How high is the wattage of a MacBook 13?

If the 15 Watts is a reasonable average number, then your cost calculations are basically correct if you are using it outdoors and are ignoring your indoor environment.
If you are using the device indoors in the winter and you normally heat with electricity the net cost is zero. Every watt the computer consumes is put into heating your home offsetting your normal heating system’s operation – so it is virtually free! .
If it is the summer and you are running air conditioning, every watt you expend must be offset by 1.5 watts of your air conditioner, so figure 2.5×0.074 = or 0185€ per day.

Of course these numbers depend upon the heating system and the A/C SEER rating etc. They are just normal average values.
I heat with wood which is free so it is worth turning off electrical appliances.

Thanks for your detailed answer LuckyGuy. Haven’t thought about the heating of my room, that’s interesting. Most of the time we don’t have a heater on or A/C not all the time free.
If somebody knows more accurate about the consumption, I would appreciate!

Perhaps this chart from the University of Pennsylvania would help?

I wasn’t sure which model you meant by ‘MacBook 13’.

Here’s a chart from Apple. (sorry – that was towers, not laptops)

Found it! http://www.apple.com/environment/reports/ – there’s archived info for older models at the bottom of the page.

You are great! Registered today and I am surprised about the nice answers!

So this means when the MacBook drains up to 60W:

60W *24h = 1440Wh
1,44 * 0,25€ = 0,36€ per day.

I think it is less, because 60W is only when you are actually working on the laptop. So it should be something between.

@1stein Your 15 watt number is probably a good average for an entire day. Your display shuts off when you don’t use it and the processor goes to standby after a set amount of time.
The idea of incidentally warming your house with electricity from appliances is very significant. I live in a cold climate. There are people who heat with LP gas swapping out their incandescent bulbs with compact florescent bulbs because they use less electricity. They think they are saving money. Where I live (Western NY) on a per KWhr or BTU basis, LP gas is more expensive than electricity here! For every dollar they save on their electric bill, their propane bill goes up $1.83 ! And they pay 4x the price for the bulbs for the privilege! Also, the bulbs are supposed to last a longer time. But if you check the specs you will see that is true only if they are used with the base down, in free space, and are not turned on and off. People just don’t get it. Sad.

@1stein: Welcome to Fluther!

You can get a free app called “coconutBattery” it has many useful built-in tools you can use. One of them is a Battery Power Usage indicator. You can play with it & see for yourself. I’ve run a youtube video, itunes (music), as well as surf simultaneously, without hitting the 15 w mark. Good luck.