Edit: I rewrote my post. To downvoters, can you comment on your reasons? As a kind reminder:
Downvote questions that don't show any research effort or don't contain enough information to be clear and answerable. These questions may also need to be closed.
The Seebeck effect is used to power space probes that travel too far for solar panels to be sufficient. In that scenario, the heat source is provided by radioactive decay, which lasts for decades. More info on Radioisotope thermoelectric generator and the example of Voyager 2.
The voyager probes had 3 RTG - 99.6kg total without the plutonium - for a rated power of 470W. Keep in mind that this device can have a structural value and therefore an inferior net weight. Also, those are values from the 1970s.
The Seebeck effect is the first option that came into my mind to produce electricity from the waste heat, of turbofans or other areas with unwanted heat (pumps, radar?). The point is to recover wasted energy (other ideas welcome).
Electricity consumption figures of the airplane systems (link provided by Bianfable) are currently around 90kW for essential load and utility systems (basis for comparison, keep in mind these values can be reduced).