- I would like to be able to run two battery chargers at the same time, hence the "tandem."
No problem. If you can power the loads of the dehumidifier and garage door you've got plenty to run multiple chargers at once.
- I expected that the garage door opener would have a surge on startup, then a lower load. I did not see that-- it reached a certain draw (listed below) and stayed at that level during operation. It's a very small garage door that runs for a short amount of time, FWIW.
Nice. That helps.
- A few devices had idle loads that I wasn't expecting: dehumidifier #1, dehum #2, and garage door.
That's normal. There's always control voltage running the system so it's awake and ready when you call for it.
- I recorded the observed watts, and also did the same W = V*A calculation using observed volts and amps. For most devices, observed and calculated were pretty close, but for the battery chargers (marked with "???") they are way off. Not sure what's going on there, might re-test.
Close enough, the difference between 150w and 151.12942w is nothing. Rounding is fine.
That's supposed to be that way for American power.
- I thought it was interesting that the "idle" volt reading was a little over 121, but with more load drawn, that number crept down a little.
Again, perfectly normal. Think of Volts like water pressure. There is always water pressure in the pipes in your house but when you flush the toilet the pressure drops a little and shocks whoever is in the shower. That's perfectly normal. 120v, 110v, 115v are all terms used interchangeably.
- In reading the manual for the garage door opener I discovered that it has the option to install a backup battery, so I might do that and leave the unit on grid power, which would take that item out of this calculation.
You could but if it's some super expensive proprietary battery that costs as much as the opener, then don't waste the money since having the rest of the system accommodate for it is almost nothing.
OK, so have you grabbed the Power Audit form from the Resources section? If you plug all the numbers into that form it will automagically calculate your inverter size, battery requirements, and array size. You've got half the math done, now you just need to get time involved. For example, your fan #1 calls for 170w at high speed, so that tells you that you need at least 170w of inverter to power it, but how long are you going to run it? If you only run it 1 hour a day that's 170Wh. But if you want to run it for 10 hours a day that's 1700w which is about 1.5 bog standard LFP batteries. That's where you're going to get the numbers for capacity that's going to determine how much battery bank you need.
I gotta say, that's the most thorough power audit I've seen posted here in a long time. ?
Just napkin mathing here, worst case scenario is that the fans (280w), the dehumidifiers (800w), and the battery chargers (400w) are all plugging along when you open the door (450w) for a total of about 2000w of load. So, a 3Kw inverter gives you plenty of capacity to run anything and still plug in a power tool or two or your MargaritaMaster-5000 without worrying too much.