HDD 16 (Degree (°C) days) for Tuscany in 2020 was 1310 (from https://www.iea.org/articles/italy-climate-resilience-policy... )

That's a 60° F HDD. Unfortunately, the HDD for the US tend to be based on 65° F. San Francisco has a mean of close enough to 3000 over 1948 - 2013 (it's probably lower now).

As these are temperature degrees, to convert from HDD 16 to HDD 65 (and ignoring that its 60 vs 65), that's 1310 * 9/5 that's 2358.

Ballpark, it needs less heating than San Francisco... though there's the "it's a castle and not well insulated" and "that's a large volume". However, it's not as big of an undertaking as one would imagine (to heat a castle) in a mediterranean climate.

I'd compare it to heating a church in San Francisco. Again, ballpark estimates put this in $1.00 - $1.50 per square foot.