According to the Aliens: Colonial Marines Technical Manual, androids (sorry, 'synthetic people') are powered by a "25 kW hydrogen fuel cell". There's no indication that they need to eat for sustenance or gain any energy by doing so.
The modern synthetic is a highly complex machine: stronger, faster and better coordinated than an average human. The basic chassis is a carbon fiber skeleton with latchment points for the artificial musculature. The muscles are vat-grown silicone colloids powered either by pumped microhydraulics or electrical stimulation. Power for the android is supplied by a 25 kW hydrogen fuel cell with a life of approximately 400 days between refueling. As in a human, the skeletal structure is inherently unstable and is effectively suspended by the musculature. No limb locking, joint motorization or gyrostabilization is present as in other anthropomorphic frames such as powerloaders; instead, the muscles must work actively to keep the chassis standing upright while active feedback systems control its stability. Though the muscles can withstand considerable wear and tear, the lack of a self-repair facility means that they eventually lose their strength and become increasingly elastic over time. It is recommended that the muscle system be overhauled on a regular basis and individual elements replaced fully every two years. Muscle layout and operation, as with certain other internal functions, are homologous with those of the human body; indeed, military synthetics are designed to be virtually indistinguishable in appearance from a human.
You might also want to note that the plan diagram of its internal 'organs' is entirely lacking anything that resembles a digestive system aside from a small container that appears to attach to the penis (and presumably the anus) to allow it to store a small amount of food during chow-time and then excrete it convincingly, should the need arises.