115VAC electricity is a sinewave with a peak of 163V. Maybe the cheapo meter simply peak-detects then reduces the reading to the RMS value of 0.707 times less.
80V/115V= 70% so when a cheapo inverter produces a squarewave with the same power as a sinewave then its peak voltage is 70.7% the peak voltage of the sinewave.
Many electronic products use a linear power supply that relies on the peak voltage of a mains sinewave. A cheap squarewave inverter has a peak voltage that is 0.71 times what the sinewave had. Then there is a sudden reduction in the power supply voltage when the UPS begins supplying the power, unless it has a modified or pure sinewave.
Here, this one is $15 and does most everything my Fluke does on low-res setting (high res has rarely (Never?) been needed.
It has true RMS, min/max, auto ranging,, auto power off, delta voltage, - even sends dc, sine or triangle waves for continuity test. Please tell me how well it works. Ships free with prime but not sold by amazon. It is sold by the "manufacturer" but that seller has 99% rating so I think you are safe. The most you can lose is $15.
Auto Ranging Digital Multimeter - Sine/Triangle Wave Out Put Voltage Tester AC/DC Voltage AC/DC Current Ohm Volt Test Diode Continuity Test - - Amazon.com
Many electronic products use a linear power supply that relies on the peak voltage of a mains sinewave. A cheap squarewave inverter has a peak voltage that is 0.71 times what the sinewave had. Then there is a sudden reduction in the power supply voltage when the UPS begins supplying the power, unless it has a modified or pure sinewave.
You seem to be mising the point?, that it's a UPS - not a general purpose inverter. A squarewave would probably be perfectly fine, but it's quite likely to use a modified sinewave anyway.
I would say yes. Both of the below meters are connected to the same source, a UPS with a MSW (Modified Sine Wave) output.
The meter on the left is an inexpensive average responding RMS indicating meter while the meter on the right is a true RMS responding RMS indicating meter so what you see makes sense to me.
Years ago a good RMS responding meter was an expensive proposition. Today they are relatively inexpensive and loaded with additional useful features. Just find one with the features you want at the price you want to pay.