No, it will not cost energy to keep you photos stored on the camera, it costs energy only at you are using the LCD screen, photographing and anything you use your camera.
Digital cameras are designed to use intermittent power, the duration that the battery works is greatly depending upon your energy management skills. You need to learn to save the battery power,
For instance:
1. Turn off the LCD and use the viewfinder
2. Minimize photo previewing
3. Do not use continuous focus when not necessary
4. Do not press the shutter button half way down for no reason.
5. Carry another battery
You can get cheap OEM battery from some website, like
http://www.canon-digital-camera-battery.co.uk/
http://www.digital-camera-batteries.org/
or get the expensive original battery from amazon
get orignal canon battery from from
http://www.amazon.com. or