Most of Telects +24V power panels are able to operate with +12V applied, however the alarming will NOT operate.  The alarm card is triggered when the voltage drops below 18V, when operating at +24V.  On a +12V system the alarm card will not function properly, nor will the LEDs. 

You can apply +24 to the panel and power devices with +12V but the same rules applies, the alarm will not work for any device that operates under +18V. You will need to check with your equipment manufacture on whether the load will operate properly with a +24V supply.