As many of you are probably aware, I have been testing a prepaid b-mobile Fair SIM, that gives me 1 GB of data to use over 120 days (approximately 4 months). This is in addition to my normal Docomo service plan, so I use two devices, one for voice (Docomo ht-03a) and one for data (Google Nexus One).
My Docomo voice plan costs me just under 2000 yen per month, and if 1 GB lasts me the full 120 days, I would pay just over 2000 yen per month for data. At the Fair SIM recharge rate of 8400 yen per gigabyte, this would work out to 4000 yen total per month. This is about half of what I normally pay.
One month ago, based on only two weeks worth of usage data, I attempted to predict how much, if any, of my b-mobile Fair SIM charge would remain on 8/10/2011, exactly 120 days after activation. It appeared that about half would be remaining. Now that I have twice as much information for the calculation, I get essentially the same answer.
So I am now convinced that one charge, on average, will last me the full four months. Without the use of wifi at both home and work, however, it would not be possible.
Before the start of next month, I will cancel my Docomo data plan for the ht-03a, which scales from 372 to 5985 yen. I considered subscribing to NTT East's Flet's spot wifi service, which has seemingly good coverage, includes all the train platforms I use, and costs only 210 yen for month for Flet's home internet subscribers, until of course I saw that for some asinine reason, it uses PPPoE authentication, ruling out usage by both iPhones and Androids. (And check out the setup guide! [PDF, J] WTF? It's wifi folks, you don't need MAC address registration, plus PPPoE authentication, and a dedicated client.)
Next, I will begin looking into the feasibility of a VoIP solution for voice, so that I can do away with carrying two phones. My gut feeling is that due to the family plan, it is probably not worth canceling my Docomo service all together. But rather than guess, I can easily use the information available in my monthly statements to calculate what I would have paid if I was using BlueSIP or b-mobile VoIP, similar to how I did here.
Here is another over complicated chart. Feel free to stop reading now.
I started using the SIM on April 12th, a few days before they went on sale. I really put it though the ringer, so to speak, at first, burning through data with skype and youtube and whatever I could think of. The Fair SIM, unlike the Aeon "Plan C" SIM (which is 4980 yen for 1.2 GB per month) is wide open and does not pass through a proxy, so I can basically do whatever I want with it (except for p2p, which I don't use anyway).
Based on my usage since Golden Week, I predict that I will have 400 MB remaining when the charge expires on August 10th. To take into account the time I didn't use the SIM, I recalculated based on an expiration date 120 days after Golden Week. This results in 300 MB remaining. (The initial heavy usage partially offsets this, but not entirely, because the blue line passes through the top of the graph on the left side; 100% offset would result in the line passing exactly through the top left corner.)
My prediction doesn't significantly change if I use only the latter portion of my usage data. Doing so is justified by the apparent trend in my daily bandwidth consumption (green line and dots). As I became accustomed to how much data my daily activities consumed, I began caring less and less about how much data I was using. Now, the only thing I do differently than before is avoid viewing pages with lots of high quality images and updating market apps over wifi instead of whenever I got an update notification.