With the recent cold weather Guernsey has required more electrical power than ever before with a peak demand of 94.1 megawatts (MW)* and a total daily usage of 1,700 megawatt hours (MWh) on Thursday 11th of February 2021. That’s 94,100,000 watts and 1,700,000,000 watt hours, but whilst these numbers might look big and impressive what is their importance and what do they mean? 

Firstly, what is a watt? Named after a Scottish inventor James Watt, wattage is a measurement of power and the unit is watt - W. 

Power is the demand for energy over a period of time, which for watts is one joule for one second (joule per second).

But what is a joule? Joules are a specific sized packet of energy that is a standardised measure so you can compare between different energy sources more easily, for example your mobile phone battery’s electrical energy vs a chocolate biscuit’s chemical energy. Thinking of biscuits, you might recognise joules from food labelas they are often shown along with calories, look for a number followed by “kJ”. This means you can calculate the number of biscuits it would take to charge your mobile phone battery. I will save you the bother and tell you its, quite amazingly, only about one tenth of a chocolate digestive (scarcely a bite!) 

So why watts and not biscuit power?

Watts are part of the SI unit system (French: Système international, or international system) often called the metric system. There are other measurements of power that use other measurement systems, such as horsepower or British thermal units, however these do not so neatly fit into calculations as SI units do and would make calculating your electricity bill in horsepower or biscuits rather more difficult. So, in summary, watts give us a convenient way to measure power, this is important as GEL has to both supply enough power to meet the demands of the Island and determine how much each customer has used so it can bill customers correctly 

So now we know what a watt is, but how does a kilowatt become a kilowatt hour and how does that relate to our electricity usage?  

On your electricity bill the units after the numbers of your electricity usage are kWh (kilowatt hour). Each kilowatt hour is the amount of power used by something that operates for 1 hour at 1 kilowatt power demand. An example would be a typical microwave oven which is 1,000 watts or 1 kilowatt (check the front of yours to see what the wattage is, typically 800 to 1,000 W), if that microwave was to run for 1 hour it would use 1 kilowatt hour (see GEL’s energy usage calculator for electrical appliances for cost calculations) LINK. We then measure your usage at your meter and send a bill. This principle can however be used to calculate the energy usage of anything not just electrical items.  

So here are a few energy comparisons: our chocolate digestive from before (347,000 joules) has enough power to heat 1 litre of water by 82 °C or about 0.6 chocolate digestives worth of power to make a cup of tea.  

The islands peak power demand on Thursday 11th February of 94.1 MW would require 271 chocolate digestives every second to meet the demand. The whole island demand for the same day would require a whopping 734,890 biscuits! That’s 46,138 packets or the weight of two elephants. (if you can find a sustainable method of generating electricity with chocolate biscuits please let us know!)          

How about 1 megawatt hour? What would be possible with that much power? That’s 3,600 Mega-joules of energy which is enough to lift 13,500 elephants up a flight of stairs. Or about enough energy for you to climb to the top of the Burj Khalifa (the worlds tallest building at 829.8 m tall) 16,000 times. It would be about the calories burnt in 344 marathons or 9,019 miles if you ran them back to back    

*A short note on kilo- and mega- these are factors of 1,000 that make the watt values easier to handle: a kilowatt is 1,000 watts and a megawatt is 1,000 kilowatts or 1,000,000 watts. GEL would need a much bigger Control Room and screens to fit all the zeros in if all the figures read in watts and not megawatts!