2.1 Energy Definitions



2.1 Energy Definitions

What is Energy?

The scientific definition of energy is the capacity to do work. Energy is transferred from one form to another through a process, and during this process we get the outcome of useful work being done. Examples of energy processes are burning wood in a stove to keep a house warm, combustion of petrol to move a vehicle and converting the energy from food into helping us function in our daily lives.

How do we measure it?Power and Energy

We need to introduce some scientific terms so we can refer to quantities of energy simply. The main point here is to understand the difference between Power and Energy. Energy as mentioned above is the capacity to do work, whereas power is the rate of doing work.

Energy (the capacity to do work) is measured in Joules (J) or Watt-hours (Wh)

Power (rate of using or producing energy) is measured in Watt (W) or kilowatts (kW)

Energy = Power x Time or Power = Energy / time

A Note on Units

There are several different ways of quantifying energy; barrels of oil equivalent, British Thermal Units (for heat) or Calories to name a few. Watts and Watt-hours are generally used as a standard way of comparing amounts of energy.

Power and Energy Examples:




Heat energy produced by an average person sitting

1,000W (1kW)

small domestic kettle

1,000,000W (1MW)

one large wind turbine

1,000,000,000W (1GW)

the Hoover dam

1,000,000,000,000W (1TW)

The worlds power consumption in 1890 [see reference 1]




Running a 50W laptop for 4 hours


A 2 kW kettle boils water in 6 minutes


A solar panel installation averages 2kw for 12 hours

6.5 million kwh

A 2.5 MW wind turbine annual output [see reference 2]

Power density

This is the amount of power per unit area and it is measured in kW/m2.


Solar radiation represents the power density coming from the sun in the form of electromagnetic radiation (energy) that falls on a surface. During summer months at noon, on a clear day, power density values from as much as 900W/m2 to 1000 W/m2 can be measured. [see reference 3]


This is the amount of energy stored in a given region or space per unit volume or mass.


The energy of a barrel of oil (158.9 litres) is 4392 kWh. Its energy density is therefore

27640 kWh/m3. [see reference 3]


The second law of thermodynamics states that when energy is converted from one form to another some of this energy will be lost. Another way of stating this is that in any energy process entropy (or disorder) is increased, which means that it is impossible to have a completely 100% efficient process. Even if all of the energy in a closed system is converted from one form of energy into another, the useful energy is less than the total. Energy which is not useful may take the form of heat loss, or may result in sound waves etc. The measure of how much energy you put in to how much come out is known as the conversion efficiency.


A thermal power station is able to extract 1691kWh of electrical energy from a barrel of

petroleum (which contains 4392kWh of energy). The station's conversion efficiency is

38.5%. [see reference 3]


Commercial photovoltaic modules can reach efficiencies of 18%, i.e. if the incident

radiation is 1000 W/m2 it will produce 180 W/m2. [see reference 3]