Wasteful means using more than is needed. Wasteful spending, a wasteful process, wasteful of time.
It is a criticism of the method rather than of the amount: a wasteful process throws away what it could have used. It takes of when the thing wasted is named, as in wasteful of energy. The -ful ending here means full of rather than a quantity.
1adjectivewastefulUsing more than is needed.
“It's wasteful to leave the lights on in empty rooms.”
wasteful is a quality.
Waste plus -ful, from Old French waster, from Latin vastare, to lay waste. Vast is a relative.
Middle English
wastful
English, today
wasteful
From Middle English wastful; equivalent to waste + ful.
Etymology from Wiktionary, CC BY-SA