While submerged (on electric motors back then) he said he would sometimes lay and read in his rack; each rack had a little electric light. I asked him whether they had to conserve electricity and were they allowed to burn the lights all the time?
He told me that the electric power needed to turn that screw one turn would power every electric bulb on the boat for a month.
A 1500 watt electric heater will keep my study, with a 10ft ceiling, warm all day. Say the heater on the electric car is a 1000 watt element. Say you drive 100 miles with the heater on, and it takes 2 hours. So that's 2000 watt-hours, or 2 kilowatt-hours of energy used from the battery on that drive.
An electric car battery has about 100 kilowatt-hours of energy aboard. So I'm guessing that if you ran the heater all the time, you'd drop your range by about 2%. If the lights are all LEDs, their power draw will be negligible.
Some smart person can probably tell me if any of my assumptions are wrong.