Power usage

I hope i am posting this in the right area and that some of you more technical guys can give a bit of advice.

I have read in various threads about the amount of power used when running FM and how warm a machine can get with which i agree when it is playing which says to me it is using a lot of electric.

In normal circumstances when i am at home this is no problem as obviously i am connected to the mains but for quite long periods of the year i live on a narrowboat and rely on what power i have in the boats battery bank. I was just wondering what sort of of power my laptop would use when playing FM compared to say when just using the net, percentage wise. When i am on the boat i tend to only play for an hour or so and have not noticed a big drain on the batteries so it does not seem to be a problem, its just an interest thing really.