Digital scales typically use one of the following power sources:

Battery-powered: Many digital scales are powered by batteries, usually standard AA or AAA batteries. Battery-powered scales are convenient because they can be used anywhere without requiring access to a power outlet. However, the batteries need to be replaced periodically, and the scale may stop working if the batteries run out of power.

AC power adapter: Some digital scales come with an AC power adapter that allows them to be plugged into a standard power outlet. These scales don't rely on batteries and can provide continuous power as long as they are plugged in. AC-powered scales are suitable for use in locations where a power outlet is readily available and are often used in commercial or industrial settings where the scale is used frequently.

Hybrid power: Some digital scales offer hybrid power options, allowing them to be powered by either batteries or an AC adapter. This provides flexibility for users, allowing them to use batteries for portability or an AC adapter for continuous power.

As for reliability, both battery-powered and AC-powered digital scales can be reliable, but it ultimately depends on factors such as the quality of the scale, the brand, and how well it is maintained. Here are some considerations:

Battery-powered scales are convenient for portable use but require regular battery replacement, which can be a hassle and may lead to downtime if batteries run out unexpectedly. However, high-quality batteries and energy-efficient designs can extend battery life and improve reliability.
AC-powered scales provide continuous power without the need for battery replacement, making them suitable for long-term use and environments where frequent measurements are required. However, they rely on access to a power outlet and may not be as portable as battery-powered scales.
In terms of reliability, both types of scales can be equally reliable when designed and maintained properly. It's essential to choose a reputable brand and model, follow the manufacturer's recommendations for maintenance and calibration, and use the scale according to its intended purpose to ensure reliable performance over time.