Your cart is currently empty!
Energy Basics
What is energy ?
In simplistic terms energy is the ability to do work. Modern civilization is possible because people have learned how to change energy from one form to another and then use that energy to do work. It may exist as potential, kinetic, thermal, electrical, chemical, nuclear, or other forms. In the International System of Units (SI), the unit of energy is the joule (J). It is a derived unit that is equal to the energy expended, or the work done, in applying a force of one newton through a distance of one metre. However, energy can also be expressed in many other units that are not part of the SI, some of these units are ergs, calories, British thermal units (BTU), kilowatt-hours (kWh) and kilocalories, which require them to be multiplied by a conversion factor to be expressed in SI units i.e the joule ( J ).
Energy is often confused with power, they are not the same thing!
Power is defined as energy per unit of time, which is the watt (W), which is can also be expressed as a joule per second. Thus, one joule is one watt-second (Ws), and therefore 3600 joules is equal one watt-hour (Wh). A joule is a very small unit of energy so it is often expressed in terms of megajoules (MJ) for example 3,600,000 J is 3.6 MJ which is the same as 1 kWh, 1 kWh is not 1 kW, however 1 kWh per hour is 1 kW i.e. 1 kW.hour divided by an 1 hour = 1 kW (the hours cancel out).
Energy is power multiplied by the run time. That is why energy is often expressed as kilowatt-hours (kWh) rather than megajoules (MJ) i.e. it is the power in kW multiplied by the run time in hours. The kWh is a convenient unit to use for energy but this unfortunately leads to confusion for many people, it does makes it very easy to calculate the amount of energy consumed by an appliance but it can also cause confusion. For example, if you were to run a 2 kW (2,000 J/s) heater for one hour (3,600 s) to heat up a room. The heater would “consume” 2 kWh of energy i.e 2 kW x 1 hour. This is a more convenient calculation than the alternative calculation expressed in SI i.e. 2,000 J/s x 3,600 s = 7,200,000 J which would usually be expressed as 7.2 MJ which coverts to 2 kWh. If you were to run the same 2000 W (2 kW) heater for 2 hours it would twice as much energy (as you might expect) i.e 2 kW x 2 hours = 4 kWh (14.4 MJ). The heaters power did’t change it just ran twice as long so it used twice the energy. It is of course the same appliance so the power remained the same. In the strict sense, it didn’t use more power”, it actually used “more energy”. The power remained the same it was just used over a longer period of time, therefore it used more energy. Alternatively, you could do the same amount (2 kWh) of “work” in half the time if say you ran a 4 kW heater for half an hour i.e 4 kW x 0.5 hours = 2 kWh. The more powerful heater would only needs to run half as long to do the same amount of useful work (heating up the room) it therefore uses the same amount of energy to do the same work. The “work done” in heating up the room is equal to the “energy used” if you assume 100%efficiency. Simplistically, the 4 kW heater would warm up the room in half the time of a 2 kW heater if you make some simplifying assumptions regarding efficiency, heat transfer etc.
Confused ?
You are not alone. Especially due to the fact that in most cases the units of energy are expressed as kilowatt-hours (kWh) and not in the more appropriate SI units i.e. joules. As previously stated the joule (the SI unit of energy) and a watt (the SI unit of power) are relatively small so they are usually expressed as mega-joules (MJ) in the case of energy and kilo-watts (kW) in the case of power. One mega-joule is 1,000,000 joules and one kilo-watt is 1,000 watts so 1,000 watt-hours is a kilowatt-hour (kWh). Both MJ and kWh are units of energy and can easily be converted using the appropriate conversion factor i.e.
1 kWh = 3.6 MJ
Example: convert 5 kWh to MJ:
5 kWh = 5 kWh × 3.6 MJ/kWh = 18 MJ
Hey dude your wrong!
Everybody knows kWh are for electricity and MJ are far gas. You can’t compare electricity with gas! Your comparing comparing apples and oranges, they’re not the same thing.
Yes they are!
They are exactly the same thing, they are both units of energy and are directly comparable after applying a conversion factor to account for the difference in units. If you ignore the differences in efficiency and use the same figures as shown in the unit conversion example above; it would takes about 5 kWh of electrical energy to heat up 100 kg of water from 22 Deg C to 65 Deg C, then same can be done with 18 MJ of gas. Energy is energy, 18 MJ (5 kWh) is 18 MJ (5 kWh) regardless of the units used; it is exactly the same amount of energy. The water doesn’t care whether if you use 5 kWh of electrical energy or 18 MJ of gas to heat it up, it will still heat up to the same temperature in the same amount of time. How long it would take is dependent on power. A 2.5. kW hot water heater would take about 2 hours to heat up the water, a 5 kW heater would take about an hour, a 7.5 kW heater would take about 40 minutes and a 10 kW heater would take about 30 minutes regardless of whether the heater uses electricity or gas as its energy source. The cost may be different due to differences in price between the energy sources, but the amount of energy required remains the same. A hypothetical cost comparison is shown below.
Energy Cost comparison example:
5 kWh of electricity @ $0.27 per kWh = $1.35
18 MJ of gas @ $0.03 = $0.54
The water doesn’t care whether you use electricity or gas but you may. The cost of heating the water directly using an electrical restive heating element at the rates shown is 2.5 times the cost of using gas to do the same thing. This example is simplistic and ignores a lot of factors like the differences in efficiency, gas is less efficient that electricity and a heat pump would be a way more efficient way to heat water than the electrical heating element in the example above. In reality almost no one would have an electric hot water heater hooked up to their regular electricity supply at $0.27/kWh, at the very least an electrical hot water heater should be connected as a controlled load “off-peak” supply which would be about $0.14/kWh this would bring the electric hot water heater cost down to about $0.70. The point isn’t whether one method of heating water is better than another or if one is more cost effective than the other, it’s just a simple example to show that energy is just energy and that you can actually compare gas to electricity in terms of energy.
Below is a great YouTube video by Technology Connections; “Power is not energy: why the difference matters” that explains it very well.
You can also easily estimate the cost and energy consumption of your appliances with this nice little interactive online calculator (link below) that estimates the energy and running costs of your appliances after you enter in the cost ($/kWh) power (W, kW, BTU/hr, hp, (ton-refrigeration) and run hours.
Hope you found the post helpful and not too confusing.