Is lazy the same as people with low energy? Or is lazy something entirely different?

Lazy is someone who just doesn't care or doesn't want to be active.

Low energy could be due to lack of sleep, poor nutrition, or an underlying health problem. It could also be due to stress, or having over-extended yourself.

The two things aren't the same. A lazy person may not have much energy but they typically don't care to do anything to change that. Someone who isn't lazy, but has low energy, will usually be bothered by that and try to make themselves be active, in spite of their body just wanting to go take a nap.