I tried running the fan, but in this heat it was useless, and the room didn't cool down at all.
Usage
Used as a noun phrase with だ / で: 〜では焼け石に水だ. It judges a measure as hopelessly inadequate, not merely small. Common with budgets, aid and last-minute efforts.
Similar words
雀の涙: a tiny amount (of money); describes the amount, not its uselessness.