House battery plan: reasonable or foolish?

[Sean]

... I think I was describing the same thing except for the fuse. I'll try it this time with a picture. Is this where the fuse should go? How about the rest?

Looks fine except the batteries should be connected "on the diagonal."  If you switch the positive side power takeoff from the right-hand battery in this diagram to the left-hand one, that would do it.  This keeps the load and charge on both batteries more balanced; the way you've drawn it, the right-hand battery will get deeper cycles than the left-hand one.

...
Convert both to watt hours.
Refrigerator: 350kWh/yr = 39.95Wh (on the average)

Actually, I think what you mean is that 350kWh per year means an average draw of 40 watts, not watt-hours.

Batteries: 210Ah x 12V = 2520Wh
To find the amount of time I could run the refrigerator off the batteries, do I simply divide the batteries' Wh by the refrigerator's Wh to get about 63 hours? This is assuming nothing else is running off the batteries. Am I doing that right?

Well, that would be correct if you could use all 210 ah from the batteries.  Unfortunately, you can't.  To begin with, this type of battery should never be drawn down below about 50% SOC, so you would have at most 105 usable amp-hours.  Secondly, you will seldom be able to charge them to 100% full; usually when charging from a generator or other high-cost source, you will stop the charge at around 80% SOC.  Lastly, batteries only have their full rated capacity when they are brand new.  I got the sense from your post that these are used batteries, so their usable capacity is likely somewhat less than the spec sheet I linked earlier.

Starting from a new, 100% full battery, you could get perhaps 1,200 watt-hours from them.  After paying the roughly 10% "inverter penalty" (the inefficiency of the system), you would get about 27 hours of fridge run time with no other loads.

Starting from an 80% full battery, you would get maybe 750 watt-hours from the batteries, for a run time of just 17 hours.

I've written tomes on this subject here in the past, and you might benefit from digging some of that out of the archives.  Lots of links in there to some very informative battery sites as well.  Just a few examples:
http://www.busconversions.com/bbs/index.php?topic=13072.msg137437#msg137437
http://www.busconversions.com/bbs/index.php?topic=4329.msg39499#msg39499
http://www.busconversions.com/bbs/index.php?topic=7452.msg72646#msg72646
http://www.busconversions.com/bbs/index.php?topic=14138.msg150489#msg150489
http://www.busconversions.com/bbs/index.php?topic=2620.msg23253#msg23253
among many others.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com