In the Render Room instead of choosing RENDER like you normaly do. (Hitting Render button on bottom left of screen.) Hit the BATCH QUEUE Tab (Bottom Left of screen). From there look to the right where it says ADD (hit this button)and browse through your files to find the file you wish to render. Set up the scence (Rendering Parts) as normal and then hit the LAUNCH button. If the power goes out simply hit the PAUSE button and then shut down. Carrara will save up to the latest square it was working on or in animation it will save up to the latest rendered image in the sequence. This way there will be no worries about losing your image.