20x4 LCD will write but then won't update
Ah ha.   I solved it by using lcd.clear() and simply repainting the entire display with every change.  I have much simpler LCD programs that don't require doing this, but it worked in this instance.

Same code as above, but with this before each power notification:

lcd.clear();
lcd.setCursor(2,0);
lcd.print("PV Control 3.9");


etc...



UPDATE:
  Now that I have two 20x4 LCD screens showing readings from two separate Arduinos (PV Controls and Weather station) I have discovered that it is very important to keep the screen writes to a minimum.  No time delays in between writing parts of the screen, for instance (like when I had Serial.print statements in there for debugging), doing all the readings and calculations before writing the screen, and timing the screen updates to be infrequent, like on the order of 65 seconds for weather and 10 minutes for the PV.  I2C does not have collision detection so it is important to try to prevent the two screens from being updated at the same time.  The I2C&amp;Power arrangement is working great.  One source of power is all it takes to activate the entire network.