I'd agree with bestbang4thebuck that an idle multichannel power amp with no incoming signal would probably consume on the order of 25W to 40W total (probably closer to 25W). Over the course of a month, a constant 25W draw is going to add up to about 18KWh. Assuming an average utility cost of $0.10/KWh (it's lower than that here, but we get our power from TVA), that's about $1.80 a month (or $0.06 a day). Even if we assume it's closer to 40W, that's still just 29KWh or $2.90/month.