>  Date:    8/1/00 6:53:12 PM Central Daylight Time
>  From:    cstann@home.com (CHRIS STANN)
>
>  [Discussing the power requirements of an RV air conditioner]
>
>  30 amps max at startup.  Much less when running.  It would be like running
>  your high beams on with aux. lights.  Yep, a 90 amp alternator would be
>  nice.

The amps don't do you any good unless they're supplied at the
necessary voltage.  The A/C wants 30 amps at 120 volts, but
the alternator's putting out 90 amps at only 12 volts.

You can trade amps for volts by running the alternator output
through some sort of power converter; what stays constant
is the wattage, which is amps x volts.  (You can only get more
watts by getting a more powerful alternator.)

So, the A/C wants 30 amps at 120 volts, which is 3600 watts.
But the alternator puts out 90 amps at only 12 volts, which is
just 1080 watts - less than a third of what the A/C needs.

Looked at another way, we could take that 12 volt, 90 amp
output from the alternator, and run it through a power converter
that would convert it to 120 volts (that's 10 times the voltage)
and 9 amps (one-tenth the amperage).  Same result as before -
less than a third of what the A/C needs.  To run the A/C (and
nothing else), you'd need a 300 amp alternator, which I suspect
might be a bit taxing for the engine.

These calculations assume a 100% efficient power converter;
real-life power converters lose a bit of power during
conversion.  Also, the air conditioner runs on "AC"
("alternating current"), whereas the alternator (oddly,
given its name) puts out "DC" ("direct current"), so the
power converter you'd use would actually be an "inverter",
which converts DC to AC (sort of) along with converting
12V to 120V.

                                      -Steven Sittser