Agree that it's probably old batteries but there also are "phantom" draws from things like CO detectors, refrigerator, any clocks, little lights like the TV, etc.. They don't add up to a lot but they are a draw which can be a problem with old batteries or ones that have been allowed to discharge too much and too frequently. Once you get new batteries you can start learning about managing your electricity including what draws the most (usually air conditioner, microwave, blender and hair dryer) and what those pesky phantom loads actually draw. Look at the plates on all your appliances to see their amps and you'll begin to figure out what is the most problematic.
ArdraF