Why does the power displayed by the inverter differ from that on the electric meter?

After installing the photovoltaic system, users can see how many kilowatt-hours of electricity are generated every day from the mobile phone monitoring APP or the inverter, and calculate how much money they can earn every day. But at the end of the month, when settling electricity bills with the power supply bureau, there will be some differences. Some users have less electricity, and some users have more electricity. What is going on?

In fact, the electricity read from the inverter may be different from the electricity read from the meter. Under normal circumstances, there will inevitably be losses from the inverter to the meter. According to the law of conservation of energy, the power generation of the energy meter is lower than the power generation displayed by the inverter.

In actual applications, due to the measurement error of the inverter and the measurement error of the electric energy, the meter power is higher or lower than the inverter display power, which is normal.

1. Principle of power calculation

To understand the reason, we must first understand how the power is calculated.

The formula for calculating power is:

W=UIT=I2RT

In the formula:

W—Power generation of the photovoltaic system, unit is KWH

U—Mains power voltage, unit is V

I—Output current, unit is A

T—Daily power consumption time of the load, unit is h

R—Impedance of the output cable, unit is Ω

Regardless of the type of instrument or equipment, the rules for calculating power are the same. The power is calculated based on the data collection of the three physical variables of voltage, current and time, and then different algorithms are used to calculate the power.

2. Inverter power calculation

In addition to the work of converting DC to AC in the entire photovoltaic system, all dynamic operation data of the power station are completed on the inverter side, and power calculation is also a part of it. The inverter collects voltage and current data through the built-in current sensor and voltage sampling circuit, calculates the power generation through the software algorithm, and displays it on the inverter screen and monitoring platform.

The calculation of the inverter power generation involves the detection of voltage and current. From the current sensor to the computing chip, there is also a hardware sampling and processing circuit. The accuracy of the current sensor is about ±0.5%. With the hardware sampling circuit and software calculation, the overall accuracy is about ±1%.

3. Electricity meter power calculation

At present, there are many types of electric energy meters, but the principle of measuring electricity is essentially the same. Taking the electronic electric energy meter as an example, the current and voltage sensors connected to the measured circuit are converted by the module converter and input into the microprocessor for analysis and processing, and the forward and reverse active and reactive electricity measured by time-sharing are displayed on the display screen.

Like the inverter, the measurement of the electric meter also has errors. Single-phase or three-phase electric meters will have accuracy levels of 0.2S, 0.5S, 1.0S, 2.0S, etc. on the nameplate, which means that the error is allowed within ±0.2%, ±0.5%, ±1.0%, ±2.0%. The accuracy level of the electric meter commonly used by residents is 1.0S, so there is an error of ±1%.

Inverter voltage sampling error, temperature influence, sampling error uncertainty, inverter and meter calibration coefficients, cable material thickness and length, terminal contact loss, etc. will affect the accuracy of power measurement. Therefore, there will always be more or less difference between the power displayed by the meter and the power displayed by the inverter, which is normal. Under normal conditions, it is generally believed that when the connection cable distance from the inverter to the meter is within 50 meters, the deviation between the power generation displayed by the meter and the inverter within 2% is acceptable.