I did the math wrong before saying 50% more (that would be 12 hours not 18 ) but 24 is 6 hours more than 18 so 33% more. As for cost, get your power bill and it should say how much per kilo watt hour. Like someone mentioned earlier- some countries charge more for what they call "peak energy", which for where I live means the time when everyone is coming home in the afternoon after work and school, turning on lights, watching TV and cooking dinner. So something like 3pm to 10 pm or something. That period costs a lot more then say 12am, when everyone is asleep and not using power.