Depending on your weight, you can burn between 75 to 140 calories in a 15 minute shower. A person always burns more calories by standing up which is required when showering. Additionally, the streching and moving required to shower yourself burns calories.
The lower the temperature of the shower you are taking, the more calories you will burn as your body burns calories in order to keep itself warm, thus raising your metabolism. Another aspect to consider in showering to burn calories is that drying yourself is also exercise especially if you have very thick or long hair to towel dry.