NVR vs DVR and Storage Sizing: Calculating Required HDD Capacity for Frame Rate, Resolution and Retention Period

Summary

Choosing the right recording system and sizing the storage correctly are fundamental to a functional CCTV system. Too little storage and the system overwrites useful footage before an incident is reported; too much and you are wasting budget. The recording device also determines the system's capability for remote access, analytics, and expansion.

The shift from DVR to NVR reflects the broader transition from analogue to IP camera technology. DVRs accept coaxial BNC connections and record analogue or analogue-HD video. NVRs accept ethernet connections and record digital IP video. In a new installation, there is rarely a reason to choose DVR — IP cameras provide better resolution, more flexibility, and easier installation over greater distances. The only exception is retrofitting existing coaxial cable infrastructure, where an HD-analogue system (AHD, TVI, CVI) with a hybrid DVR may be more cost-effective than re-cabling.

Storage sizing is driven by four variables: bitrate (how much data each camera generates per second), number of cameras, recording hours per day, and retention period. Most modern NVRs and cameras display bitrate in the configuration menu — this is the most accurate input to the calculation. Alternatively, use manufacturer published bitrates for the camera model at the target resolution and frame rate.

Key Facts

• NVR — records IP cameras over ethernet (cat5e/6 or Wi-Fi); cameras typically PoE-powered from the NVR switch or separate PoE switch; more flexible; better analytics integration
• DVR — records analogue (BNC coaxial) cameras; useful only for retrofits of existing coaxial cable infrastructure
• Hybrid DVR/NVR — accepts both BNC and ethernet; used in mixed systems; more expensive; increasingly niche
• PoE NVR — NVR with built-in PoE switch; single ethernet run from camera to NVR carries both data and power; most practical for small to medium installations
• Channel count — NVRs are sold in 4, 8, 16, 32, 64-channel versions; match to number of cameras + 20–25% spare for future expansion
• Maximum resolution — check the NVR's maximum resolution per channel and total bandwidth; some cheaper NVRs limit resolution when all channels are active simultaneously
• RAID — some NVRs support RAID 1 (mirrored drive); if one HDD fails, footage is retained on the second; important for critical applications; adds cost
• HDD type — use surveillance-grade HDDs (WD Purple, Seagate SkyHawk); standard HDDs are optimised for desktop use (mostly reading); surveillance drives are optimised for continuous writing; standard HDDs will fail faster in CCTV applications
• H.264 vs H.265 compression — H.265 (HEVC) produces approximately half the file size of H.264 at the same quality; all modern NVRs support both; use H.265 to reduce storage requirements
• Smart encoding — some NVRs/cameras use background compression (lower bitrate for static areas, higher for motion areas); can reduce average storage by 30–50%
• Remote access — most modern NVRs support remote access via manufacturer's P2P cloud service or direct DDNS; requires internet connection and port forwarding or P2P registration
• Motion-triggered recording — only records when motion is detected; reduces storage used but can miss slow-moving intruders or result in footage gaps; continuous recording is preferred for security systems
• Retention period — set in NVR settings; NVR overwrites oldest footage when HDD is full; default overwrite when full is acceptable; alert on low storage is useful
• Cloud backup — some systems upload clips to cloud storage; useful as backup if the NVR is stolen; storage costs apply; check compliance with UK GDPR for cloud storage jurisdiction

Quick Reference Table

Bitrates vary significantly with scene complexity, frame rate, and camera quality. Verify actual bitrate in camera configuration.

Detailed Guidance

The Storage Calculation

The fundamental formula:

Storage (GB) = Bitrate (Mbps) × Cameras × 0.0075 × Hours/day × Days

Where 0.0075 converts Mbps to GB/hour (1 Mbps = 0.45 GB/hr; simplify to 0.0075 GB per Mbps per minute ≈ 0.45 GB/Mbps/hr).

More precisely:

• 1 Mbps = 7.5 GB/hour? No — 1 Mbps = 1 Megabit per second = 450 MB/hour = 0.45 GB/hour
• More precise formula: HDD (GB) = Bitrate (Mbps) × 0.45 × Hours × Cameras × Days

Worked examples:

Example 1 — 4 domestic cameras, 30 days:

• 4 cameras, 4MP each, H.265, average bitrate 3 Mbps each
• Recording 24 hours/day
• 30-day retention
• Storage = 3 × 0.45 × 24 × 4 × 30 = 3 × 0.45 × 24 × 120 = 4,860 GB ≈ 4.9 TB
• Rounding up + 25% overhead: specify 6TB HDD (one drive) or 2×4TB RAID 1

Example 2 — 8 commercial cameras, BS 8418, 31 days:

• 8 cameras, 4MP each, H.265, average bitrate 4 Mbps (higher quality for ARC verification)
• Recording 24 hours/day
• 31-day retention
• Storage = 4 × 0.45 × 24 × 8 × 31 = 4 × 0.45 × 24 × 248 = 10,771 GB ≈ 10.8 TB
• Round up + overhead: specify 2 × 8TB HDDs (RAID 1 for redundancy) or 1 × 14TB

Example 3 — 16 camera commercial, motion-only recording (12 active hrs/day):

• 16 cameras, 4MP each, H.265, 3 Mbps
• Motion-triggered recording equivalent to 12 hrs/day active recording
• 28-day retention
• Storage = 3 × 0.45 × 12 × 16 × 28 = 3 × 0.45 × 12 × 448 = 7,258 GB ≈ 7.3 TB

Frame Rate Selection

Frame rate (frames per second, fps) significantly affects both image quality and storage. Key considerations:

25fps (full rate):

• Smooth, natural motion; useful for tracking fast-moving subjects
• Maximum storage consumption
• Required for BS 8418 recommended quality at the ARC
• Standard for most commercial security applications

15fps:

• Acceptable quality for most security purposes; slight jerkiness in fast motion
• Approximately 60% of storage of 25fps (not 60% exactly — compression algorithms handle motion differently)
• Suitable for internal fixed cameras with slow-moving subjects

6fps:

• Noticeably jerky; still adequate for identification and evidence
• BS EN 62676 minimum for standard CCTV
• Approximately 25% of storage of 25fps
• Appropriate for areas with infrequent activity (storage rooms, perimeter areas)

Variable frame rate: Many modern NVRs support different frame rates per channel, and some cameras support reduced frame rate during periods of no motion (e.g. 1fps when static, 25fps on motion). This significantly reduces average storage without sacrificing critical event quality.

NVR Selection Criteria

Channel count: Never fill every channel at installation. Allow at least 20% spare capacity for future cameras. An 8-camera site needs a 10 or 16-channel NVR. Expanding the channel count later requires either purchasing a new NVR or adding a sub-NVR — expensive compared to specifying slightly more capacity upfront.

Maximum total bandwidth: Aggregate all cameras' bitrates and check against the NVR's maximum bandwidth:

• 8 cameras at 4MP, 4Mbps each = 32 Mbps total
• Check the NVR can record 32 Mbps simultaneously — some cheaper NVRs throttle bandwidth when all channels are active, reducing quality

CPU/processing power: For systems with multiple channels of high-resolution cameras plus AI analytics (person detection, vehicle detection, face recognition), processor performance matters. Entry-level NVRs struggle with simultaneous 4K decoding for display and recording. Specify NVRs from reputable manufacturers with sufficient processing power for the intended use.

Operating system: Most NVRs run embedded Linux. These are generally stable and resistant to malware. Avoid connecting the NVR to the internet with default passwords — CCTV NVRs are frequently targeted by botnets (Mirai and successors). Always:

• Change default admin password immediately
• Update firmware to latest version
• Disable UPnP on the router
• Use manufacturer P2P service rather than direct port forwarding where possible

Remote access:

• P2P cloud access (manufacturer service): easiest for customers; no network configuration needed; dependent on manufacturer service uptime
• DDNS + port forwarding: more control; requires static IP or DDNS subscription; network configuration required
• VPN: most secure; requires VPN server at site (on router or NVR); recommended for high-security applications

HDD Selection and Maintenance

Surveillance-grade HDDs:

• WD Purple: designed for 24/7 write workloads; available in 1–22TB; standard for most NVR applications
• Seagate SkyHawk: equivalent; available up to 16TB; some models include health-monitoring software
• Seagate SkyHawk AI: for AI-enabled NVRs with analytics processing; higher vibration tolerance
• WD Purple Pro: for enterprise NVRs with RAID; rated for heavy workloads

Standard desktop HDDs (WD Blue, Seagate Barracuda) are not rated for continuous write operation. They may work initially but typically fail within 1–2 years in CCTV service. The cost saving over surveillance HDDs is minimal; the cost of failure (replacement, lost footage) is significant.

HDD sizing tips:

• HDD manufacturers quote capacity in decimal GB (1TB = 1,000GB); operating systems display in binary GiB (1TB = 932GiB). Factor in approximately 7% difference when sizing.
• Leave 10–20% of the HDD unused (most NVR firmware does this automatically when set to overwrite mode)
• For 24/7 commercial applications, HDD life expectancy is 3–5 years; plan for replacement

RAID considerations:

• RAID 1 (mirroring): two identical HDDs; one mirrors the other; if one fails, recording continues; requires 2 HDDs of equal size; effectively halves the usable storage
• RAID 5: 3+ HDDs; one drive equivalent of redundancy; most efficient for larger arrays; requires 3+ HDD NVR
• For most small commercial and domestic systems, RAID is not specified — the risk of HDD failure without redundancy is accepted. For critical applications (police-monitored, evidence continuity required), specify RAID 1.

When Footage Runs Out — Retention Checks

Set a reminder to verify actual footage retention after installation:

1. After 30 days, check the NVR is still recording to the expected timeframe — if storage has run out faster than calculated, reduce bitrate or frame rate
2. Verify that overwrite mode is enabled — some NVRs default to "stop recording when full" which means the system stops recording silently without alerting the customer
3. Confirm the retention period meets the customer's requirements and any insurer specification

Frequently Asked Questions

Can I use a standard PC instead of an NVR?

Yes — PC-based software (Milestone XProtect, Blue Iris, iSpy) can record IP cameras. Advantages: more flexible, easier to expand, better analytics integration. Disadvantages: less reliable than embedded NVR (Windows updates, software crashes), requires more maintenance, higher power consumption, more expensive for the same reliability level. For professional installations, a purpose-built NVR is generally preferred. For home use with existing PC infrastructure, PC-based VMS software is a viable option.

How do I estimate storage when I don't know the camera bitrate?

Use these conservative estimates as a starting point:

• 2MP H.265: 1.5 Mbps
• 4MP H.265: 3 Mbps
• 4K/8MP H.265: 5–6 Mbps

Then add 50% buffer to account for scene complexity variation. After installation, check the actual bitrate in the camera menu and recalculate. If actual storage will run out before the retention period, reduce bitrate in camera settings (test quality before signing off).

My customer wants 90-day retention — how do I achieve this without spending a fortune?

Options to reduce storage requirements for long retention:

1. Use H.265 encoding (if not already) — halves storage vs H.264
2. Reduce frame rate for non-critical cameras to 6–8fps
3. Enable motion-based quality enhancement: full quality on motion, low quality (1fps) on static background
4. Add a second HDD (most NVRs support 2+ HDDs) rather than replacing the first
5. Use an external HDD or NAS for secondary storage with footage archive

Regulations & Standards

• BS 8418:2015 — minimum retention requirements for ARC-monitored systems; [verify in standard — typically specifies retention period for verified activations]

• UK GDPR / Data Protection Act 2018 — storage limitation principle; retain no longer than necessary; maximum period requires documented justification

• ICO CCTV Code of Practice — guidance on retention periods; typically recommends 7–31 days for most commercial applications

• BS EN 62676-1-1:2014 — recording requirements for CCTV systems; minimum frame rate at 6fps

• WD Purple HDD Specifications — surveillance HDD technical data

• Seagate SkyHawk HDD Specifications — surveillance HDD technical data

• ICO CCTV Code of Practice — retention period guidance

• IPVM Storage Calculator — independent storage calculator (subscription)

• cctv camera types selection — camera resolution and bitrate inputs for storage calculation

• bs 8418 registered cctv — retention requirements for BS 8418 monitored systems

• gdpr cctv ico obligations — legal retention limits and data subject rights

• nsi ssaib approval guide — documentation requirements for approved CCTV installers