A power reserve indicator is a complication that shows the wearer how much energy remains stored in the watch's mainspring — essentially a fuel gauge for a mechanical movement. As the mainspring unwinds over its operational cycle (typically 40 to 80 hours for a standard movement), the indicator moves from full to empty, warning the wearer when winding is required.

The mechanism behind a power reserve indicator samples the state of the going barrel — the drum containing the mainspring — either by measuring the rotation of the barrel arbor relative to the barrel itself, or via a differential gear arrangement that compares input and output rotation. This differential signal is transmitted to a display, most commonly a linear scale with a needle pointer, though some movements use a sector display, a retrograde hand, or a disc visible through an aperture.

Power reserve indicators have a long history in pocket watches, where they were practical necessities for instruments that might spend days in a waistcoat pocket unworn. In wristwatches they are a mixed proposition. On a manually wound watch, knowing when to wind is genuinely useful; on an automatic worn daily, the mainspring is rarely depleted and the indicator is more decorative than functional. For this reason, power reserve indicators tend to appear most often on dress watches, complicated pieces, and independently made movements where the technical exercise of building the complication is part of the point.