An unmanaged HP procurve switch is rated for, as an absolute maximum, 15W. A worst-case scenario for electricity pricing on the mainland UK is 20p per kW/hour.
24 * 365 * 0.2 * 15/1000 = £26.30
I'm not sure a maximum electricity price of £25 a year is significant when considering moving to a gigabit network for home use.
A gigabit switch coupled to a considerably cheaper 100mbit adsl router, vs an all-in-one effort, looks like a really easy choice to me. Even more so if you already have a router you're happy with.