Is it more eco-friendly to see movies in a theater or on DVD?

Green Lantern opens in theaters on June 17. How does Slate's Green Lantern recommend watching it? Is it more eco-friendly to catch the movie in a theater or to wait for the DVD?

In a theater if you can walk there, but on DVD if you'd have to drive. At least, that's the Lantern's best guess. It would be nice to divide a commercial cinema's total energy consumption by the number of moviegoers, compare that with the energy consumption of a home theater, and declare a winner. Unfortunately, none of the theaters the Lantern contacted was forthcoming about their energy use. Either they're not tracking their consumption or they don't want to disclose it. In any case, the information vacuum means a lot of speculating and an uncertain answer.

Let's start with the projection and display technology. If you're even thinking of comparing your home movie setup with a real theater, let's assume you've chucked your cathode-ray tube and sprung for a high-definition unit. (Let's also assume that you're using the TV's built-in speakers.) The tech website CNET has helpfully averaged the energy consumption of home-theater equipment it has tested. Plasma models clock in at 301 watts, while the more miserly LCD televisions consume an average of just 111 watts. So over the course of the 105-minute Green Lantern, an average plasma TV would consume 0.53 kilowatt-hours of energy, while the LCD would use 0.19 kilowatt-hours. (A kilowatt hour, the primary measure of energy consumption over time, is what a one-kilowatt machine consumes in an hour.) Let's take the average and assume your television uses about 0.36 kwh to display the film.

DVD players consume an average of 13 watts, or 0.02 kwh over the course of Green Lantern. Streaming the movie from a wireless router onto a DVR would be slightly less efficient, pulling 40 watts, or 0.07 kwh during the film. (If you think the environmental cost of delivering your rental DVD will tip the balance, think again. As the Lantern previously explained, such shipments consume very small amounts of energy.) Nevertheless, you're probably running your DVR and router 24/7 regardless. So let's go with the DVR number.

Combining these numbers, the Lantern estimates that watching the movie at home will use roughly 0.43 kwh and be responsible for just more than half a pound of carbon dioxide released into the atmosphere.

The technology at commercial cinemas eats up lots more energy than your puny flat screen. The precise amount can vary by projection bulb and by how brightly the cinema chooses to screen the film. Usually, it falls between 3.1 and 10.5 kilowatts. A typical machine projecting at a medium brightness for the running time of Green Lantern would consume about 9.6 kwh.

The energy consumption of a theater's sound system largely depends on its age. In the last 15 years or so, manufacturers have cut the amount of energy their systems consume in half, mostly by converting electricity to noise more efficiently. The best models now consume about 1.7 kwh during the course of a 105-minute movie. Unfortunately, few theaters have adopted the new technology. More probably, your local theater is using the previous-generation system, which would use about 2.8 kwh. The combined projector and system therefore account for 12.4 kwh, and 16 pounds of CO₂.

If we're just talking about sound and light, the theater wins out as long as it can pack at least 29 people into the screening. And for a flick with such a great title, that seems like a sure thing, right?

Once we consider heating and cooling, though, the calculation gets much more complicated. Even with a known outside temperature and a desired inside climate, the energy requirements for heating and cooling vary with the efficiency of your window A/C unit, insulation, the number of people in the cinema, the size of your living room, and so on.

Even with those complicating factors, a reasonably full cinema will still probably win the comparison. Movie theaters are pretty HVAC-efficient. They have no windows and plenty of insulation to keep the sound from bugging the H&M shoppers next door.