I will opine on this, as well. I was born and raised in the San Francisco bay area... your chickens do NOT need supplemental heat in this area. I would be more concerned with the humidity level rather than the heat. The drier, the better. Knowing the fog increases the humidity in the area, which is what truly contributes to the chill rather than air or wind chill factors, you'd be better off using a dehumidifier to keep them warm.
Chickens body temperatures are in the 100s, normal. As someone stated, they put in a sealed oil heater in one corner of their coop, and the chickens avoided that area... because it was too warm for them, even with one in full molt (which also makes me wonder... you stated your chicken is only 8 months old, but going through a molt already? It's usually not until their second fall that they go through a molt). The best test? As the ones found with the heater, it's like finding the sweet spot in a brooder, where the chicks are distributed evenly under the heat, rather than huddled together to stay warm, or are spread out away from the heat source. Your chickens will show you if they are cold or not.
That being said, I now live in North Dakota, where we go into the negatives in the winter, for weeks at a time. So, I just installed a Comfort Cozy radiant wall heater... it doesn't operate on the floor, has a zero clearance back, even though I have it on a frame that keeps it several inches away from the coop wall, so it is above the level of the chickens' access. It is plugged into a ThermoCube tap, that doesn't allow it to activate until it goes below 45F. So, when the ThermoCube activates, the heater comes on, heats the room to an ambivalent temperature of 59F (the lowest it will go) on 750 watts (low setting). But when the room temperature rises to 45F, the ThermoCube deactivates, thereby turning the heater off... fail-safe mechanism. I only use the heater, really, to keep the room warm enough to keep water liquified, not to heat the chickens, because they'll do just fine. They go outside into the snow on their own during the day, so I know they can tolerate the wind chill factors here. I plan on installing a couple of solar panels next Spring/Summer on top of the coop (an 8x12 shed) so that I can use that as the electrical source instead of running an extension cord from the house, making the coop independent.