20%, though this time of year it rarely drops that low unless I'm trickle-charging my car off it. (My car more often uses a NEMA 14-50 on a separate TOU EV meter.)
For the winter months in particular though, I wish there were a "smart" automated way to set the reserve to the lowest point where it could anticipate charging to ~90% the next day, based on weather conditions, day-of-week usage patterns, etc. In stormy winter conditions this might be 70% - 80%. In sunny summer conditions, 10-20%. This would give maximum reserve in the event of a power outage, while also maximizing cycling and minimizing grid usage.