Catalyzes the NADP-dependent reduction of long-chain acyl-ACP to the corresponding fatty aldehyde. Involved in the biosynthesis of alkanes, mainly heptadecane and pentadecane, by producing the fatty aldehydes used by aldehyde decarbonylase. a long-chain fatty aldehyde + holo-[ACP] + NADP(+) = a long-chain fatty acyl-[ACP] + H(+) + NADPH a long-chain fatty aldehyde + holo-[ACP] + NAD(+) = a long-chain fatty acyl-[ACP] + H(+) + NADH Belongs to the short-chain dehydrogenases/reductases (SDR) family.