24V AC-AC Power Supply Outputs ~80V Unloaded
80 volts peak-to-peak is...

\${ 80V \over \sqrt 2 \times 2} \approx 28.284V\$

... 28.3 volts RMS. This is an error of...

\$1 - {24V \over 28.284V} \approx 0.1515\$

... 15.15%. That is a bit high, but may be in spec for the equipment it's meant for, since the voltage will drop a bit when the supply is loaded.