How does superman have powers inside?

If superman's powers come from the sun how does he have them while indoors.How does superman have powers inside?The light of the sun still filters through windows and such. However, even when he's in a pitch black room, he still retains the energy from the sun in his body. He, like most living things on this planet, can retain the suns energy in his cells. He is, in a way, like a living battery. The problems begin to occur when he's away from the sun for too long.