How Much Storage Does a Security System Actually Need?
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How Much Storage Does a Security System Actually Need?
"How long does it keep the footage?" is the question every client asks. The honest answer is: exactly as long as the drive you put in allows. Which makes it a decision you make at install time, and an annoying one to revisit later.
Here's how to size it properly.
Storage depends on bitrate, not resolution alone
This is the part most storage guides get wrong, including a lot of manufacturer charts.
Resolution alone doesn't determine file size — bitrate does. Two cameras at the same resolution can generate wildly different amounts of data depending on frame rate, bitrate settings, and how much is actually moving in the scene. A static back hallway and a busy loading dock at identical resolution are not the same storage load.
So work from the bitrate your recorder is actually configured for. The math is simple:
1 Mbps ≈ 10.8 GB per day, per camera
| Bitrate per camera | Storage per camera per day |
|---|---|
| 1 Mbps | ~10.8 GB |
| 2 Mbps | ~21.6 GB |
| 4 Mbps | ~43.2 GB |
Total storage = cameras × bitrate (Mbps) × 10.8 × retention days
Example A: 8 cameras at an average 2 Mbps, 30 days retention
8 × 2 × 10.8 × 30 = 5,184 GB ≈ 5.2 TB
Example B: 8 cameras at an average 1 Mbps, 30 days retention
8 × 1 × 10.8 × 30 = 2,592 GB ≈ 2.6 TB
These are estimates only. Actual retention varies with frame rate, codec settings, variable bitrate, scene activity, audio recording, and recorder overhead. Always round up and leave margin.
Where compression fits in
H.265 (HEVC) can meaningfully reduce storage and bandwidth requirements compared with the older H.264 standard at similar perceived image quality. But the actual savings depend on resolution, frame rate, bitrate settings, scene activity, and recorder configuration — so treat it as a lever rather than a fixed multiplier.
In practice, that lever gives you a choice: hit the same retention target on a smaller drive, or get longer retention out of the same drive. SVD's DVR line records in H.265.
The important point for quoting: don't promise a specific retention figure based on a compression assumption. Calculate from the bitrate the recorder is actually configured to use.
Motion-activated recording changes the math
If the system records only on motion rather than continuously, storage requirements drop substantially — but by an amount that depends entirely on how much motion the scene actually has. A back stockroom might record two hours a day. A retail floor might record ten.
Motion recording is a legitimate way to extend retention. Be careful about promising a specific retention number based on it, though: the figure will move with the client's actual activity, and during a busy season retention shrinks exactly when they're most likely to need it.
For clients who need guaranteed retention — insurance requirements, compliance, incident investigation — size the drive for continuous recording. It's the only way the number is reliable.
What retention period do clients actually need?
- 7–14 days: the practical minimum. Enough to catch an incident that gets reported quickly.
- 30 days: the most common request, and a reasonable default. Covers incidents discovered on a delay — inventory shrink, a complaint filed a few weeks later.
- 60–90 days: required in some industries or by some insurers. Ask before assuming 30 is enough.
The failure case is always the same: the incident happened five weeks ago, and the system only holds four.
Capacity is only half the decision
Sizing capacity gets you the right number of terabytes. It doesn't get you the right drive.
A 16-camera system writing continuously is a sustained, multi-stream workload, and a desktop-class drive with plenty of capacity will still struggle with it. Capacity and drive class are separate decisions, and you need both right. SVD DVR systems ship with WD Purple surveillance-class drives, rated for continuous multi-stream recording.
Bottom line
Calculate from bitrate, not resolution. Multiply cameras × bitrate × 10.8 × retention days, round up, and add margin. Ask the client what retention they actually need before assuming 30 days — and if they have a compliance or insurance requirement, get the number in writing before you spec the drive. Then make sure the drive is rated for the write load, not just the capacity.