How much energy does a "brick" power cord take?

Unplug it if you wish, but as I wrote the power adapter uses nowhere near a few W per hour. Brewing a single cup of coffee uses several times the energy an already charged MBP uses overnight. Sending a text message probably costs more. Washing your socks definitely costs more. I haven't verified the toilet flush comparison but I suspect it's also a lot more than leaving a MBP plugged in overnight.

The energy not used will have to be used when it is eventually charged, so it saves nothing. Power consumption is not reduced, it's merely deferred to the next day. Furthermore, energy not consumed and subsequently lost to heating your home overnight has to be made up by something else.

There is no free ride with energy usage, only "feeling good" about saving it. For many people feeling good is more important to them than actual facts. If that describes you, go ahead and unplug it.

Was just about to post but John summed it up. The energy you don't put in at night when demand is low will be outweighed by doing it by day, when demand is high.

Rethink your approach.

I said demand, not rates, best to use what I say rather than filling in the words for me.

If you want to save energy then look at your lightbulbs, each one of which uses far more energy than your Mac.

Yes, I measured it myself some time ago using a 17" MacBook Pro and verified that Apple's published values are accurate. It's barely anything. The power adapter is nearly 90% efficient, and some power loss will occur whether or not the MBP is plugged in. Even if the battery were not electrically connected to anything it will still lose charge internally, at some small rate.

Csound1 is correct, electric demand is lower overnight than during the day. Some utilities charge commensurately lower rates overnight, in some cases substantially lower, to encourage usage during off-peak periods. The electric distribution network is engineered for peak demand, so anything that forestalls having to invest in capital equipment just to meet peak demand is in every consumer's best interest.

Limiting high demand appliances such as electric water heaters or clothes dryers to overnight operation, for example, is a great idea. Anything else is a great idea too. Macs don't represent much in terms of overall energy usage but if you are going to choose charging one overnight vs. during the day, choose overnight.