CCTV Storage Calculator: NVR Drives, PoE and UPS

The interactive planner on this page takes camera groups (count, bitrate, recording hours, motion share), retention days, NVR drive layout, bays and drive size, PoE classes and switch budget, and UPS equipment, and shows each formula with your numbers.

Example: 8 cameras, 30 days

• 8 cameras at 4 Mbps, recording 24 hours a day, write 43.2 GB each per day and 345.6 GB together.
• 30 days of footage is 10.37TB; with a 10% planning reserve, plan for 11.4TB usable.
• With 8TB drives: 2 drives as independent disks (no protection), or 4 drives in RAID 6 (survives any 2 drive failures).
• PoE: 8 class 3 cameras need up to 103.6 W at the cameras; a switch that reserves by class sets aside 123.2 W of its PoE budget.

Decimal TB as drives are sold; binary TiB as many NVRs and operating systems show it. Bitrate is the camera’s own setting, not derived from resolution. Every estimate here is for planning: confirm bitrates, power and drive support against your equipment datasheets.

Storage per camera by bitrate (continuous recording)

GB/day = Mbps × 86,400 s ÷ 8 ÷ 1,000. Motion recording writes only part of the day: multiply by the share of time actually recorded.

Days one drive holds at 4 Mbps per camera (continuous)

Whole days, decimal capacity, no reserve. Real NVRs keep a little space for their own database; leave headroom.

Drives for 11.4TB usable (the example above)

Count of drives, with usable capacity in brackets. Check the NVR’s bay count and maximum drive size before choosing.

PoE classes: power at the switch port and at the device

IEEE 802.3af/at/bt values. The difference is the most that can be lost in 100 m of cable at full class power.

Bitrate decides storage, not resolution

Storage is bitrate multiplied by time. A camera recording at 4 Mbps for a full day writes 4 × 86,400 seconds = 345,600 megabits, which is 43.2 GB.

Resolution only sets a range. Two 4 MP cameras can record at very different bitrates depending on frame rate, codec (H.264 or H.265), the camera’s quality setting, how busy the scene is, and low light, where image noise can raise the bitrate considerably. That is why the planner asks for bitrate and treats its resolution presets as starting points. The camera’s web page or the NVR’s channel settings show the configured bitrate: use the maximum for variable-bitrate (VBR) streams if you want a worst case.

Continuous, scheduled and motion recording

Scheduled recording is modelled exactly: hours per day. Motion or event recording is modelled as the share of those hours actually written, including the pre- and post-event buffers the NVR keeps. That share is an estimate. A camera watching a busy entrance can record most of the day; one watching a back corridor may record a few percent. If you are unsure, plan with continuous recording: it is the upper bound.

Required, raw and usable capacity

The planner keeps three numbers apart. Required is the footage plus any overhead and reserve you chose. Raw is the manufacturer-rated capacity of every drive you buy, hot spares included. Usable is what the layout leaves for footage: all of it with independent disks, less with RAID.

Many NVRs write to independent disks and overwrite the oldest footage when full. That gives the most capacity, but a failed disk loses whatever was recorded on it. NVRs that support RAID 5, 6 or 10 trade capacity for surviving a drive failure; see the RAID calculator for the full arithmetic and the hot spare guide for what a spare does and does not add.

TB on the box, TiB on the screen

Drives are sold in decimal terabytes; many NVRs and operating systems show binary tebibytes. 11.4TB is 10.37 TiB: nothing is missing, it is the same number of bytes. The planner shows both. More detail in TB vs TiB.

Drive bays and drive size limits

An NVR can only use the bays it has and the drive sizes its firmware supports. Both are on the datasheet, often as "up to N SATA HDDs, up to X TB each". Enter them and the planner refuses a plan that does not fit instead of quietly suggesting one. If no size fits, the options are larger drives (if supported), a layout with less overhead, shorter retention, lower bitrates, or expansion storage.

PoE budget is not switch power consumption

A PoE switch has two power figures. Its PoE budget is how much it can deliver to cameras and access points across all ports. Its power consumption is what it draws from the wall, which includes the switch itself and losses in its power supply.

Some switches reserve the full class power for each device, whatever the device actually draws: Cisco documents this as the default for its Catalyst switches, which lower the reservation only when the device reports its needs over CDP or LLDP. Check how your switch allocates. That is why the planner budgets by class unless you choose measured draw. A class 3 camera reserves 15.4 W even if it uses 8 W; 8 of them reserve 123.2 W.

Check two things on the switch datasheet: the total PoE budget, and the highest class each port supports (802.3af, at, or bt). A PTZ camera or a heated housing may need PoE+ or PoE++ ports.

UPS: size on watts, and treat runtime as an estimate

Add the NVR, its drives, the switch’s own draw, the PoE power it delivers (converted back to wall power) and anything else on the UPS. Count PoE cameras once, through the switch. With example values (NVR 20 W, 2 drives at 6 W, switch 20 W, 76.1 W of PoE at 90% efficiency) the load is 136.6 W, so a UPS rated at least 171 W keeps it at 80% of the rating.

Size on the UPS watt rating: the VA figure alone can mislead. Runtime depends on the battery, its age, temperature and the UPS cut-off, so the planner only gives an upper bound from battery energy (216 Wh, such as two 12 V 9 Ah batteries, at 90% inverter efficiency gives at most 85 minutes here) and points you to the manufacturer’s runtime chart.

Check these on the datasheets before you buy

Cameras: configured bitrate (or maximum for VBR), PoE class, maximum power with IR and heater on.

NVR: channel count, incoming bandwidth (Mbps), bays, maximum drive size, supported RAID levels, power without drives.

Drives: capacity, operating power, and whether the NVR maker lists them as compatible.

Switch: total PoE budget, per-port class, power consumption without PoE.

UPS: watt rating and the runtime chart at your load.

Common planning mistakes

Using the main-stream resolution instead of the bitrate. Bitrate is what fills disks.

Forgetting night-time. Many cameras use more bitrate in low light; measure at night.

Mixing up TB and TiB. A shortfall of about 9% that is only a change of unit.

Counting PoE cameras twice on the UPS. Their power is already inside the switch’s load.

Treating RAID as a backup. RAID keeps recording through a drive failure; it does not protect against deletion, theft of the recorder or a failed controller.

8TB surveillance drives

In-stock surveillance-class drives of the example size, with live prices. This is a drive class, not a compatibility check: confirm the model is on your NVR maker's compatibility list.

• WD80PURZ - Western Digital Purple 8TB 5400RPM SATA 6Gb/s 128MB Cache 3.5-inch $300.00
• Seagate ST8000VX010 SkyHawk 8TB 5400RPM SATA 6Gb/s 256MB Cache 3.5-Inch Hard Drive $492.63
• ST8000VX0002 - Seagate Surveillance 8TB 7200RPM SATA 6Gb/s 256MB Cache 3.5-inch Hard Drive $583.12
• WD84PURZ - Western Digital Purple 8TB CMR SATA 6Gb/s 128MB Cache 3.5-inch Surveillance $468.42
• ST8000VE001 Seagate SkyHawk AI 8TB 7200RPM SATA $521.05
• WD8001PURP - Western Digital Purple Pro 8TB SATA 6Gb/s 7200RPM 256MB Cache 3.5-inch $537.39
• ST8000VX004 - Seagate SkyHawk 8TB 7200RPM SATA 6Gb/s 256MB Cache 3.5-inch Surveillance $555.68
• WD81PURZ - Western Digital Purple 8TB 5400RPM SATA 6G/s 256MB Cache 3.5-inch Surveillance $571.68

Shop for this plan

• NAS and surveillance drives
• PoE network switches
• UPS and battery backup

Frequently asked questions

Sources

• Texas Instruments: TPS2373 datasheet: PoE overview (IEEE 802.3bt class power) (accessed 2026-10-23)
• DigiKey: Power over Ethernet standards and capability (accessed 2026-10-23)
• Cisco: Troubleshooting Power over Ethernet (PoE), Catalyst 9200 (accessed 2026-10-23)
• APC by Schneider Electric: Watts vs VA: what’s the difference anyway? (accessed 2026-10-23)

Related tools and guides

• RAID calculator
• RAID hot spares and capacity
• TB vs TiB
• PoE switch buying guide
• APC UPS sizing guide
• UPS battery backup calculator

Updated 2026-10-22.