What is Jevons paradox?

The Jevons paradox says that when a technology makes a resource more efficient to use, total consumption of that resource can rise rather than fall. It matters for energy and materials policy, because efficiency gains alone do not guarantee lower use.

How it works

Efficiency lowers the effective cost of what the resource provides, such as light, heat or travel. A lower cost raises demand, since people buy more of something that is cheaper. Two forces act in opposite directions: each unit of service needs less of the resource, but more units of service are wanted.

If demand is sensitive enough to the lower cost, the second force wins and total use of the resource goes up. If demand barely responds, total use falls. The result therefore depends on how strongly demand reacts to price, which is the thing to estimate before relying on efficiency to cut consumption.

Example

Say a car uses 10 liters of fuel per 100 km and a driver covers 10,000 km a year, using 1,000 liters. A new engine cuts use to 5 liters per 100 km, so the cost per km halves. The driver now drives 25,000 km a year. Fuel used is 25,000 divided by 100, times 5, which is 1,250 liters. Efficiency doubled, yet consumption rose by 25 percent. The numbers are an illustration of the mechanism.

How to model it in Machinations

Create a Register for efficiency set by a Lever, and a Register for cost per km that falls as efficiency rises. A Source produces driving distance at a rate that rises with lower cost through a state connection. A Converter turns distance into fuel use using the efficiency, and a Pool tracks total fuel. Play with efficiency at 10 and at 5 liters per 100 km and chart the fuel Pool to see whether the rebound outweighs the saving.