Residential voltage varies in 24-hour increments

This isn’t really Home Assistant question, but since HA is how I discovered it I figured I’d start out by asking here. I’m in the United States, and my residential voltage is varying by about 5-6 volts. The weird part is that this variance is occurring in 24 hour increments. On one day, voltage will be around 123v, and the next day, voltage will be around 117 volts. The change occurs around midnight each day. Does anyone know what could be causing this phenomenon? And is it an indication of a problem either in my home or on the lines near my home?

Here’s a graph that shows it pretty well. Also, this particular graph is from a CyberPower UPS. But, I also have a couple of z-wave plus that report voltage and those graphs are very similar, so I don’t think it’s an indication of a bad UPS.

That variance is perfectly normal and well within tolerance of the net. Over here its 10% so could be 207 or 253 volts. Every electrical appliance you have has been designed to deal with this.

But what i wonder: that graph is that from the output or input?

Its definitely “within tolerance” but is a bit odd that it drops and comes back alternating every day.

Mine is below (in US as well)- you’ll see voltage drop during the day as everyone turns on their AC and electrical demand increases, then voltage increases at night when less power is demanded and people turn things off.

I really don’t know what would cause yours though. Either something in your house or neighborhood runs every other day and draws power, or the utility switches generating sources every other day? Those are just my best guesses. I wouldn’t worry too much unless you are experiencing surges or any other electrical issues though. A whole house surge protector is always a good idea too to protect smart home devices.

The graph that I posed was from the input of a UPS, however, as I mentioned I have a couple of z-wave plugs on a few other appliances that corroborate what the UPS is showing. I’ve also done a few random checks with my multimeter and each time I’ve checked, it matches what the UPS and plugs are showing.

I was fully expecting to see variances throughout the day, but I am just stumped at the strange 24-hour cycle. My only wild guess is that the utility is cycling between two pieces of equipment every day at the nearby substation.

I do have a whole-home surge suppressor built in to my main electrical panel.