How much natural gas will American consumers want to buy in a given year? You might at first think that we can answer this question by adding up the amounts each American household and business consumes in that year. But that’s not enough to answer the question, because how much natural gas Americans want to buy depends upon the price of natural gas.

When the price of natural gas falls, as it did from 2006 to 2013, consumers will generally respond to the lower price by using more natural gas—for example, by turning up their thermostats to keep their houses warmer in the winter or switching to vehicles powered by natural gas. In general, the amount of natural gas, or of any good or service that people want to buy, depends upon the price. The higher the price, the less of the good or service people want to purchase; alternatively, the lower the price, the more they want to purchase.

So the answer to the question “How many units of natural gas do consumers want to buy?” depends on the price of a unit of natural gas. If you don’t yet know what the price will be, you can start by making a table of how many units of natural gas people would want to buy at a number of different prices. Such a table is known as a demand schedule. This, in turn, can be used to draw a demand curve, which is one of the key elements of the supply and demand model.