An electric heater might have a power rating of 1 kW so if you leave it on for 1 hour it will use 1kWh of energy, 2 hours then 2kWh. The k here stands for kilo (1000) and the unit most normally used is kWh (1 kWh is 1 kilowatt hour and is often called 1 unit). The units are not confined to electricity, you can relatively easily calculate for example that 1kg of propane contains 13.9 kWh of chemical energy, which will be released upon combustion. It is useful to do this to compare different sources of energy with electricity.
A Watt, by the way, is named after a Scottish bloke called Watt, who lived long before all the climate change stuff, so lets blame him for all of this since he can do little about it if we do and he did help with the industrial revolution.
What about kWh, MWh, GWh and TWH? Well these are just orders if magnitude of energy, k is kilo, M is mega, G is Giga and T is Tera.
- 1 kiloWatt -1000 Wh
- 1 megaWatt - 1,000,000 Wh
- 1 gigaWatt - 1,000,000,000 Wh
- 1 teraWatt - 1,000,000,000,000 Wh
Now can we have some perspective please? What do these numbers mean?
Examples of 1 Wh
A modern LED light in your house required about 5W to power it, so if you leave it on for 12 minutes it will consume 1 Wh of energy.
Examples of 1 kWh
Thing like fan heaters require between 1 and 3 kW to power them, so if you leave it on for an hour it will have consumed between 1 an 3 kWh, depending on the setting you have the heater on.
Examples of 1 MWh
A high-speed electrical train is a good example here. On average they consume about 25 kWh (0.025 MWh) of energy for every kilometer they travel, so for 1 MWh of energy the train can travel 40 km. It is about 400 km from Newcastle upon Tyne to London, so the train would require about 10 MWh of energy.
Examples of 1 GWh
Since Watt was Scottish lets look at Scotland. The total electrical energy consumption of Scotland in 2012 was about 25,000 GWh, divide that by 365 in a year and we get 68.5 GWh per day and then by 24 hours in a day, 2.85 GWh. So to give you and idea of a GWh, Electrical energy requirement in Scotland is about 1 GWh every 20 minutes on average.
Examples of 1 TWh
Again using Scotland it has a population a little over 5 million and requires 25 TWh of electrical energy each year.
Now here is the real big number. Total energy demand in the world in 2015, this includes all sources of energy, electrical energy and burning fuels directly was 160000 TWh. This equates to 21700 kWh per person on average given that in 2015 there were about 7,350,000,000 people, With the USA using 88000 kWh per person and Africa on average 7000 kWh per person.