This link pegs the efficiency increase from an electrical WP at ~5-10+% w/ a car that gets ~27mpg. Even w/ HD diesel running at 60+% load
there's a benefit but it's really small, less than a percent. Most of the advantage seems to come from not running the system when it isn't needed. Not having to remove a ****-ton of belts, reservoirs, and an engine mount would be a double bonus IMO (Camry needs a new WP)!
