What is the appropriate translation for 'energy'?
The translation of energy to Chinese is really a tricky one, or very "context sensitive". I will try to give example one by one.


体力 Body(Physical) Power


  我太累。 I'm not too tired, I just don't have as much energy as I used to do what I want to do (like play basketball for 8 hours straight).


[繁] 我太累, 體力不夠了,不復當年啊。
[简] 我太累, 体力不够了,不复当年啊。

Enerygy -&gt;  体力   body(physical) strength / body energy
            不够了 no enough / run out

            不復   not as (good as)
            當年   years ago / in the past


So 我的能力不够 should be 我的体力不够.

The following example fall in the same usage


  The players aren't lazy, they just don't have enough energy to keep up with the other team.

精神 Will Power / Brain Power


  I have other responsibilities, so I don't have the time or energy to sit beside you and manage you.
  
  [繁] 我有其他事情要處理, 沒有那麼多時間精神跟在你後面管著你。
  
  [简] 我有其他事情要处理, 没有那麼多时间精神跟在你后面管着你。


事情要处理  things that have to be taken care of / responsibilities (责任)
精神       energy / will power (mental state related)

PS

体力 is always referring to physical/body strength/state. As 体 mean body.

精神 can be referring to mental state only or both.

[繁] 他今天精神很好
He look good today.
He is in good shape today.


精力 can refer to both physical or mental state, or both at the same time. Mostly interchangeable with the above two.


  [简] 我太累, 体力 不够了,不复当年啊。
  
  [简] 我太累, 精力 不够了,不复当年啊。
  
  [简] 没有那麼多时间 精神 跟在你后面管着你
  
  [简] 没有那麼多时间 精力 跟在你后面管着你