If the harness is hardwired into his nervous system thus allowing him to control the tentacles like one would an arm or leg...why would he need a power source for it? Otto doesnt need AAA batteries for his flesh limbs, does he?
There is a clear difference between metal arms and fleshy limbs; as a human you wouldn't have the electrical output in your nerves to be able to power the tentacles BUT this brings up an interesting possibility with Otto perhaps examining Electro and finding a way to produce enough natural power for the tentacles.