The appeal of 42 volts comes down to one equation:
volts x amps = watts
where volts represent electrical force, amps measure current, and watts equal electrical power
In other words, for the same watts, as volts rise, amps fall.
Add up the power consumed by all the electrical accessories in a new car—the power windows, the defroster, the heated seats—and the total will probably be between 1.5 and 2.0 kilowatts, depending on the sticker price, says Hansen. To supply 2.0 kilowatts of power, a 14-volt alternator must be capable of churning out more than 140 amps. Not a problem, especially with new water-cooled designs. But size up a 14-volt alternator to feed the 3.0 kilowatts of power expected in cars built later this decade, and you're looking at 200 amps, electrical loads that cause clattering alternator noise and require tree-trunk wiring harnesses.
Now install a 42-volt alternator in that 3.0-kilowatt car, and the required amps plummet to 70. As amps drop, so do wire sizes—by as much as half. This change also frees up electrical capacity for more gadgets. Electric water pumps, electric air-conditioning compressors, electric brake calipers, electric cam-timing adjusters, even electromechanical engine valves that replace camshafts with sturdy solenoids to open and close the valves, have all been built and tested with flywheel-size starter-alternators supplying the power.
Electrified, these accessories could be managed more effectively by a computer while freeing the engine of their parasitic losses and helping out fuel economy.
But a 42-volt Mercedes that was due in 2005 has been put on hold, says a company spokesman, and only one car to date has made it to dealerships with a 42-volt electrical bus. It's a gasoline-electric hybrid version of the Toyota Crown sedan, sold in supercar quantities—just over 2000 copies in three years—only in Japan.
"The bloom is off the rose now," says Mike Matouka, a staff development engineer at GM who in 2001 helped convert the electrical systems of some research cars to 42 volts.
Switching to 42 volts involves more than just shrinking wires and tallying up the cost saving. Faster corrosion of electrical components and sustained arcing are two major reliability issues; the latter is a safety issue as well. Electrical equipment has to be protected against much higher voltage spikes, such as when an alternator becomes disconnected from the battery, and many devices in an automobile just prefer fewer volts.
Light-bulb filaments, for example, grow too long and flimsy to survive potholes if designed to handle more than 14 volts. Microchips sizzle if they see more than 5 volts, and smaller, low-amp electric motors must be wound with special extra-thin wire that increases cost. 
