The economy is, according to the Oxford Learner’s Dictionary, “the relationship between production, trade and the supply of money in a particular country or region”, so we can say, for example:
Economics is a science that studies economies and develops possible models for their functioning, e.g.
Economic, without an “s”, is an adjective meaning “connected with economy”, e.g.
Economical, on the other hand, carries a somewhat ﬁgurative meaning of “not requiring too much of something” (such as money, space, time, etc.), e.g.