Runs fine, won't start on battery.

In the middle of deliveries at work today I left the electrics on while I dropped someone's order off, and when I came back the battery was too low to start - after some choice words (I'm going through a lot lately and today is the worst so far) I got it bumpstarted and dropped off what I had, then rode home and got my car - I hate driving my car. 

The battery is a NAPA wet battery, but it's only been drained once before when I left my lights on overnight - feels too soon to have it be dead. The acid levels might be low, so I'll check that, but if they aren't - what should I be looking for? Check stator output, reg/rec? It runs on its own, but maybe doesn't charge the battery - I dunno. Just fishing. 

You're on the right track.  If you have a manual it should detail the correct tests for stator and voltage regulator.  I know there's a video on Youtube for a Z1 that might give you some idea of what you should be checking.  As for the battery, charge it up and check your fluid specific gravity.  

IMHO the quality of lead acid (wet) batteries has taken a nose dive in in the past few years. I had one about 2 years back that I drained trying to start the bike. It just would not charge on a smart charger but on a very basic dumb charger I got it up to a max of 12.5 volts which I would call a battery on life support. It lasted a few more weeks before it totally died

My suggestion is to put it on a charger, wait an hour after reaching full charge greenlight or whatever and measure voltage with a meter. If  V >12.5 then great. check bike charging system - see manual for tests.  If V <12.5 then toss it.

 

You should check the voltage you get at the battery with the bike running.
Put your probes on the battery posts. Not the terminals.
Should be over 13 volts

Turn on the high beam. Does the voltage drop?
It should not drop.

Raise the rpm up to 4000. Does the voltage increase?
It should increase to over 14.

Charging wet cells is fun.
I had to do that last month.
Constant current, constant voltage, specific gravities, equalizing charges.
It's easy once you figure it out.

batteryuniversity.com/learn/article/char...he_lead_acid_battery
if you get bored

A lot of the battery brands are made by the same company.
Your NAPA battery is made by the same company that made my Autozone Duralast battery.
Clarios--Formerly Johnson Controls.
jgdarden.com/batteryfaq/batbrand.htm
 

Popped it on the charger last night (levels are fine) so we'll see. I caved and ordered an SH775 reg/rec just in case, it'll be cheap insurance even if the battery is the issue. We shall see. 

If unsure of the battery's condition, I would charge it with a good motorcycle smart charger then do a load test.
I'm currently running a Scorpion AGM battery from batterystuff.com that is now 3 yrs 9 mos old.   
I keep it hooked up to a Ctek US 0.8 smart charger when not riding & test the battery w a VC97 digital multimeter.
My previous scorpion agm lasted exactly 2 yrs, but I only hooked it up to an old schumacher mc charger overnite about once a month.

Interesting findings. I rode to work today with the headlight fuse pulled and had no issues - and the built-in voltmeter is reading higher than ever, though not perfectly accurate. Picked up a new multimeter on the way home (no idea where the old one went?)

I rode maybe 30 miles total (odometer broken) but started about 10 times with no issues. Never left the electrics on. 

Off: 13.1v
On: Drops to 12.7, steady
Idling: Slowly climbs to 14.1 (@1000rpm)

Headlight fuse back in:
Off: 13.1
On: Dropping past 12.5, didn't let it go further
Idling: ~12.6v
Turned off: climbed to 13.1

So I think my battery may be okay, but either my charging system is pretty weak or my headlight is drawing too much. Don't have the energy to test my stator output or R/R tonight.  

Yeah, something is amiss.
If you run that headlight, you may end up with a dead bike.
Hopefully, it's just a bad connection in the charging system.

When you are ready we can try to troubleshoot it.

Appears the battery is good it’s recovering on its own.  If a battery will not recover to a 12.6 volt minimum the battery is suspect. If won’t recover at all then it’s junk.