Since Animorphs take place mainly on earth, our main characters use the everyday power sources with which we are all familiar. The Andalite in the Animorphs is very familiar with nuclear power, including both fussion and fission, so we can safely assume that nuclear power is used for at least some of the Andalite's technology. Since the Yeerks copied all their technology from the Andalites, we can also assume they use nuclear power to some degree. Since they have sneered at our "primitive fossil fuels", and show no signs of using other power sources that we may use in the future (like solar or wind power), they have probably either discovered a new power source that nobody has mentioned, are unwilling to give the details of their technology to us "primitive humans", or are just using atomic power and it is not a big enough deal to be a major conversational topic.