What did you switch from and to? Even Tesla's Charging Calculator is far less optimistic than that:

https://www.tesla.com/charging-calculator

"Seattle" may be a critical bit here. The Fed thinks the "U.S. City Average" cost of electricity is far higher than yours:

https://fred.stlouisfed.org/series/APU000072610

Based on how many of my friends followed their first EV purchase with an electrical outlet upgrade - even those with very short commutes - I suspect your "120/15 is only fine for normal commuters" is still a tad optimistic.

EDIT: Re-reading comments here - I'd bet a leading reason to upgrade from 120/15 to 240/50 charging is to get much more of your charging done within the 12am - 6am "lowest rate" time window. Or whatever that window is, locally.