Battery usage is normal even when in standby. Even though the screen is off, doesn't mean the device is off. When in standby, the iPad is doing little things, like trying to keep track of the time (we all know the iPad1 fails miserably at this), checking the internet for its apps like email, etc. Putting it in Airplane Mode will make the battery last longer. I haven't used my iPad 1 since I got my iPad 3 a month ago, yet I've kept it in standby (screen off) in airplane mode and I checked it last night and the battery is currently at 40%.
So even though the screen is off, the iPad is still ON.