When engine powered vehicles first started out it made some sort of sense to compare their output to something that people generally knew: the output of a horse.

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For example, you might compare data on cash transactions or electricity consumption with official output figures.

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It's impossible to add up all the indirect environmental costs of a windmill, but a good proxy for this arithmetic is simply to compare the resources consumed (in dollars) with the output produced (in dollar value of the electricity).

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