Heating Prius in winter, 12v charger

I would not use a 12v heater, the Prius battery is smaller than a standard car. It doesn't turn an alternator, so it doesn't need to be big (amp wise).

Use an extension cord, tethered to your house + outdoor timer, with a 120vac car cabin warmer.

I use the EBH + cabin warmer, tethered to a 3-prong extension cord.

 

If you are mostly concerned about total energy cost/emission issues, consider just plugging the grill well. The car will get acceptably warm in ~ 2 miles @+20f. It will probably use less energy net/net than block/cabin heat arrangements.

I have never done an energy cost analysis, but a block heater may increase your initial cold start gas mileage, is the net energy used less than just cold starting? Same with cabin heat.

If you just want to warm yourself, consider heated seats.

 

With gas @ 1$/L, electricity @ 0,08kWh, 10km commute, using the block heater for 3,5h in the morning saves me about 10¢/day. I still need to have more data point to make some definitive conclusions, but it is already clear that a block heater, used cleverly, does make a business case for itself.

My fuel consumption went from 4L/100km @ 0°C to 5.8L/100km @ -20°C. It seems that the car heater demand makes most of the difference. I hope a cabin heater will help by letting the engine get up to temp faster, rather than stealing its heat to move it to the cabin. Will be installing one in the next days.

 

I tried search at the 3-gen forum and did not found anything close to this Idea, So...
I took another approach on cabin heating. Instead using 12V heater I went to use 220V electrical heater (living in Europe). During warmup the Prius also charges the HV battery, so there is electrical power in excess. I just opened HV battery junction box and connect 2x2.5mm2 cable using round 6mm terminals and additional M6 nuts since the main cable studs are pretty long. At the other end of the cable I put standard 2-nest extension outlet. This way if current is consumed trough extension cord it will be detected by HVB computer and taken into account. The easiest way to use heater on 220V DC is this mini portable "Handy heater". Inside there is circuit that rectifies the AC voltage into DC and uses a relay to turn on or of the heating element and drives the 12V 80mm fan (had to be plugged it in only one way to made it work 'because there is one-wave-single-diode-rectifier). This unit worked flawlessly for me but is only ~350W. I used Torque app to monitor the current when I turn on the heater (in fact it is always ON and when reaches the temp of 20C it shuts down automatically) and there is some 1.5A consumption. Considering the charging current during engine warmup is about 20A this is very small portion and in about 5 minutes the HVB is above 60% full.
So I went big. Took an verry old 230V AC 2kW fan heater, removed the AC fan and in this location 140mm 12V 8W 180 m3/h PC fan was installed. I took an old 12V 1A wall power adaptor (which also can work at 220V DC) disassembled the housing and installed the PCB into the heater to power the fan itself. The only problem is that heater does not have thermostat (usual bimetallic) and I hat to turn it off manually when the cabin reaches proper temperature. So goodbye cold winter mornings

 

Still heating the cabin in cold days this way. This 2kW electrical heater makes me very happy