Usually the meter on my 78 registers right at 16 for voltage. Lately it has been reading at about 13 to 14, slightly lower. Sometimes after I crank the engine and start driving at night the display lights will be a little dimmer than usual (shows 13 / 14 volts). Then after a few minutes the volts go up a bit and the lights are close to normal.

Maybe it's just me but I swear that there is a slight loss of power as well. I'm concerned because usually I get a reading of 16 or so, right at where the meter stops measuring.

The alternator was replaced a couple years ago. It is an absolute OEM / Nissan part. Could it be my battery? I need to check how old it is, but what do you guys think?

Bad connections or internal regulator on it's way out. My gauge in my 78 reads just short of 16v when the car is running w/o headlights, ac, etc. but when I check it with a voltmeter it's about 14.2-14.3 so those gauges aren't reall that accurate. My 810 reads right at 15v but agiain, when I check it with a voltmeter it's at 14.1-14.2.