Take-Back by the Numbers

Here’s how the evaluation of savings from compact fluorescent bulbs can go wrong. The root problem is the take-back effect. But evaluators amplify the problem by basing calculations on new bulbs and not on the ones replaced (since they don’t know what was replaced).

• Replace a 40-watt incandescent bulb with a 100-watt-equivalent compact fluorescent lamp (CFL).

• The CFL uses 23 watts.

• The actual savings is 17 watts.

• CFL program evaluation assumes that when a 100-watt CFL is used, it replaces a 100-watt incandescent bulb (they can’t tell, so they make this guess).

• Replacing a 100-watt bulb with a 23-watt bulb saves 77 watts.

• Calculated savings: 77 watts. Actual savings: 17 watts.

In addition, because the light is cheaper, people may leave it on more, and the savings could actually be zero. This is an extreme case, but it happened in my kitchen. Most efficiency gains are not lost to the take-back effect—at least not right away.