CCTV hard disk: how much storage you need and how many days it records

Your recorder keeps video on its hard disk until the disk is full. Then it either writes over the oldest video or, if it is set that way, stops recording. So the number of days you can look back through is a rolling window. It depends on how fast your cameras fill the disk and how big the disk is.

How fast the cameras fill it comes down to bit rate, the amount of data each camera sends every second. Bit rate goes up with resolution, because a 4 MP camera sends more than a 2 MP one. It goes up with frame rate, because 25 frames a second sends more than 15. It also depends on compression. H.265 needs roughly half the data of H.264 for a similar picture. In CP Plus's own storage calculator, the H.265 figure for a 2 MP camera is half the H.264 figure. A busy scene also sends more data than an empty corridor.

Recording type matters as well. Continuous recording writes all day. Motion recording writes only when something moves.

• 1. Find each camera's bit rate in megabits per second (Mbps). Many recorders have an information screen that shows it. On CP Plus recorders it is Main Menu, Info, BPS on a DVR and Main Menu, Operation, Information, BPS on an NVR. If you cannot find it, use the typical figures below.
• 2. Work out one camera's data for a day. One megabit per second, recorded for a full day, is 10.8 GB. So multiply the bit rate by 10.8. A camera at 1.5 Mbps writes about 16 GB a day.
• 3. Multiply by the number of cameras.
• 4. Divide the size of the disk in GB by that number. A disk sold as 4 TB is 4,000 GB. The answer is the number of days.
• Treat the answer as the best case. A recorder also uses some space for its own files, so the real number can be a little lower.

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Take 8 cameras, each 2 MP, recording all day in H.265 at 15 frames per second. CP Plus's storage calculator uses about 1.5 Mbps for that setting, which is 1,536 kbps. Each camera writes about 16.6 GB a day, so the 8 cameras write about 133 GB a day. This is how long each disk size lasts:

• 1 TB disk: about 7 days.
• 2 TB disk: about 15 days.
• 4 TB disk: about 30 days.
• 6 TB disk: about 45 days.
• 8 TB disk: about 60 days.
• With 4 cameras at the same setting, every figure above doubles. With 16 cameras, which write about 265 GB a day, a 30 day window needs about 8 TB.

Here are the same 8 cameras on a 4 TB disk with different settings. The bit rates are the ones CP Plus's calculator uses for each setting. They are planning numbers, not promises, and your cameras may send more or less.

Motion recording writes only while something moves. CP Plus's calculator lets you pick how much of the day the cameras record: 10, 20, 25, 50, 75 or 100 percent. The last three lines below show the first setup if the cameras really record for only part of the day. That is the catch. A camera that faces a road, a tree or a flickering light can trigger all day and save almost nothing, so check what a normal day actually uses before you trust the saving. Check too that playback shows the start of an event, not just the middle. Keep all-day recording on the cameras that matter most, such as a cash counter, a gate or a store room, and use motion recording on the quiet ones.

• 2 MP, H.265, 15 frames a second: 1.5 Mbps per camera, 133 GB a day, about 30 days.
• 2 MP, H.265, 25 frames a second: 2 Mbps per camera, 177 GB a day, about 22 days.
• 2 MP, H.264, 25 frames a second: 4 Mbps per camera, 354 GB a day, about 11 days.
• 4 MP, H.265, 15 frames a second: 1.9 Mbps per camera, 166 GB a day, about 24 days.
• 8 MP, H.265, 15 frames a second: 3.7 Mbps per camera, 320 GB a day, about 12 days.
• 2 MP, H.265, 15 frames a second, recording half the day: about 60 days.
• The same, recording a quarter of the day: about 120 days.
• The same, recording a tenth of the day: about 300 days.

CP Plus recorders have a built-in estimate. Go to Main Menu, Storage, Rec Estimate. For each channel you set the resolution, frame rate, bit rate and record time. Then the By Space tab (called Known Space on an NVR) shows how long the disk you pick will record, and the By Time tab (Known Time on an NVR) shows how big a disk you need for the days you want.

CP Plus also has an HDD and bandwidth calculator on its website, at cpplusworld.com/hdd-calculator. You enter the number of cameras, compression, resolution, frame rate, how much of the day is motion, and the disk size, and it gives the recording days. Other camera brands offer similar calculators.

A recorder writes to its disk all day, every day. A desktop drive is built for lighter use. Seagate says it plainly: using a regular desktop hard drive for surveillance is not recommended, because desktop drives are designed for intermittent use, not continuous 24/7 operation. Western Digital says an average desktop drive is built to run for only short intervals, not the 24/7 always-on environment of a high-definition surveillance system.

Both companies sell drives made for recorders, WD Purple and Seagate SkyHawk. Both list a workload rating of up to 180 TB of data written a year, which each describes as three times a desktop drive, and both say a drive supports up to 64 cameras. Check the specification sheet of whichever drive you buy.

Before you buy, check your recorder's manual or datasheet for the largest disk it takes and how many disks fit. CP Plus's quick start guides say to use the recommended HDD from the manufacturer, and some CP Plus recorders are sold as 'without HDD', so you add the disk yourself. Switch the recorder off and unplug it before you open the cover.

Think about how long it takes you to find out something went wrong. A shop that closes for a long weekend needs at least that many days. A site that counts stock once a month needs about 30. A claim reported late needs the days in between.

How long you are required to keep video depends on where you are and on your business. Ask a lawyer.

Your real number may not match the sum, so look at the recorder.

• 1. Open the playback screen. On CP Plus recorders that is Main Menu, Playback on an NVR and Main Menu, Video on a DVR.
• 2. Pick a camera and look at the calendar. On CP Plus recorders a date with video has a solid dot under it.
• 3. Go back to the oldest date with a circle. Count the days from there to today. That is how far back you can look. Check a few cameras, because they can differ.
• 4. If the oldest date is much newer than your sum says, go to Main Menu, Storage, Basic. The Auto-Delete Old Files or Delete Expired Files setting may be set to fewer days than the disk could hold.
• 5. On the same screen, check what happens when the disk is full. Overwrite makes the recorder write over the oldest video, which is what you want. Stop makes it stop recording when the disk is full, which leaves a gap with nothing recorded.
• 6. To see the disk itself, go to Main Menu, Info, HDD on a DVR or Main Menu, Operation, Information, HDD on an NVR. It shows total space, free space and status. A disk that is nearly full is normal when overwrite is on.

A disk can fail without warning, but there are signs to look for. Most recorders can raise an alarm for no disk, disk error and low space. On CP Plus recorders these are HDD events in the abnormality settings, so switch them on. The light marked HDD on the front also turns red when a disk error occurs or free space is low.

• The recorder reports a disk error or no disk, or the status on the HDD screen is not working normally.
• There are gaps in playback, or the oldest date is later than it used to be.
• The recorder makes clicking or grinding noises, or it freezes or restarts when you open playback.
• A disk check shows problems. On a CP Plus NVR go to Main Menu, Storage, HDD Detect, Manual Detect. On a DVR go to Main Menu, Storage, Disk Check, Manual Check. Then open the detect or check report, which shows the S.M.A.R.T. report, the health record the drive keeps about itself. Key area detect checks the video already on the disk. Global detect scans the whole disk, takes a long time and can affect recording. These checks are on some models only.
• Some NVRs also have HDD Health Detection under Main Menu, Operation, Information. CP Plus says it supports Seagate SkyHawk drives of 4 TB and higher.
• If you think the disk is failing, do not wait. Copy off any video you must keep. CP Plus recorders can save recordings to a USB pen drive. Then replace the disk with a surveillance drive the recorder supports. A new disk is formatted from the recorder's own menu before it records, which erases it, so do that before it holds anything you need.

• Use H.265 if the recorder and cameras offer it. CP Plus's NVR manual lists H.265 as the recommended setting.
• Lower the frame rate on cameras that watch slow scenes. 15 frames a second is plenty for many views.
• Turn on the smart codec option if it is there. CP Plus describes it as reducing the bit stream for non-important video to save space. On a CP Plus NVR it is under Main Menu, Camera, Encode.
• Lower the resolution or bit rate on cameras that do not need detail.
• Use motion recording on quiet cameras, keeping all-day recording where it counts.
• Add a second disk if the recorder has room for one.