The non-rivalness of pure public goods means that the demand for public goods must be thought of differently than that for private goods. Suppose apples sell for 50 cents each. We know that we would purchase apples as long as our marginal utility for additional apples exceeds 50 cents each. So If I would buy two at that price and you would buy 3, then market demand is 5.

Suppose instead we are interested in the demand for a shared good (another way of thinking about a public good). Even though televisions aren't typically considered public goods, within a household it is reasonable to think of them this way. Suppose you and your SO (significant other) are considering buying a big-screen TV together. If you value the TV at \$350 and your SO values it at \$400 then the total household value would be \$750. So, in your home 1 such TV would be demanded if they sell for no more than \$750. Suppose you would value a 2nd TV at \$150 and your SO would value a 2nd at \$200, then you would demand 2 TVs if the price were no mor than \$350, and so on.