I have been reviewing 倹約 when the word 節約 came to my mind. They share a common kanji, they both mean "to save, to economize" (of course, after putting する). How do these words differ? I checked this link and found this difference. What are your thoughts on this one?
I agree with this explanation. 倹約 is only for saving money (in the sense of "spending less money"), whereas 節約 is used with money, energy, time, fuel or any other types of resource. 倹約(する) refers to a policy or lifestyle, and it cannot be used with an individual episode. For example, you can say クーポンで1000円節約した ("saved 1000 yen") but not 1000円倹約した.
Also, 倹約 tends to be associated with financial hardship or a frugal life. But that does not necessarily mean 倹約 itself is a negative word; 倹約家 is at least much more positive than ケチ.