can I record 7 hours of WTV.files on a 25GB BD-R disc?

It varies by channel and content (bitrate is variable) but I have found that one hour of HD programming in WMC is about 6 GBytes so you are looking at about 3 to 4 hours per disc at best.

I see. You are recording SD analog, kind of rare these days.

Your numbers seem in the ball park. You'll have to convert those WTV files into a AVCHD or some other fomat that the Sony BD player supports, and then burn it. I see no reason why it shouldn't work.

Protected content (like HBO) will only play on the PC you recorded it on. The .wtv format contain DRM...

BD-R discs and drives are just a storage, not a player. Unconverted, you will need WMP or WMC on the PC to load to play those WTV files. BD players or BD playback software can't handle those wtv files. In order to play them in standalone BD players, you need first convert wtv file into a conventional video file then use proper software to author the disc into a BD movie disc.

He is recording on analog only. So no DRM.

You could use a program like Handbrake to convert the .wtv files to .m4v or .mkv files, then burn those files onto a disk to play in a standalone BD player. However, YMMV as to whether this disk is playable on different machines - I did this to some 720p HD .wtv files; they played fine on my standalone BD player, but had a 4 to 5 second audio lag when played in another BD player.

If this does work for you...I imagine you could easily get the 5 GB .wtv files compressed down to about 1 GB per hour as .m4v or .mkv files - you could get about 20 hours or so on a BD disk then. (25 GB BD-R actually holds around 23 GiB - you know, the whole "gigabyte vs. gibibyte" thing.)

Why don't you simply get an inexpensive USB hard disk, save the WTV files on the hard disk, and them replay them from your computer using WMC?

I can't imagine you want to be transcoding analog video files.

What hardware are you using to capture and digitize these analog cable signals? Didn't it come with software to both record and play back the recordings? And does it allow you to record in other formats that might be playable on your BD player?

By the way, if you really want to use disks, it might be a lot cheaper to use DVD-Rs than blu ray disks. They'll hold less and you'll use more disks, but they'll work just as well.

I tmay be a stupid question, but can one write DVD format movies on a BluRay, so that one could cram a number of DVR's on one physical medium?

Will a stand alone BlueRay player play them back? I was under impression that BluRay players will play DVD's, but will they play a BlueRay Disk with DVD (MPEG2 basically) movies on it?

I've never tried it, but there seems to be a lot on the internet written about that. It's possible, but I think the problem is authoring the BD in a manner that creates a menu providing access to the multiple DVD videos. There seem to be a number of software programs that will allow you to do this. Sony DVD Architect Studio is one. And here is a link to another that is supposed to work (note, I haven't tried this):

http://adubvideo.net/how-to/converting-multiple-dvds-avchdbluray-multiavchd

I know that you can do the reverse and record HD video on a DVD disk that will playback on many BD players.

MPEG2 (both SD and HD) are suported by BD. You can make a BD movie disc out of those MPEG2 streams in BDMV format. No need to author them in DVD movie format.

Don't you need to author it somehow to create a menu structure to access the multiple videos?

You need author it regardless. BDMV has strict requirements for folder layouts and file sysem (UDF 2.5) and files that needed to be created. Menu or not is optional. You can set multiple video files as chapters so you don't have to create menus.