In Germany, my rule of thumb was that a constant 1W load is 1€/year. This year, I had to update it, it is now more something like 3€/year. The increase is so important I am still not sure if my calculations are right!
1W x 24h x 365 = 8760Wh
My current price: 0.33€/kWh (100% renewable)
8.76kWh x 0.33€/kWh = 2.89€
