The content of this article was originally posted back in 2018 in this longer ‘An explainer about electricity demand [take 1]’ article … I’ve copied the first two sections out here today as their own discrete article given that I’ve needed to refer back to this repeatedly in the period since 2018, and will likely need to continue to so so in future.
Unfortunately there are still a significant number of people who seem to still confuse the difference between a megawatt (a measure of rate of energy production/consumption/transfer) and a megawatt hour (a measure of volume of energy, particularly electrical energy).
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Increasingly I am wondering why this is the case:
Scenario #1) Sometimes it will be a result of genuine lack of awareness;
Scenario #2) Sometimes it might be a case of wilful blindness;
Scenario #3) But also sometimes I wonder if there are cases of deliberate misrepresentation (a desire to confuse or misinform the reader).
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(A) The difference between a MW and a MWh
Given the number of times we’ve seen people confused about this over the years, back in 2018 we thought we’d start by explaining the difference between the two fundamental metrics:
1) one megawatt-hour (MWh), which is a measure of volume/quantity; and
2) one megawatt (MW), which is a rate (or a volume over a time).
To help readers visualise the difference, we’ve included the following animated image showing the analogy of flow into a bucket of water:
Image #1 – A MW measures the rate of energy supply (or consumption) and a MWh measures the volume
Does this help to clarify the difference?
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(B) Different measures of rate, and volume, of energy delivered/consumed
Since 2000 we’ve been striving to make the energy sector more understandable to a diverse range of people. This has been primarily in Australia’s National Electricity Market – but has also been elsewhere, on occasions.
Across the world there are a couple different units used to describe the same volume of energy, summed up in the following table:
| Table 1 – different units of measurement | |||
| Measures of volume/quantity | Measures of rate | ||
| 1MWh | 1 MWh = 1,000,000Wh
The “watt-hour” (symbolised Wh) is a unit of energy commonly used in the electricity sector. |
1MW | A megawatt is the rate at which energy would need to be supplied/consumed on average over an hour to deliver/consume a quantity of 1MWh. |
| 3,600MJ | 1MWh of energy is the same quantity as 3,600MJ (or 3.6GJ)
3.6GJ The “joule” is the SI unit (i.e. International System of Unit) used across many countries for energy more broadly (i.e. not so much in describing electricity). |
3,600 MJ/h | A rate of 1MW is equivalent to 3,600MJ/h (just expressed in different units).
3,600MJ/h In SI units, it is more common to see rates of energy expressed in “joules per second”. One watt is just an energy delivery/consumption rate of one joule per second. |
| ~3.4MMBtu | 1MWh of energy is the same quantity as 3,412,141 Btu
The Btu (British thermal unit) is used in a number of countries for energy more broadly where they do not follow the SI units– most notably including the USA. An added point of confusion here is that the “M” in this unit represents the Roman Numeral “M” (i.e. 1000) and not the more widely used descriptor of 1 million (i.e. 1,000,000) with SI units. |
~3.4MMBtu/h | 1MW of energy is the same rate as 3,412,141 Btu per hour |
Please do point out any errors I have made in the table above.
Perish the thought that journalists or anyone else would want to spread fake news but it is easy to inflate the contribution of humungous batteries by measuring them in MW so we can be impressed by a 500MW battery that is going to substitute for a 2000MW coal power station. Or the total MW of all the big batteries on the ground at present or in the pipeline.
Of course giving the MW number to indicate the amount of storage is like giving the length of an area without the breadth.
We just continue to have a disgraceful level of energy literacy in this country that can be traced back to a couple of generations of people who had little exposure to basic science and physics at high school. It allows much of the evangelical spin put into the public domain about big batteries and how everything can be replaced by windmills and solar panels to be swallowed hook line and sinker by an unknowing populous – oh, and how it will all be cheaper than it is today. If we want to decarbonise then the sooner the facts get out there the better so we can have a sensible conversation about cost and how it will be funded.