The energy the cameras use depend on the model of cameras. A cheap CMOS sensor unit like in many low end webcams uses just a few dozen milliwatts. A CCD based camera like in a better webcam or point and shoot camera may use over a few dozen up to a few hundred milliwatts. A camera with a wireless function is going to use a few hundred milliwatts at least. A camera with a mount that can do any of the following -- focus / zoom / pan / tilt -- will use several hundred milliwatts to several watts of power. A camera including LIGHTING such as IR or visible light will use several hundred milliwatts up to several hundred watts depending on the lighting (LED vs. halogen vs. metal vapor or whatever).
Storage for storing 1 month of videos depends on the
a) resolution (320x200, old basic TV resolution, good quality analog TV resolution like 720x480 or 640x480, HDTV resolution over 1024x768 or better) , etc.
b) color depth (B&W vs color)
c) frame rate -- still pictures taken every 1 second, 10 seconds, 30 seconds, 60 seconds, only when something moves / the scene changes much, use less storage. Storing full motion video all the time at 5FPS, 10FPS, 15FPS, 30FPS, 60FPS takes much more space unless you are deleting portions of video where there is little motion / scene change activity, of course if the environment is always changing like a highway traffic camera then you'll always have lots of video to store.
d) What to get for day / night / rain / fog implies you'll do video of the outdoors. If you do that through a window and have the cameras indoors that is very different possibly than requiring cameras that are mounted in weather-proof outdoor enclosures. Actually the cameras may be similar but the weather proof camera enclosures and the cable runs for them and power runs for them / installation costs tend to cost more because of the added hardware and installation factors. No camera is going to be able to record through any substantial distance of heavy fog. Rain is usually not so much of a problem over short distances like 50 feet. For outdoor all weather use the enclosure / camera lens must be designed so that condensation / rain drops / dirt do not block the view. Such enclosures with covers / shields / environmental seals cost more of course. Night time photography requires either very sensitive CCD or hiqh quality / sensitivity CMOS sensor cameras, or active IR or visible illumination. Over distances of a few feet like a very small room or hallway you can use IR or visible LED based lighting. Over a large area of outdoors such as a part of a parking area or back yard you'd need significant artificial lighting enough to fairly brightly illuminate the area to the extent just as if you were taking a picture with a good digital handheld camera and wanted the lens size vs. exposure time to be reasonable. Typically low pressure sodium / mercury vapor lighting is economical when you need a few hundred watts per bulb or more. For larger size areas you'd typically need multiple lights over the location, and possibly multiple cameras or at least less cameras but with integrated pan and maybe zoom functions so they can sweep over the area and maintain a useful resolution detail at the maximum viewing distance.
Also for storing a month of pictures or video you will want to digitally compress the data and store it on a computer hard disk. For only a few / several cameras you can use relatively low cost PC based hardware and even many free software packages (though configuring/installing them via a consultant may not be free) to help you do the storage and retrieval. If you have more than several cameras then the system gets to be more complex and you should find some help in designing it.
Wireless cameras come in two kinds... one is an analog "video" camera that just sends wireless analog video in a typical "analog TV" NTSC/PAL signal over to a receiver to generate analog video you could record with a typical VCR or whatever. The other kind is a computerized digital camera which sends some kind of digital pictures over a wireless link which is either something non-standard or which may be standard like 802.11 wireless networking. If you have digital still pictures coming straight from the camera or digital video in a standard compressed format like MPEG-2 or Motion-JPEG or whatever then you will have some kind of PC based software to handle further storage / compression of the video or still images. Some systems offer a dedicated PC based or proprietary retrieval / storage / display system controller. Others offer custom software packages for a properly equipped PC to receive and store / replay the video / pictures.
The very best thing from a storage perspective is to keep a very limited number of pictures or full speed video portions, and to automatically delete all the times when the picture stays basically the same with little interesting changes. Some networked digital cameras or their systems can do this automatically. Sometimes it is possible to keep a short period of some minutes / hours of better speed / resolution video and then after that period reduce / delete everything that does not change interestingly and store it in a lower resolution, lower frame rate, etc.
Check out places like this for more info:
http://www.polarisusa.com/
http://www.axis.com/
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