The word "Energy" is a noun. But in the sentence "Tired mothers were waiting," the word "tired" is an adjective describing her state of energy.
In science, we give names to the units used to describe energy, such as:
Joules (used in many scientific measurements)
There are many other units used when describing amounts of energy;
newton-meters (equals 1 Joule, but expressed as a product of force over distance)
electronvolt (eV is used for very tiny measurements of energy on the molecular scale in physics and chemistry)
ergs (for very small energy measurements in physics)
British thermal units (BTUs used to measure heat or heat content of a fuel)
kilowatt hours (kWh are used for electrical energy. Your electric bill uses these)
calories (the energy to raise one gram of water 1 degree C)
Calories (for food energy we use the same word with a capital C, which is actually kilocalories)
The description of energy is used in so many different areas of study, that different names and sizes have been established. Most are created to be an appropriate size to the science they are used in, so we don't have to deal with too many zeros.
In science, we give names to the units used to describe energy, such as:
Joules (used in many scientific measurements)
There are many other units used when describing amounts of energy;
newton-meters (equals 1 Joule, but expressed as a product of force over distance)
electronvolt (eV is used for very tiny measurements of energy on the molecular scale in physics and chemistry)
ergs (for very small energy measurements in physics)
British thermal units (BTUs used to measure heat or heat content of a fuel)
kilowatt hours (kWh are used for electrical energy. Your electric bill uses these)
calories (the energy to raise one gram of water 1 degree C)
Calories (for food energy we use the same word with a capital C, which is actually kilocalories)
The description of energy is used in so many different areas of study, that different names and sizes have been established. Most are created to be an appropriate size to the science they are used in, so we don't have to deal with too many zeros.