The quality or state of being economical; using money or resources carefully and efficiently without waste.
From 'economical' (adjective meaning thrifty or efficient) plus the suffix '-ness' (which creates abstract nouns describing a quality or state). Ultimately from Greek 'oikonomia' (household management).
The economicalness of LED lightbulbs over incandescent ones is a perfect example of how a small change—using less electricity—compounds into massive savings over time, both for wallets and the planet.
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