Energy and power units.

If you seek information from a respectable website like some of the pages of the DOE/EIA, you will be mystified perhaps by the varying units. You will find MMBtu, megawatts, megawatt-hours, perhaps gigawatts, kilowatt-hours (kWh) maybe barrels of oil, tons of coal, Mcf or MMcf of natural gas. In chattier websites and news briefings, you will encounter nonsense like "providing electricity for [N] thousand homes". Treat those with the utmost suspicion.

In Europe you stand a good chance of encountering MJ, i.e. megajoules. Wikipedia even has a site (quite helpful, actually) with megajoules, gigajoules, terajoules and petajoules.
A petajoule = 1000 terajoules = 1000,000 gigajoules = 1000 million megajoules. A megajoule (MJ) is a million Joules, while the more familiar unit one Watt = one Joule per second. The Joule is the SI (scientific international) unit for energy (also known as work). The Watt is the SI unit of power. One Watt = one Joule per second. So one Joule = one Watt.second .

Multiple power by time, to compute expended 'energy' or 'work' done.

So a kilowatt is a thousand times the power of one Watt, and a kilowatt-hour (kWh) is the energy of a thousand Watts running for one hour, or one incandescent 100W bulb running for ten hours. It probably costs you less than ten cents from your electric company.
To convert a kilowatt-hour to Joules, consider that it is a thousand Watts times 3600 seconds.
It is therefore 3,600,000 Joules, which can be written 3.6 million Joules, or 3.6 megajoules (3.6 MJ)

A gigawatt-year is the annual energy production of a typical base-load coal or nuclear power unit. A gigawatt is one million times kilowatts.
A year is 365 times 24 hours.
So a gigawatt-year is 365 (days) times 24 (hours) million kWh. which is 8760 million kWh.
In SI units, a gigawatt-year is 8760 times 3.6 million megajoules.
That's near enough 31.5 thousand million megajoules, which can be called 31.5 petajoules, or 31,500 terajoules. These are the standard units for large amounts of energy.
One petajoule equals 1 000 000 000 000 000 joules (10 to the power 15).

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