I'm confused about optimizing 7 watts as important -- rough numbers, 7 watts is 61 kWh/y. If you assume US-average prices of $0.16/kWh that's about $10/year.
edit: looks like for the netherlands (where he lives) this is more significant -- $0.50/kWh is the average price, so ~$32/year
edit: looks like for the netherlands (where he lives) this is more significant -- $0.50/kWh is the average price, so ~$32/year