How to measure laptop current draw?

I'm interested in quantifying how much my laptop costs to run (as I know the kWh cost). However, I need to know the wattage of my laptop in standby or usage mode. Is there some reliable software that can read current draw? I can see that on the transformer for the laptop, it reads 2.25A at 20V. However, I doubt that if I have my computer idling, it will be drawing 2.25A (am I correct in assuming that 2.25A is the max amperage this transformer can provide?). One way that I thought of, was to charge the battery to 100%, then let it run down to 75% and measure the time taken whilst idling or when

Question by John Hon on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.