節約 and 無駄: saving resources and waste
Compare conserving resources with using them wastefully.
Conserving what you use
節約 (せつやく) means conserving money, time, or resources. 「電気を節約しています」 says that the speaker is saving electricity. It does not mean using no electricity at all.
「時間の節約になります」 means that something helps save time. 節約 is not restricted to cutting financial costs; check which resource the context concerns.
Wasteful use
無駄 (むだ) refers to something useless or needlessly consumed. 「時間を無駄にしました」 says that the speaker wasted time. The combination 無駄にする expresses wasting something.
「無駄ではありません」 denies that something is pointless. Do not reverse it into a claim of waste; preserve the direction of the evaluation.
A plan to reduce waste
「無駄な出費を減らして、節約したいです」 expresses a wish to save by reducing unnecessary expenses. It does not report successful savings yet. What counts as unnecessary depends on the speaker's purpose and circumstances.
「節約しすぎて困りました」 means getting into difficulty from economizing too much. The presence of 節約 does not ensure a positive outcome.
Does 「節約のために歩きました」 describe a plan to walk later?
Show the answer and why
No. It describes walking in the past to save resources. Saving was the purpose; walking was the completed action.