Yet another of the security industry's seemingly simple questions. And another tough reply...
Storage is consumed by the data created by each of the cameras. It is also impacted by the degree of reliability you decide to build into the system. Lastly, the amount of storage devices is impacted by the physical constraints it has, i.e. data throughput as well as camera licensing maximums.
First, and most importantly, the
data. H.264 video is created, in mpst cases, at a variable rate. As the scene contents change, so does the bit rate. The more "activity" in a scene, the higher the data bit rate, and subsequently more storage is required. Some solutions deply a CBR, or constant bit rate. CBR certainly helps to nail down the storage required for a system, but it may actually cost you more in storage because the "rule" is being made that each camera will operate at "x" bit rate or "y" bit rate where "x" and "y" are fixed. This makes for a simple storage calculation. Ask us for a storage calculation here.
When deciding on reliability, the industry is most definitely at a cross roads. Security end users have been deploying systems without RAID protection for several decades. Now that the convergence of IT and security is upon us, IT wants nothing to do with low reliability storage, so they demand a RAID implementation rather than JBOD (Just a Bunch of Disks). When deploying a RAID option, most often RAID 5 with hot spare, the system is allocated at least one hard drive that is set aside for the potential failure of an active hard drive. This allows the system to continue operating properly without data loss. As a result, there is less total storage available. In other words, if a storage array has eight hard drives that are each three terabytes, and one is set aside as the hot spare, the total usable storage is approximately seven hard drives multiplied by three terabytes which equals approximately 21 usable terabytes of storage. Check out the Fundamentals of IP Video here.
Then there is the often overlooked, or ignored, rule that limits the amount of data throughput allowed by a single server. This is a potential bottle-neck in the system. Most VMS systems will advertise 200 or 300 cameras per server. That may work just fine as far as licensing, however, once the bit rates are calculated, you'll ususally see a much lower camera count per server. This bandwidth consideration becomes even more important as the resolution of your cameras increase. For example, if you have 100 standard definition cameras, you'll likely have no problem storing the video generated on a single server. Take that same camera count and change the cameras from standard definition to Ultra 4K (UHD). Now you'll be looking at some real life data problems. Here is where bandwidth management is key! More on bandwidth reduction here.
In the end, storage calculation can be as simple as putting a few variables into a spreadsheet. The hard part is deciding or estimating what the bit rate will actually be once the cameras are deployed. You can consider a bit rate sampling if the site is existing. If this is a new construction project, you'll need to rely on your trusted adviser to assist you.
