# Re: Air conditioning question, more

```Mike says:
>Input from members of this esteemed list finds HP draw of an a/c compressor
>to be in the region of 5 HP, or thereabouts.
>
>At 746 watts = 1 HP, that's 3,730 watts, so lets use 4KW instead. As Avi
>pointed out, 4,000 watts at 13 volts is 307 amps - not practical, and
>that's why I proposed to use an alternator wired to produce 110 volts -
>4,000 watts is then 36 amps;

It is worse.  You are assuming 100% efficiency.  The alternator is less than
100% efficient, and the electric motor is also less than 100% efficient.
Using a generous 75% efficiency factor for each of alternator and motor :-)
one gets:

Need 5hp or 4kw at output:

4000 / ( 0.75 * 0.75 ) = 4000 / .5625 = 7111

You would need 7kw worth of effort from the engine to run this new system.

I don't see how this conversion to all electric helps, since you are just
shifting the load from a compressor to an alternator.  The off-to-on
transition might be a bit smoother with the all electric system, but the
load will still go up and down with the compressor running or not running.

If you really want to remove the transition from off-to-on, you can charge
up a bank of 12V batteries from a high output alternator, and use a 110V
motor to run the compressor.  You would need an inverter designed for the
4kw load.  The alternator would continuously charge the batteries and the
load for the intermittently running compressor motor would be handled by the
batteries and the alternator combined.  They do this all the time on yachts
:-)

Humorously,

Bernard Littau
Woodinville, WA
'88 5kcstq

```