Fuel cell
From Kerbal Space Program Wiki
A fuel cell converts liquid fuel and oxidizer into electric charge. While the fuel cell is a simple way to generate electricity, it's consumption of valuable fuel makes it a poor choice for long-duration flights that are dependent on specific fuel levels. The fuel cell is best used for short flights that require a recharge of energy about halfway through the flight when other means of electricity generation are not available. It can also be used when the fuel is replenished by an ISRU Converter.