next month’s bill. Net energy metering is very common in the United States, and many state power companies must provide net electricity metering for users who have installed grid-connected photovoltaic systems.
The simplest way to achieve net energy measurement is to allow mechanical meters to rotate in both directions. In this scheme, the electricity generated by the photovoltaic system either directly supplies power to the load or outputs electricity to the grid. The latter reverses the electric meter and reduces the actual amount of electricity recorded by the electric meter. In the evening, as the grid supplies power to the load, the meter will rotate forward, thereby increasing the amount of electricity recorded by the meter. In this scheme, the electric meter is an effective net input electric meter, and the user only pays for the part of the electric energy that is supplied by the grid and recorded by the electric meter.
The disadvantage of this metering scheme is that the user neither knows the exact power generation (in kWh) of the photovoltaic system, nor the exact power consumption, and does not record the electrical energy generated by the photovoltaic system that directly supplies the building load.
If the local distributor requires this kind of metering scheme, it is recommended that the installer install a separate electric meter (if the inverter does not have an electric meter), and record the exact electricity generated by the photovoltaic system. Furthermore, the system performance can be analyzed, and by comparing with the value of the output meter, it can be determined how much electrical energy is directly supplied to the building load.
Total energy metering
Total electric energy metering is a method used by power companies to separately measure the total amount of photovoltaic power generation and electricity consumption on site. This method is often used in Europe, Australia and other places where the total electricity feed-in tariff is used: the total energy meter uses two turntables (one for electricity and one for power generation); or two mechanical meters are installed, each of which can only rotate in one direction. The output meter records the power generation delivered to the grid after the photovoltaic system is sent out during the day, and the input meter records the exact power consumption from the grid.
In this scheme, the output electric energy is subtracted from the input electric energy, and the user pays for the net input (net metered electric energy); or the output electric energy and the input electric energy are priced separately, so that the user obtains a profit. Total electric energy measurement can also use electronic input and output two-way metering meters.