Difference between mechanical and digital meters

The mechanical energy meter is a current meter, and the digital meter is a power meter. The readings should be consistent at standard voltage (220v). However, the allowable deviation of power supply lines in China is ±5%, that is, the voltage fluctuation between 209V and 233V is possible.

At different voltages, the readings of the two meters will be biased, mainly due to changes in the apparent power of the load. For the convenience of calculation, we use 200V power supply to make a description. Under the condition of constant load, the current of a rated power of 220 watts is 1A, and if the voltage is insufficient, it drops to 200V, the current becomes 0.91A. In this case, the power reading of the mechanical watch is 0.91 × 220 = 200 watts, which is 0.2 kWh per hour, while the power reading of the digital watt-hour meter is 0.91 × 200 = 182 watts, and the hour is 0.182 kWh. The difference between the two is 9.89%. In the case of voltages above 220 volts, the power reading of the digital energy meter is higher than that of the mechanical power meter. For example, for a 240 volt supply, the same load current is 1.1A, and the mechanical power meter has a power reading of 232 watts and 0.232 kWh per hour. The digital energy meter counts 264 watts and 0.264 kWh per hour. The difference between the two is 13.79%. Therefore, when the low voltage power supply, the user suffers, but although the calculated power of the mechanical meter is lower than the rated power, although the watch is slow, the user still suffers. In the case of high voltage, although the reading of the mechanical energy meter is higher than the rated power, it is still lower than the actual power. Although the watch goes faster, the mechanical energy meter users still earn. The basic response of the digital energy meter is the actual power used, which is consistent with the use.

To put it simply, for a mechanical watch, if the supply voltage is low, the reading on the same load meter is low, but the actual power consumption is less, and the user actually suffers. On the contrary, if the power supply voltage is too high, the meter will go faster, but the user's actual power consumption is much larger, so the user actually earns.

Take the above figures, the appliance with a rated power of 220 watts, if the power supply is 200 volts, the user only uses 182 watts, but the mechanical meter count is 200 watts, although the watch is slow, it loses 18 watts. That is, when the power supply is 200 volts, the 220 watt bulb uses 182 watts of electricity, but it has received 200 watts of money.

If it is 240 volts, the user uses 264 watts, but the mechanical meter counts 232 watts. Although the watch is fast, the user earned 32 watts. That is, at 240 volts, the 220 watt bulb used 264 watts of electricity, but only received 232 watts of money.

For a digital electricity meter, no matter how the power supply voltage changes, it reacts to the actual power consumption. Whether the reading is fast or slow, the user does not suffer.