Dear Pablo,

My parents are from the typewriter generation and insist on printing every document before they read it. Their argument is that reading it on the computer uses more energy than printing it out and turning the computer off. Is this true?

It certainly depends on the document and how you print it. A one-line e-mail would not make sense to print, where a 100-page reference document that is printed double-sided may make more sense to print. But let’s try to back that up with some numbers.

My laptop uses about 30 watts (more during start-up). In the time it takes to read a page (8.5 x 11), let’s say two minutes, the computer will use 0.001 kWh (kilowatt-hours) of electricity. For a 100-page document this adds up to 0.1 kWh of electricity, costing you less than 2 cents on your electricity bill. The generation of electricity creates about 1,000 pounds of greenhouse gases per MWh (megawatt-hour), or 1 pound per kWh, depending on where your electricity comes from. This means that reading a 100-page document on your laptop causes about one-tenth of a pound of greenhouse gas emissions. Pretty small. But how does that compare to paper?

The U.S. paper industry had emissions of 17.2 MmT (million metric tons) of CO2 in 1994, the most recent year that both numbers are available. In the same year, pulp and paper production in the U.S. was 59.05 MmT (million metric tons). Dividing these two numbers gives us the amount of CO2 per unit of paper: 0.29. This means that for every pound of paper, just under 0.3 pounds of GHGs are released. A 500-page ream of 20-pound paper weighs 5 pounds, so 100 pages weighs 1 pound. If you print double-sided, you only need 50 pages, or one-quarter pound of paper. Based on the numbers calculated above, one-quarter pound of paper causes about 0.145 pounds of greenhouse gas emissions, or 45 percent more than using the laptop. And this doesn’t even include the energy used by the printer or the computer during printing, which probably adds more than one-tenth of a pound.