A power indicator device (1) of a thermoelectric generator of a watch. The indicator device includes a first gas fluid reservoir (2), a second gas fluid reservoir (3), and at least one tubular conduit (4) connecting the two reservoirs. The first reservoir is placed on an upper surface of the thermoelectric generator in a watch case, while the second reservoir (3) is placed on a lower surface of the thermoelectric generator in contact with the back of a watch case. The tubular conduit is of the capillary type and comprises in an intermediate portion (9) a liquid forming a barrier to the gas fluid of the two reservoirs and allowing, in a display portion (24) visible from the outside of the watch case, to provide an indication of the power of the generator by measuring the temperature difference of the two reservoirs.