Argonne National Labs measured an Insight's 12V loads at 200-220W at cruising:
http://www.transportation.anl.gov/pdfs/HV/437.pdf
page 11, fig 17.
Ergo disabling automatic background charging free up around 200W, or 300W with the fan on, or about 400W in the winter. This power will be drawn out of the main battery instead of from the crankshaft, and can be paid back when it's convenient instead of when I'm trying to lean burn.