What is energy return on investment (EROI), and how is it interpreted?

• A. EROI = usable energy obtained ÷ energy invested in obtaining it; higher values indicate more favorable energy sources.
• B. EROI = energy invested ÷ usable energy obtained.
• C. EROI measures only the environmental impact.
• D. EROI is always less than 1 for viable energy sources.

Answer: (A)

Energy Return on Investment compares the usable energy you get from an energy source to the energy you must invest to obtain it. It’s calculated as usable energy obtained divided by energy invested in obtaining it. When the ratio is greater than one, you end up with net energy, and a higher number means you’re getting more energy back for each unit you put in, which makes the source more favorable from an energetic standpoint. Keep in mind that the exact value depends on what you count as energy invested and usable energy, including extraction, processing, and delivery costs, so system boundaries matter. EROI does not measure environmental impact, and it isn’t limited to being below one—many viable energy sources have EROI well above one.