REFLECTOR: A/C (was Re: GRT Displays overheating - Anyoneelseseethis?)

[Mark Magee]

Sid,
This site shows proper inverters for wattage:

http://www.donrowe.com/inverters/inverter_faq.html#size

If my A/C pulls 400W, I would think a 1000W sine wave inverter would easily
handle the surge when the motor starts and run at 400W at max load. Show me
how I'm wrong. I thought I had this figured out, but don't want to proceed
if there's something I'm not seeing. This site seems to think 400W could be
handled by a +or- 1000W surge inverter.
Specifically:
----------------------------------------------------------------------------------------------------------------------------

*FORMULA to convert AC Watts to DC Amps:*

*AC Watts divided by 12 x 1.1 = DC Amps*
(this is the size vehicle alternator you would need to keep up with a
specific load; for example, to keep up with a continuous draw of 1000
watts, you would need a 91 amp alternator)


---------------------------------------------------------------------------------------------------------------------------------------------------
So 400W/12x1.1= 36.6W Load VDC at running power. The startup load up to
1000W the battery and inverter can take that momentarily to cover the surge.
I have brainfarted before, keep me honest here Sid.
Seems to me as my 400W A/C will work easily on a 1000W Sine Wave inverter.

Mark
N34XL

On Wed, Jul 11, 2012 at 1:45 PM, sbjknox <sbjknox at earthlink.net> wrote:

>
> ----- Original Message ----- From: "Mark Magee" <edjonesbrady at gmail.com>
>
>
> Subject: Re: REFLECTOR: A/C (was Re: GRT Displays overheating -
> Anyoneelseseethis?)
>
>
> [snip]
>
>  I pulled apart a small roll around 4,000 BTU A/C that pulls 4 amps on the
>> AC
>> I Used camping with the kids. I think 4000 BTU's of A/C would marginally
>> cool
>> an XL cabin on a 100F OAT day. The good part is the whole unit can be run
>> on an inexpensive 1000W sine wave inverter that factoring in 8% efficiency
>> loss it would still only pull about 50 amps on the 12V. I'm hoping a
>> second
>> alternator, possibly a  supplenator would fit the bill and double as a
>> back
>> up alternator. As well I might install some 110V outlets for use in flight
>> after the A/C was shut down in the cool air.
>>
>
>
> Some rather creative math going on here...
> Strictly speaking, you can't equate BTU and Watts but common useage of BTU
> when refering to air conditioning actually is BTU/hour so that 1000 Watts =
> 3415 BTU/hr and 4000 BTU/hr equates to about 1171 W.
> But the real issue is with the current at 12Volts.  Even if you had a sine
> wave inverter with 92% efficiency (which I believe is overly optomistic)
> the current at 12V is
> (1171W)  /  (12V) / (.92)  =  106 AMPs!
>
>
>  I think the whole arrangement with alternator might add 12 lbs.
>>
>
> For 4000 BTU/hr, not likely...
>
> However,  your 4000 BTU roll-around unit could not have produced anywhere
> near 4000BTU/hr if it was only pulling 4A from the 115VAC line.  (4A)  x
> (115V) = 460 W.
> (460W)  x  (3.415BTU/hr/W)  =  1571 BTU/hr  at 100% efficiency.
>
> So, IF your roll-around unit would   "...marginally cool an XL cabin on a
> 100F OAT day.", then a much smaller (than 4000 BTU/hr) A/C might work and
> your current draw at 12V would perhaps be acceptable.
>
> Sid Knox
> Oklahoma
>
> Velocity  173 RG  N199RS
> Starduster  N666SK
> KR2         N24TC
> W7QJQ
>
>
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