The prayer of a person in right relationship with God is effective

Scripture reference

James 5:16–18 (ASV 1901)

16Confess therefore your sins one to another, and pray one for another, that ye may be healed. The supplication of a righteous man availeth much in its working. 17Elijah was a man of like passions with us, and he prayed fervently that it might not rain; and it rained not on the earth for three years and six months. 18And he prayed again; and the heaven gave rain, and the earth brought forth her fruit.

Stories

Realizing that they had erroneously cut off from God to focus only on controlling rampaging locusts from eating their crops, Chen called his entire family and led them to make their ways right with God before praying for a paralyzed child in the family. Their prayer was answered as the boy who could not leave his bed was out running!
For George Müller, nothing was too big or too small to be a subject of prayer. Fervently, he besought God, not only for men to give to the work but to know when to accept or not accept gifts—however large they maybe.