Megawatts And MegaWattHours

Posted on Sat 03/09/2019 by


By Anton Lang ~

These two similar sounding terms are perhaps the most misunderstood things in the whole electrical power generation debate, and while there are some important things in this debate, these two terms are in that small group of the most important of them all.

Firstly, the simple explanation for both terms.

Megawatts means the design specification maximum power that the generator can actually deliver. This is what I refer to as the Nameplate for the generator. The acronym for Megawatts is MW.

MegaWattHours is what that generator, while it is actually working, delivers in power to the grid over a period of time, here hours, and that period of time can be an hour, a day, or a year. The acronym for MegaWattHours is MWH.

I will explain it in a little more depth below, and show you, with the use of some graphs what the difference actually is.

What made me think of it is that this week, here in Australia, there was a proposal put forward to construct two new technology coal fired power plants in New South Wales. That proposal was for a total of 2000MW of coal fired power. I did the math for the yearly output for these two coal fired plants, and found that they will deliver power to the grid in that State, a total power (in MWH) a little more than the total delivered power from EVERY wind plant in Australia combined, FIFTY ONE of them in all, and that wind power has a total Nameplate (Megawatts) of 5661MW. So then, how can a coal fired plant deliver more MWH than ALL those wind plants when the wind plants have a Nameplate that is 2.83 times larger than those coal fired plants? I will also explain that below as well.

I might think that this proposal is more along the lines of a thought bubble to gauge public opinion, and that they most probably will not be constructed, but what it does do is to get people talking about it at last, something sorely needed in all of this latest and most current electrical power debate. The proposal did have the desired effect though, as Greens politicians went ballistic into apoplectic fits of rage, saying that this has zero chance of coming to fruition.

So then, Megawatts and MegaWattHours.

It’s an electrical principle, so it’s a little difficult to explain for the person not trained in electrical theory, hence it is so often misunderstood.

I have two graphs below which I will use to explain the difference, and as with all the images I use in my Posts, if you click on the image, it will open in a new and larger window, so you can better see the detail.

Now, for the sake of comparing ‘apples with apples’, I have used a wind plant and a single coal fired Unit both of the same Nameplate (MW) for my examples.

The wind plant is the Macarthur wind plant in the State of Victoria, and this is the largest wind plant in the Country. This plant was opened in 2013, so it is still only 6 years old. It has 140 individual wind towers at the site. Each tower has a single generator at the top of the tower, and each generator has a Nameplate of 3MW, so the total design specification, (MW) the Nameplate for this plant, is 420MW. The wind turns the three large blades out the front of the generator, and that is what drives the generator.

The single coal fired Unit I am using as my example is Unit 1 at the two Unit Millmerran power plant in the State of Queensland. This Unit is the closest I can find to the same Nameplate as for the Macarthur Wind plant, and it has a Nameplate of 425MW. This plant was first opened in 2002, so it is now 17 years old. It is a Supercritical plant, so one level of technology lower than these proposed new coal fired plants. This plant is only one of four of these Supercritical plants in the whole of Australia, and all of them are located in that State of Queensland, the others at Callide, Kogan Creek, and Tarong North. The remaining coal fired plants still operational in Australia are all either one level lower than this one, (Critical) or two levels lower, (SubCritical) the older of the currently operational coal fired plants here in Australia.

For further equal comparison, I have selected the same day for these graphs, March 6th 2019, barely three days ago now, and that date is shown at the top of each graph.

The graph at left is for the Macarthur wind plant, and as you can see from the list under that graph, all of those 51 wind plants have their boxes unticked, so the only one shown is that Macarthur wind plant, indicated on that list as Macarth1, shown near the bottom right of that image.

The graph at right is for the Millmerran coal fired plant, Unit 1, and that box is the only one ticked in that list under the graph, shown here as MPP_1, also shown near the bottom right of the image.

What both images show is the total power delivered across the whole 24 hour period of that day, 6th March 2019. I have also added the time indicator for Midday, so it shows the power being generated at that single point in time, and that is shown at the left of the main body of the graph, 374.3MW for the Macarthur wind plant, and 426.8MW for the Millmerran Unit 1. Note the scale change with each graph indicating MW, (the left vertical scale) and for the wind plant it shows a maximum of 400MW, and for the coal fired Unit, it shows a maximum of 2000MW on that left vertical axis on both graphs.

Now, (nearly) everybody knows that wind power is variable with the speed of the wind blowing at any one point in time, and the graph for Macarthur wind plant shows that variability, with the output varying up and down across the day.

With the coal fired plant, whilst ever they feed in coal at the front end, the total design specification power (MW) is delivered from the generator, so that is why that graph is a virtual straight line across the page at the maximum design specification for that Unit.

To calculate the total power delivered across the day, I took separate readings at every hour mark across the day for both the wind and coal fired graphs, and added each of them individually for a total for each graph. I then divided that total by the number of readings to give me an average. I then multiplied that average by 24 (the hours in a day) to give me a total generated, and delivered power for that day in MWH.

The total power delivered across the day by Macarthur wind plant came in at 4152MWH.

The total power delivered by Millmerran Unit one across this same day was 10200MWH.

See the difference there? The coal fired Unit with the same Nameplate (MW) generated and delivered 2.46 times the power. (MWH)

There is a relationship between the Nameplate (MW) and the generated power (MWH) and that relationship is referred to as the Capacity Factor, and that is usually calculated across the whole 12 Month period, and this is the Industry Standard. The most current average Capacity Factor for all the wind power plants in Australia across the whole year is 30%, meaning that across that year, all of those wind plants will deliver 30% of their Nameplate in total power that they all generate.

Now, for this example above, I have actually used a good day for Macarthur wind plant, because on this day, that Capacity Factor for Macarthur Wind was 41.2%, well above that year round average of 30%. That can be better shown by what happened on the following day, as these two graphs below show the output for both of these same plants for that following day, 7th March 2019.

Now, using the same maths, the total power delivered from Macarthur wind plant was 345MWH, (so, the Capacity Factor was only 3.4% for that day) while for the Milmerran coal fired Unit, it was the same total as it was the day before, 10200MW, so on this day, that coal fired Unit delivered 29.6 times the power in MWH.

So, while there is a difference between MW and MWH, that changes on a daily basis, as you can clearly see with just these two sets of graphs.

So then, that brings us to what I mentioned above, how these two proposed new tech coal fired plants will deliver more power across a year (MWH) than every wind plant in the Country, even though the Nameplate (MW) for wind power is 2.83 time larger.

This proposal for those two new coal fired plants is for USC coal fired plants, and here USC stands for UltraSuperCritical, and that is one level of technology higher than the existing most modern plants in Australia, those four SuperCritical plants in Queensland. Those new USC plants are now being referred to as HELE power plants, and that acronym stands for High Efficiency Low Emissions.

Now, unlike older technology coal fired plants, these new tech ones will operate at close to their maximum all the time that they are delivering their power. The only time they are off line is when they are scheduled for maintenance when their power output goes back to zero. Because they are so efficient, they will operate at (or close to) their total Nameplate (MW) for all the time they are on, and you can see that in both of those graphs for the Millmerran plant above, and that will be close to, or above 90% across each year for at least the first few years, and all those existing new tech plants in Queensland are still operating at or above 90% after many years of operation, and, as you can see from this Unit at the Millmerran plant, it is still delivering its maximum power after 17 years of operation.

So, with a Nameplate of 2000MW, then across the whole year these plants will deliver 15.8TWH, and here TWH stands for TeraWattHours, and a TWH is one million MWH.

Now, all of those 51 wind plants currently in Australia have a total Nameplate ofs 5661MW, so that’s 2.83 times the Nameplate for these proposed new coal fired plants. When you take that relationship between Nameplate and generated power, (the Capacity Factor) then for all these 51 wind plants, then the total generated power across the whole year is 14.9TWH.

So, these proposed new tech coal fired plants will deliver 6% more power (MWH) over each year than all the currently existing wind plants in the Country.

THAT is why the difference between Megawatts and MegaWattHours is so important.

Anton Lang uses the screen name of TonyfromOz, and he writes at this site, PA Pundits International on topics related to electrical power generation, from all sources, concentrating mainly on Renewable Power, and how the two most favoured methods of renewable power generation, Wind Power and all versions of Solar Power, fail comprehensively to deliver levels of power required to replace traditional power generation. His Bio is at this link.