Cost of running a laptop..

[marine]

Correct. If you can afford donator, you can afford to run a laptop.

I didn't pay for it, I won it ^^

[Stephen!]

Buy a power usage unit then you can prove it to him

[HotelUser]

Buy a power usage unit then you can prove it to him

That's not a bad idea, or, couldn't you check the capacity of your battery and then calculate the amount lost after use based off the percentage remaining?