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I would like to ask some clarifying questions :

1) Are we looking into video storage for nest camera?

2) what is the time frame we are looking here for storage calculation?

3) Are we calculating across world or just US?

Assumptions:

1. We are looking into video storage only

2. Restrict calculation to 1 yr storage and US only

Equation = % of household using nest * storage for a day * no. of days of video stored in an year * no. of nest cameras per household

Let's calculate each part of equation separately -

Since the camera is expensive which \$300 dollar each + storage cost, not everyone can afford it

minimum = 1 house hold; max = 50M household;

% of household using nest = 5% of 100 = 5M

Generally people who use camera security install 2 cameras ; one for outdoors and 1 for indoors

no. of nest cameras per household = 2

For storage; since security camera video ~ YT video by size ; 4 min YT video = 160 MB

1 hour of Video = 160/4*60 = 2400MB ~ 2.4 GB

Nest has 5 days/10 days/30days 24*7 pricing model for video storage

on an average people use 12 days of storage 24*7 per month (geometric mean of 5 and 30)

Final storage computation in an year = % of household using nest * storage for a day * year * no. of nest cameras per household

= 5 M(% of households) * 2 (no. of cameras each household) * 2.4 * 24 GB (GB per hour * no of hours)

* 12 (only 12 days rolling video is stored for the user in an year)

= 6912 * 10^6 GB ~ 7000 PB in an year

the number seems a little high, reasons for this:

1) the % of household using nest is high

2) video quality might be a little low for nest devices compared to YT videos

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data storage is less if the cam is in a low activity area.
+1
Hi sosha
Great answer. I really enjoyed reading it. I think you did a great job of walking the interviewer through an estimate. One potential area of improvement would be to mention that the size of video is much less when there is low activity (and reduce size of video per hour to 25% of current estimate for 22 hours of the day). Also, we can consider the impact of file compression. If Nest has any compression algorithms, this will reduce the size of the video as well. Finally, note that the number you calculated is independent of "year". What you're really looking at is how much space Nest needs at any point in time and as long as you know that Nest stores 12 days of historical data, you have the duration information you need to calculate the number.