节约的形容词用英语是 "economical"。
来源：
"Economical" 是由英语单词 "economy" 和后缀 "-al" 组成的。"Economy" 意为节约，"al" 是形容词后缀。
例句：
"This car is very economical on gas." (这辆车的油耗很省。)
"She is an economical cook and always makes sure to use up all the leftovers." (她是个善于节约的厨师，总是确保把剩菜剩饭都用完。)
"He is an economical person, he always tries to find the most cost-effective solution." (他是个节俭的人，总是试图找到最划算的解决方案。)