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Homeowner baffled after washing machine uses 3.6GB of internet data a day
(www.newsweek.com)
This is a most excellent place for technology news and articles.
Depends on what it's sending. Something like a washing machine would only need a few dozen kb/day, but something like a temperature sensor could send MBs, depending on update freq.
Regardless, I want to fully control any smart device in my home, which is why I tend to not buy smart stuff.