Which of the following is NOT a unit of energy?

Correct — D. Newton is the SI unit of force, not of energy: 1 N = 1 kg·m/s².

Energy is force acting through a distance, so the energy unit is the newton-metre, which is given a name of its own — the joule.

The other three all measure energy: the joule directly, the kilowatt-hour as power multiplied by time, the calorie as heat.

Why the others are wrong

• (a)
  Joule — Joule is the SI unit of energy itself. One joule is one newton-metre, the work done when a force of one newton moves its point of application through one metre.
• (b)
  Kilowatt hour — Kilowatt hour is power multiplied by time, which is energy: 1000 W × 3600 s = 3.6 megajoules. It is the unit an electricity bill is written in.
• (c)
  Calorie — Calorie measures heat, which is a form of energy — roughly the heat needed to raise one gram of water by one degree Celsius, about 4.2 joules.

Every derived SI unit comes from a defining equation, and writing that equation is the fastest way to sort a list of units.

Force comes from F = ma, so the newton is kg·m/s². Energy comes from work = force × distance, so the joule is kg·m²/s². Power is energy per second, so the watt is J/s.

Once the dimensions are in front of you, an intruder in a list of energy units is visible without any memorised table.

Key facts

• 1 newton = 1 kg·m/s², the SI unit of force.
• 1 joule = 1 newton-metre = 1 kg·m²/s², the SI unit of energy and of work.
• 1 kilowatt-hour = 3.6 megajoules, since 1000 W × 3600 s.
• The calorie is a heat unit of about 4.2 joules.

Common traps

• Calling the kilowatt-hour a unit of power because the word kilowatt is inside it.
• Rejecting the calorie because it is non-SI when the question only asks for a unit of energy.
• Seeing newton-metre as torque and stopping there — the newton on its own is still force.

No directly related past PYQ was found.