Using the P3 Kill-a-Watt I got for Christmas, I know that the tower, SimpleTech NAS, monitor, printer, switch, and the UPS itself all drew a total of 1.43 KwH in 24 hours. So lets consider what it would cost to run this setup for a year.

1.43 KwH * 365 days = 521.95 KwH/year
Electric Rates for each KwH after the first 800 KwH: $0.06535
521.95 KwH/year * $0.06535 = $34.11/year or
1.43 * 30 days = 42.9 KwH/month * $0.06535 = $2.80/month