Have you considered a trail cam? They run off batteries, up to one year, and have a 32 GB SD card storage. PIR activation and IR illumination at night. Hunters put these out in woods to see if deer or game use the trails. After a while the hunter will come by and pull the SD card and check the pictures that the camera has taken.

We did an install for a HOA with one that wanted to get pictures of cars coming and going in the neighborhood. No power and Internet there also.

[Link: walmart.com]

Sorry, re-read your op and you state that you currently have a standard DVR installed.