Why is the reading of the photovoltaic electricity meter less than that of the inverter?

1-1. AC line loss at the output end of the inverter: The reading displayed on the inverter screen or monitor is the inverter’s power generation. The generated power must be transmitted according to a part of the AC cable. There is some power loss on the AC cable. The magnitude of the loss depends on materials (copper, aluminum, length, diameter, etc.). In conclusion, the key reason why the inverter reading exceeds the meter reading is AC line loss.

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1-2. There are always deviations between the meter and the inverter: According to the application situation, the measuring instrument will also have deviation levels of 0.2%, 0.5%, and 1.0%. The corresponding accuracy levels (0.2s, 0.5s, 1.0s) will be marked on the nameplate of the measuring instrument. The accuracy of civil measuring instruments is generally 1.0%. The accuracy of the commonly used inverter current sensor is about 0.5%. The deviation of the hardware sampling circuit and software development technology can reach 1%.

2. Possible reasons for excessive reading differences
2-1. According to grid requirements, the inspection period for civil meters is 8 years. Although the service life of this device can reach 10-15 years, if the device is used for more than 8 years, the failure rate will increase. If the timer is suspected to be faulty, please contact the grid company immediately for maintenance.
2-2. There are three types of inverter failures:
  • Panel/screen problem: The voltage and current at the AC end are measured with a clamp ammeter to determine if there is a large deviation in the AC current and voltage displayed on the inverter screen, and calculate the AC power to determine if the actual deviation is large.
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  • Monitoring problem: If the difference between the online monitoring display and the inverter panel display is large, first check if the monitoring is online. In addition, since the monitoring has a fixed loading interval time, it may also be inconsistent with the inverter display value. You can see the output measurement information of the previous day. If there is a large deviation, contact the manufacturer immediately.
  • Counter problem: If the current measurement accuracy of the inverter current sensor exceeds 0.5%, the quality of this module is incorrect and the manufacturer should be asked to replace it.
3. How much difference is normal?
If the measurement error of the inverter is 1% and the meter is 1%, then the normal deviation degree is 2%. Assuming that a 10KW power station transmits 40 kWh per day, a 2% deviation is a difference of 0.8 kWh per day, and the deviation for a month is about 24 kWh, which is a reasonable range.
If two adjacent power stations are 10kW and use the same components and inverters. If the contrast between the inverter or meter reading level exceeds 2%, it is likely that one of the power stations has large line losses, unreasonable lines, etc. It is best to conduct a comprehensive inspection of the power station.
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4. How to make the readings more consistent
AC cable directory: Check if there is a problem with the AC current line. The AC cable is too long, the load capacity of the AC cable does not meet the requirements, and the specification of the AC cable does not meet the engineering requirements. The difference between the inverter and the meter may be a problem with the AC cable.
Inverter metering and correction: The inverter power generation metering can be corrected by the manufacturer’s correction tool. It is not best to do this without a reason, because continuous modifications will be required in the next generation.
Summary
In short, the settlement of photovoltaic electricity bills is based on the electricity meter provided by the grid company. If the owner of the power station finds that the difference between the inverter and the grid reading is large, they must first check all the problems of the power station, and then check the equipment problems of the cable and inverter.

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