To first order, long exposures are probably less power-hungry - much of the sensor power is burned in the image readout, and longer exposures mean you're reading images less often.
When collecting light, an image sensor pixel isn't really using up any active power (each pixel is basically a capacitor collecting electrons generated by light hitting the silicon).
When collecting light, an image sensor pixel isn't really using up any active power (each pixel is basically a capacitor collecting electrons generated by light hitting the silicon).