Does the 12v port turn off?

Yes, it is only active when it is POWERed ON. Typical TOYOTA - my Dad's old CAMRY did the same, and if I was in it with my GPS with a dead battery, it would turn it OFF if we stopped briefly - the PRIUS was the same.

I've seen 12V blankets for camping.

 

What's your situation: is the car in a garage? Would you be able to use the block heater? Maybe a 120 volt cabin heater could be wired too, on a timer. There's a thread here discussing that, a member from Finland was using a cabin heater?

 

The manual says it outputs 10 Amps so 120 watts would be absolute maximum. 100 watts would be safer.

 

Just nagging: The Prius 12 volt battery is so small, you only want to use an electric blanket in Ready.

(I think the Prime has heated seats in the upper trim levels. Several of the aftermarket leather seat covers can do heated seats)

 

I do not have a Gen 4, but Gen 2 and 3 both turn off the 12 volt socket.

 

The Gen 4 turns off the socket on the top of the console. I am not sure about the socket on the back of the console for rear passengers.
I bought one of these.

 

Well a block heater will get the cabin up to comfortable temperature "sooner", ie: 4~5 blocks vs a mile or two. It'll raise coolant temp by about 25C (maybe 40F?). It doesn't deliver a warmed up car, but speeds up the warm-up process.

The other item, an electrical cabin heater, is a little heater fan, something you might mount and wire in on the center console. I took a look, there's some recent discussion on it here, but couldn't find it so far. Maybe google "priuschat cabin heater" or similar.

Both could be run on a timer, say a couple of hours before start up.

 

Yes. All circuits go dark when the car is turned off.

 

The sockets are indeed switched with the ignition, so no worry about the battery dying.

They are however, exceptionally low-rated (no "cigarette lighter" socket has been wired robustly from ANY manufacturer for 3 decades or more, but the Prius uses 20ga wire IIRC, which makes even the 120w rating dubious), so if you go down the route of a heated blanket/12v space heater, run a dedicated line from the battery, and switch it with a relay run from the center console wiring. The wiring for the center console can easily be accessed by removing the small piece in front under the lid, then pulling up the entire cup holder assembly.

As for me? I run a regular 120v 1500w space heater, set in the driver footwell, with the cord run out through the door jam. On the lower, 600w setting, at 45F ambient (as I see your heated garage is at), setting a timer for an hour before departure should leave the cabin (and more specifically, the driver seat) toasty.

 

Except, I believe it works in ACCESSORY Mode too.

 

I guess I wasn't exactly clear on that. When I said ignition, I meant either of the modes, either Ready or Accessory.

And of course, Accessory mode will run down the battery by itself.

 

May not, my late 2017 Prius 2 will turn off the accessory mode after about 20 minutes...