I finally got around to buying an alarm clock made to run on 12vdc. This one has a 9v battery backup, but unlike the ones I had that run on mains power, the battery backup still has a display when on 9v. The Radio Shack alarm clocks I used for many years have no display on backup power but still keep time and sound an alarm for whatever time you set it before the mains power failed.
Attempts to run the Radio Shack clocks on inverters failed because even my sine wave inverters are not 60Hz. One is 59.1Hz and the other wanders all over the place.
The 12v clock is made in China of course, but it has a nice fit and finish and even if it didn't come with instructions the switches, buttons and labels are enough to figure out how to use it. It's advertised as VERY LOUD and it is, even on medium. It's also really annoying. The alarm sound is almost identical to the sound smoke detectors make when they have a low battery, except this alarm chirp lasts for about 1/2 second. The snooze is a bit short, 5 minutes, but after hearing that nasty chirping alarm you're probably going to be wide awake anyway. There is no AM on the display, only PM. You can also choose military time (24 hour display). The back light is on only when the alarm is sounding or anytime you hit the snooze bar. If you hit the snooze bar it lights up for about 3 seconds.
It's no where as big as the pictures make it look on the web. The MacBook Pro it's pictured with is 13"
I paid just over $30 for the clock including UPS ground shipping. It keeps time correctly connected to my 12v off grid bank. It uses 3.6ma steady according to my Harbor Freight meter so it won't run your batteries dead even if it's plugged into the dash of your car and left there for days.
