What the Storage Saving Really Depends On

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

What the Storage Saving Really Depends On

A 1080p doorbell camera streaming at 3 Mbps costs about $265 a year to record continuously, versus about $5.52 a year for capturing just 30 minutes of motion-only video a day — the chapter’s own worked example for data minimization. That comparison is offered as proof that collecting less footage is also the cheaper choice. This audit asks the question the pricing invites: does the roughly 97.9% storage saving come from the camera’s bitrate or the price of storage, or is it a property of the duty cycle alone?

Companion to the chapter Privacy and User Consent in IoT — every number here comes from that chapter.

Ada: The “Putting Numbers to It” box prices a 1080p doorbell at 3 Mbps: about $265/year for 24/7 recording, dropping to about $5.52 with 30-minute motion-only capture. The inputs are labelled illustrative, so rather than just re-add them, let me find what the saving actually depends on.

  • Daily volume: (3 Mbps x 86,400 s) / (8 x 1024) = 259,200 / 8,192 ~= 31.6 GB/day. Annual 24/7 cost at $0.023/GB: 31.6 x 365 x 0.023 ~= $265.
  • Motion-only for 30 of 1,440 minutes a day: (30 / 1440) x 265 ~= $5.52, a 1 - 30/1440 ~= 97.9% cut.
  • The useful part is the algebra: the surviving fraction is motion_cost / full_cost = (active/1440 x full) / full = active / 1440 exactly - bitrate and storage price both cancel. So 30 / 1440 = 2.08% of the cost remains no matter the resolution or the price; the ~98% saving is a property of the duty cycle alone.

That is the data-minimization thesis made precise: capturing 30 useful minutes instead of 1,440 cuts storage about 1440 / 30 = 48x regardless of how you tune quality or shop for storage - and the same logic shrinks the 86,400 / 10 = 8,640 GPS-style points per day that make “anonymous” data re-identifiable. Collecting less is the one lever that improves cost and privacy together.

Every number above is taken from the chapter’s own material and re-derived step by step.