Hi Catalina - I'm in MN too -
It's going to be -20 tonight - that's real temp, not wind chill (which is expected to be -40). Skies are clear and it will be very cold. Today, during the warmest, sunniest part of the day, it was -1.
I have one 250 watt heat light and it keeps the coop about 10 degrees warmer than the outdoor temp. That's not going to help me too much tonight. I'm very glad that I built the coop with insulation, sealed up most all of the drafts and have plenty of deep litter. Still, my heart breaks. My DH has forbidden me from bringing them inside tonight (I only have 4), tho I did ask. I'm sure they will be fine, but man, that's COLD.
Personally, I don't think you need heat in Seattle. Chickens are tougher than I originally gave them credit for. That said, I still wouldn't go with an unheated coop here in MN...