electricity production trivia

I was curious about some details of electricity production in the US, so I dug up some data.
The total production over the year in 2007 and 2008 was about 4,156 billion kWhr and 4,110 billion kWhr. ( Production was actually down in 2008 slightly from 2007, mostly in the last half of the year )
The maximum instantaneous production in the US in 2007 was about 1 billion kW.
There are 8760 hours in the year, so over the course of the year we run our generators at about 47% capacity.

Why is it so low? Because max capacity is needed for only a brief period in the middle of the day in the highest demand days in august.

Over the year the big 3 of energy production are used at a fraction of their capacity. All the other sources are around 10% of total production ( hydro, wind, geothermal, petroleum products, etc ) so we’ll ignore them for now.
Coal runs at 73%, Natural gas is 25% and nuclear is at 92% of generating capacity.
Why? Nuclear has the highest up front cost and the lowest fuel cost, and natural gas has the lowest up front and highest fuel cost with coal in the middle.

July and August are the months with the highest electriciy demand. During August 2007 we used 58% of capacity.
April is the month with the lowest demand, during April we average using 42% of capacity.

In August, coal averages 83%, natural gas 42% and nuclear 99% of capacity.
In April, coal averages 64%, natural gas 21%, and nuclear 78% of capacity.
Why do we burn natural gas at all if we have unused coal capacity? Because coal and nuclear are slow to start and stop, you dont cycle them on and off during the day, they take hours to heat up and cool down. You only stop a coal fired boiler if you are going to idle it for a lot longer than one day.
Natural gas is used to fill in the peak of the day, while the coal and nukes probably run all 24 hours.

I have been unable to find real data on the production/consumption curve during the day.
So picking August when we run 58% capacity here are 3 possible curves for an August day.

power consumption curve estimates

power consumption curve estimates

The green line would be constant 58% through the day. We know this doesnt happen because nobody would have built that other 42% if it was so.
The peak happens sometime early afternoon, and when it gets too close to 100% or exceeds it you get brownouts. All the lines have the same area under the curve, I have no idea which one is closest to reality. The blue line bottoms out at 37% which is where you would be if you only ran nuclear and coal and everything else shut down ( which is actually unlikely: hydro, geothermal, wind and others dont shut down either )
The point of that? To show that in the hours outside of about a 4 – 6 hour range in the middle of the day there is tons of excess capacity.


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