Sleeping computer vs. shutting off – power usage

Sleeping computer vs. shutting off – power usage

Hello. Sorry if this is a silly question, but I was having trouble finding anything via Google, because it wouldn't bring up exactly what I was looking for. So anyway, my roommate has a desktop Mac computer. She has a habit of *always* (and I mean always) having it on sleep. She never fully shuts it down. Does always having it on, even if it's sleeping (it goes to sleep automatically after a certain bit of time) use a lot of energy compared to shutting it down? Is this something worth calling her out on? (She claims to be really "green," but this seems sort of questionable to me as far as being "green," ha.)
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