Assuming they last the predicted 10 years. Someone else linked http://www.philadelphiacontroller.org/publications/other%20r... which says:

"Issue: Contract compactor specifications on expected life of product vague and not supported by data.

"The contract included compactor specifications that included an expected life of 10 years. When questioned about data or research to support this expected life, the BigBelly Solar CEO stated the life expectancy was an estimate based on the type of products and materials used in the compactors. He further stated that the compactor had a life expectancy that was estimated similar to an automobile; in that some repairs and parts may need to be made during the normal life cycle, but full replacement may not need to be made for theyears. When asked if there was any testing data to support the 10 year life expectancy, he indicated that he was not aware of any specific data."

Thanks Dan - didn't see that in the article, but if that's really the return on capital, its a significant one. I'd poke a little bit more as to what the real opportunity cost was here (ie, did you absolutely need to spend the $2.2M to get the fuel savings, or could a more efficient system of checking them with cheap transmitting devices work), but that's context that should have been in the main article.