Tuesday 4 December 2012

How does demand for electricity vary?

We’ve all heard about the surge in demand for electricity that comes at half time during the big match as everyone brews up a cup of tea, but how much higher is the peak demand for electricity than the lowest level on a normal day?  Have a guess.
A lot is written about the “how much” of energy use, but the “when” is also of great interest –particularly in relation to energy supplies like photovoltaic solar power and wind power that you can’t just switch on and off.
National Grid (the company in charge of getting electricity from the generator to the user in the UK) publishes detailed statistics on electricity demand.

 Electricity Demand Follows a Predictable Daily Pattern


The first graphic shows a plot of the half-hourly instantaneous power consumption of the UK electricity grid for 5th January and 29th June 2011.
Both are working days, but obviously one is in the depths of winter, and one in summer.  What’s striking is how similar the curves are. 
From about 6am, the country begins to wake up and switches on heating, kettles and toasters.  Power demand rises to about 10am as people head off for work and school, and then flattens for the rest of the working day.  The daily peak in demand follows at about 5:30pm as people arrive home and switch on domestic appliances, while offices and shops remain open.  This peak is much more pronounced in the winter as people are more likely to be indoors and have the lights and heating on.
From 5:30pm, demand falls as the offices and shops close and then people turn off lights and appliances and head off to bed.
But demand doesn’t fall so much as you might think through the night, many industrial processes continue 24 hours, and people time the use of appliances and storage heaters to take advantage of cheaper overnight electricity.
 In fact the ratio of highest to lowest demand on these two days is only 1.6.

...and a Weekly Pattern


The second chart shows the energy demand from two weeks from the summer of 2011.  A saw-tooth pattern is evident, with lower demand at weekends as offices and businesses are closed.  Weekend demand is around five to ten percent lower than week-day demand.

...and a Seasonal Pattern


The third chart shows the daily energy demand for the whole of 2011, and shows the difference between summer demand and winter demand.  Electricity demand in the winter is around 30% higher than the summer.  Summertime air-conditioning loads are more than offset by the increased use of electrical space heating, lighting, and clothes drying in winter.
So, was your guess close?  If it wasn’t you’re in good company.  A straw poll around the office yielded guesses that ranged from peak demand being six times more to a hundred times more than the lowest demand on the same day.