40 amps is less than the charging capacity of the stock alternator in my 73 Super Beetle (which is 45 amps). Consider that the early Beetles in general came with almost NO electrical assesories, NO power-assisted components, and no A/C.
When I added a high powered stereo and Halogen headlamps, I had the alternator beefed up to 75 amps.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car