I'm looking for somekind of antonym for "energy-intensive". I'm describing something that works well without using a lot of energy. I feel like the adjective I'm looking for also works for money.
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I suggest economical, as it can relate to money, time, or energy.
Economical: giving good value or service in relation to the amount of money, time, or effort spent. "a small, economical car" synonyms: cheap, inexpensive, low-cost, budget, economy, economic;
I also tend to think of "economic" as being more related to money or business than "economical," which is why I wouldn't suggest "economic" in your case.