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Is there a rule in English regarding when to use the word "energy" and when to use "power"?

For example:

  • I don't have the energy to deal with the problem now.

  • It takes a lot of brain power to understand the problem.

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Energy is expendable. Power is inherent.

If you say you don't have the power to do something, it usually implies you don't have the inherent ability (or authority) to do that.

  • I don't have the power to lift this.

  • I don't have the power to suspend you.

If you say you don't have the energy to do something, it implies you can't do it currently, but you may have in the past and/or you can in the future.

  • I don't have the energy to walk another mile (but I have the power, and can walk after I've rested for a bit)

  • I don't have the energy to argue with you (I'm able to, but I'm not feeling up to it)

Basically, no energy means you're temporarily not able. No power is more permanent.

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  • Though when used technically they tend to become interchangeable. 'The UK intends to guarantee its future energy needs by an expansion of its nuclear power sector.'
    – WS2
    Commented Apr 29, 2015 at 11:51
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    @WS2: Technically, all physics students would disagree with you. Power is the rate of doing work, definitely not interchangeable with Energy
    – Tushar Raj
    Commented Apr 29, 2015 at 12:28
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    Perhaps I should have said 'When used industrially...'. There is no doubt that we do talk about an organisation's energy needs and its power needs, interchangeably. Sometimes we will say renewable power and sometimes renewable energy.
    – WS2
    Commented Apr 29, 2015 at 14:37
  • More common in Hebrew to say I don't have power to do that than I don't have energy.
    – ronenfe
    Commented Jul 1 at 10:14
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In physics, energy is the ability to do work (or the work done), while power is rate of energy output. So if I push a boulder up a hill, I expend the same amount of energy regardless of how long it takes me. However, doing it in a shorter time period requires more power.

I know you're probably not concerned with this technical distinction. But it is good to know. For example, now you know that it is correct to say "The car is very powerful: It can accelerate from zero to sixty in three seconds," and "It required a huge amount of energy to erect the ancient pyramids."

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