The basic idea was this: I would sit in the car like I was stuck in traffic the entire time, keeping the cabin warm enough not just for me but for theoretical passengers as well, and see how much the range dropped in that time. The Mustang Mach-E I tested was a 2021 Premium AWD model with an extended range battery (88 kWh), which the EPA estimates at 270 miles of range on a full charge. But the delivery person who dropped it off told me it was a pre-production unit without all the software updates, so he said to expect a lower top range. 

When I got in the car to start the test at 5:45 a.m., the charge read 100% but only 186 miles of range on the instrument cluster. I was warned the range might start even lower in the extreme cold, and my garage where the car was plugged in is barely insulated and certainly not heated. (Ford's specific guidance is that storing the car between 32 degrees and 113 degrees "is most beneficial for the high voltage battery.") The temperature was 13 degrees (-1 windchill), which was ideal as the temperatures in Virginia got about as cold as that.

Instead of simply backing out into my driveway and beginning the idle, I treated the situation as if I were getting onto the highway. I took the car out for a spin first, stopping at the drive-through of a local Starbucks, and then making my way back to my house where I would "sit in traffic" for the next 12 hours. I didn't wear particularly warm clothing or bring any blankets or sleeping bags, I simply dressed like I thought I'd be getting out of the car soon. 

Before receiving the Mach-E, I had a chance to chat with Ford's Dickson to get some insight into the vehicle, hear her predictions (she wasn't sure how it would fare) and get some pointers about running the EV in the freezing cold. First, she said to precondition the car while it was still plugged into the wall, which is standard practice for EVs. Basically, you want to warm up the battery and the car interior while you're drawing power from the electrical grid instead of the car's own battery. So I did that. Second, she said it would be much more efficient to stay warm using the heated seats and steering wheel instead of heating the entire cabin though the air vents. I did that also, to an extent. 

I did use the heated seats and steering wheel, but I also tried to keep the cabin warm enough that even backseat passengers would be comfortable, so I played around with using only the footwell heating as well as the full-on blast from the footwell and dashboard vents, toggling between 70 and 75 degrees. Beyond the heat, I tried to replicate the in-traffic experience as much as possible, driving around the block a few times over the half day to simulate spurts of moving traffic, using the lights and radio at certain intervals, and even turning the car off for periods of 15-30 minutes a handful of times to conserve energy. 

Here's how the battery range held up in three hour intervals:

6 a.m.: 100% charge, 186 miles of range, outside temperature 13 degrees

9 a.m.: 92% charge, 164 miles of range, outside temperature 14 degrees

12 p.m.: 87% charge, 157 miles of range, outside temperature 18 degrees

3 p.m.: 81% charge, 144 miles of range, outside temperature 22 degrees

6 p.m.: 75% charge, 132 miles of range, outside temperature 24 degrees 

The dashboard of the 2021 Ford Mustang Mach-E electric crossover SUV, including the heated steering wheel and giant touchscreen

Did the EV Hold Its Own? 

If we interpolate the findings of this test into the situation in Virginia, it's clear this particular vehicle would have likely been fine. I lost 25 percent of the range over 12 hours while keeping the cabin toasty, so it stands to reason that over 24 hours I wouldn't have lost much more than 50 percent. I also took the Mach-E for a drive after my test just to make sure it would be able to keep going after idling for so long, and it drove off normally, just as if it hadn't been standing ice cold in the snow for half a day.