New MacBook Air Battery Life

Hi,

I just bought a new MacBook Air this week. Today when I left the house, the battery was around 70/80%. All of the applications were closed. When I came home this evening, my computer was at 18%. Is this because I didn't manually put it to sleep? This has never happened to me on my old computer, which was a five year old Air. Has this happened to anyone else? Is this just a defective computer? I'm very upset, I just spent a lot of my own money on something that isn't doing it's job. The battery is supposed to last up to 12 hours!

Thanks.