Electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) to give energy used (in joules, kilowatt-hours etc.). Meters for smaller services (such as small residential customers) can be connected directly in-line between source and customer. For larger loads, more than about 200 ampere of load, current transformers are used, so that the meter can be located somewhere other than in line with the service conductors. The meters fall into two basic categories, electromechanical and electronic.
Analog and digital panel meters are instruments that receive information from an input signal and measure and display that information. Panel meters are often mounted inside of instrument panels. They are frequently used to measure temperature but can also measure speed, pressure, current, and a number of other variables. Chances are that you’ve used an air conditioner that allows you to see the temperature on the instrument panel; this information is displayed with a panel meter.

While they were once widely used and still remain appropriate in certain circumstances, analog panel meters have been largely replaced with digital panel meters, which are far more accurate and easier to read. Analog panel meters can be used for measuring weight, speed, or acceleration but you’ll need a digital panel meter when what you’re measuring requires a high level of accuracy. Digital panel meters are able to measure in different scales at once, unlike their analog counterparts. Furthermore, because digital panel meters give you a digital reading you can increase accuracy by switching the output to a smaller or larger measuring unit.