Question

A satellite doesn't need energy to revolve the earth . Why ?

Answer 1

The satellite doesn't need energy to revolve around earth because it gets the solar energy from the sun so it needs no force to revolve around the earth

so energy is 0

Answer 2

Here is your answer

Because they were initially provided enormous energy by their launching vehicle; enough to achieve stable orbit, but not enough to achieve escape velocity and leave Earth behind.

Since space is (mostly) empty, there is essentially no friction to slow orbiting spacecraft, with the exception of those within LEO (Low Earth Orbit), which is a region so near to Earth that the exosphere exerts enough drag to gradually decelerate objects. These thin atmospheric effects, coupled with orbital decay resultant from Earth's unneven, “lumpy” gravity, do demand that spacecraft occasionally fire their thrusters in order to remain stable.

In the case of spacecraft orbiting Earth well above LEO, occasional thruster firings are also required, not due to atmospheric factors, but gravitational perturbations by Earth's Moon.

I hope that helps