Rob -

Check AC 43.13-1B. Lots of good info there (small sample below).

Ted Stanley


Chapter 11
Section 3

Output Rating. The generator or alternator output ratings and limits
prescribed by the manufacturer must be checked against the electrical loads
that can be imposed on the affected generator or alternator by installed
equipment. When electrical load calculations show that the total continuous
electrical load can exceed 80 percent output load limits of the generator or
alternator, and where special placards or monitoring devices are not
installed, the electrical load must be reduced or the generating capacity of
the charging system must be increased. (This is strictly a "rule of thumb"
method and should not be confused with an electrical load analysis, which is
a complete and accurate analysis, which is a complete and accurate of the
composite aircraft power sources and all electrical loads) When a storage
battery is part of the electrical power system, the battery will be
continuously charged in flight.