What the Storage Saving Really Depends On
What the Storage Saving Really Depends On
Ada re-derives this chapter’s own numbers step by step, at full precision
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.
See the relationship before changing it
The figure reads from left to right. The blue card is recorded motion time. The middle card applies this page's rule. The green card is continuous-storage share. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only recorded motion time, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 30 minutes/day.
- 2
Name the relationship. share = active minutes / 1,440 minutes x 100
- 3
Substitute with units. 30 / 1,440 x 100 = 2.08%
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change recorded motion time
Try Predict the direction of share = active minutes / 1,440 minutes x 100. Test another recorded motion time, then compare continuous-storage share.
Observe Recording fewer daily minutes cuts storage in the same proportion, independent of bitrate or price. Reset recorded motion time to 30 and compare continuous-storage share.
Explain Recording fewer daily minutes cuts storage in the same proportion, independent of bitrate or price.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
Ready: use the stated baseline inputs, then compare each displayed result.
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, a1 - 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 / 1440exactly - bitrate and storage price both cancel. So30 / 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.