Mega Confusion

Today's question: Why, when following Yukon's Energy's power-generation arithmetic, do I sometimes find projects measured in megawatts and sometimes in gigawatt hours? What's the difference? For instance, as I understand it, Mayo B will produce between 5 and 15 megawatts, or 36 gigawatt hours over a year. Does this mean that the upgraded dam would produce between 5 and 15 megawatts at any given time, and that it would pump out a total of 36 gigawatt hours over the course of one year? (With 1,000 megawatts equal to one gigawatt, as I understand?) I'd still be at a loss to do the math to get from one number to the other.

This is a great question, as people are often confused by the term "watt" and its multiples (kilowatt, megawatt, gigawatt) versus "watt hour" and its multiples.

Watts, kilowatts, megawatts and gigawatts all refer to measurements of power; specifically how much power a particular piece of equipment has the ability to produce at any point in time. In the case of the Mayo hydro plant, we currently have the ability to produce five megawatts (five million watts) of power with our two existing generators. With Mayo B we'll have two new generators with the combined ability to produce an additional 10 megawatts of power.

Watt hours, kilowatt hours, megawatt hours, and gigawatt hours all refer to energy. Energy is power multiplied by time. So when one of our existing hydro generators runs for an hour (let's use our largest Whitehorse unit, a 20-megawatt generator, as an example) the power consumed is 20 megawatt hours (20 million watts x one hour) of energy.

But there's a twist. Even though that generator might have the ability to produce 20 megawatts, that doesn't mean it will do that 24 hours a day, 7 days a week, and 365 days a year. Other things come in to play. For instance, the amount it can produce over time will depend on how much water is available. In Yukon the same amount of water is not available all year round. Also, we regulate the flow of water to the generating unit so that it will only produce what is needed to meet the demand, and that fluctuates based on the time of day and the time of year. 

This is why, when Yukon Energy is referring to the size or strength of our units, we use the term megawatt, but when we are calculating how much energy will be available to customers over one year, we use gigawatt hours per year (one gigawatt hour being the same as one billion watt hours).

To convert between the two, remember that Power x Time = Energy. So if a 10-megawatt hydro unit ran for one third of an hour, it would produce 3.33 megawatt hours of energy (10 x .333). Part of doing energy planning is calculating how much energy each of our units will be able to generate over the course of a year.

I hope this helps.