Measurement units of power and energy

Energy = Power x Time

This means that any indication of an amount of energy must always have a suffix that points to a certain lapse of time: seconds, or hours or years.
Energy and power are two totally different concepts. Wrong use can lead to great misunderstandings.

The basic units are:

• For energy: Joule
• For power: Joule per seconde or Joule/sec. This is also called a Watt
• So 1 Watt is 1 Joule/ seconde or 1 J/sec.

For greater and much greater measurement units the following decimal prefixes are used:

De meest gebruikte achtervoegsels die de tijdsduur aanduiden zijn:

1 hour is 3,600 seconds en 1 year is 8,760 hours

For transforming amounts of energy expressed in Joules it is convenient to know:

1 PJ = 31,7 MW year

When the amounts of energy are expressed in kWh's, the numbers are normally very high, and will become much easier to imagine by transforming them into kilowatt-years or megawatt-years. At the same time, it will become possible to show in one glimpse the average power with which this energy was produced over a year's time. This is why it is always recommendable to transform amounts of kWh's produced (or consumed) during a year into kW-years by dividing the number of kWh by 8,760. The big advantage here is that this number, in kW-years or mW-years, also points to the average power with which the amount of energy was produced or consumed during that year.

I will now give three examples of the convenience of this method:

Example 1:

We are presented with a steam turbine with an electricity yield of 4,818,000,000 kWh's (kilowatt-hours) over a certain year. Nobody will be able to conclude from this enormous number what the average power is that this turbine has produced this amount of energy with over that year. This is why one needs to divide this gigantic number by 8,760 (the amount of hours contained in a year) by means of a calculator, coming to an amount of 550,000 kilowatt-years (kWy).
And that shows immediately how that total amount of energy in that year was produced by an average power of 550,000 kW. Or 550 MW. The power of a medium-size steam turbine. And now, suddenly, the produced energy as well as the average yielding power have become intelligible numbers.

Example 2:

A wind turbine specified as a "large 3 MW" is reported to have yielded 6,570,000 kWh's of electricity during a certain year. That, again, sounds like an impressive amount. But it remains unclear how much it is exactly.
So let us transform that number into kilowatt-years again by dividing it by 8,760, working out at 750 kilowatt-years. This is how we see immediately that this wind turbine announced as a "3 MW", which means a so-called "3,000 kW wind turbine", produced electricity by an average power of 750 kW, or 0,75 MW.

Example 3 :

It is said that in 2006 and 2007, the Netherlands' electricity consumption expressed in MWh's was approximately 113.88 million MWh's. No one can get an idea of how much that really is. Therefore, let us transform the information from mWh's into megawatt-years. Again, by dividing by 8,760, the number of hours per year. That makes 13,000 megawatt-years. By omitting the suffix "years" we can see straight away that the total electricity consumption is generated by an overall average power of all supplying power plants over the year of 13,000 MW. An intelligible number from which conclusions can be drawn.

The cumulative increase over a number of years of processes that rise every year, like, for example, energy or electricity consumption, or the increase of the population, etc.

In order to anticipate the consequences of yearly increases in certain processes it is useful to dispose of the cumulative increase after a number of years for a few of these yearly rises. The following numbers are calculated for yearly increases of 2, 3 and 4 per cent after intervals of 5, 10, 15 and 25 years.

Cumulative increase in the case of a 2 % yearly increase

After   5 years 1,02 ⁵  = 1,104

After 10 years 1,02 ¹⁰ = 1,219

After 15 years 1,02 ¹⁵ = 1,346

After 25 years 1,02 ²⁵ = 1,64

Cumulative increase in the case of a 3 % yearly increase

After   5 years 1,03 ⁵  = 1,16

After 10 years 1,03¹⁰ = 1,34

After 15 years 1,03 ¹⁵ = 1,56

After 25 years 1,03 ²⁵ = 2,09

Cumulative increase in the case of a 4 % yearly increase

After   5 years 1,04 ⁵  = 1,22

After 10 years 1,04¹⁰ = 1,48

After 15 years 1,04 ¹⁵ = 1,80

After 25 years 1,04 ²⁵ = 2,66

These numbers make it clear how rash it may be not to take into account these increases after a number of years in the case of the future of some processes such as the rise of energy consumption or the population. Definitely in the case of yearly rises of 3 and 4%, that increase grows very fast after a few years. Considerably more than proportionate to the number of years.

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