Take for example a satellite designed to take pictures of the Earth. In this example the camera is the payload. In addition to the payload, a communication system is needed to receive instructions from ground controllers, and to transmit its pictures back to Earth. A navigation system is needed to point the camera. All of these parts share a common need for electrical power.
Satellites orbiting the Earth use solar power.
Sunlight falling on large panels covered with thousands of solar cells produce the electrical power needed by the other systems. A three foot square solar panel can produce about 100 watts of electrical power. That's enough power to run a small camera and modest communication and navigation systems. However, the intensity of sunlight fades as one travels further and further from the Sun. Our solar panel must now grow to unwieldy 15 foot square.