Re: power inverter/ a/c unit

Posted by John T on July 21, 2015 at 18:12:13 from (216.249.74.3):

In Reply to: power inverter/ a/c unit posted by schrade on July 21, 2015 at 17:31:24:

LETS DO A BIT OF PRACTICAL ENGINEERING

A typical (not all Billy Bob) RV rooftop AC unit may draw 12 amps at 120 VAC when its compressor is running. That computes to around 1440 watts (Power in Watts = V x I) HOWEVER it may take 4 to 6 times that current initially when starting the compressor and depending on design (I advise a Pure Sine Wave PSW Inverter instead of a cheaper Modified Sine Wave) and its surge capacity SO IT MIGHT TAKE A MINIMUM OF 2500 TO MORE LIKE A 3000 WATT INVERTER TO ALLOW IT TO START UP.

Next, I recommend true deep cycle golf cart batteries (two sixes in series for 12 volts) NOT semi deep cycle RV/Marine batteries like sold at wally World.

To operate a 12 VDC to 120 VAC Inverter if the 120 VAC AC requires 12 amps, that means you would be drawing over 120 amps out of your batteries (Inverter isn't 100% efficient, has heat losses).

A typical Trojan 6 volt deep cycle golf cart battery has an energy storage capacity of around 210 Amp Hrs, but its best to NOT discharge them over 50% of that capacity or 105 Amp Hrs

THEREFORE if you had two six volt Trojan gold cart batteries in series and were drawing 120 amps out of them to power the Inverter to run the rooftop AC, in LESS THEN ONE HOUR your batteries would be discharged 50%. If you used four batteries in series/parallel, you could double the time (maybe get 100 minutes) the AC ran prior to a 50% discharge.

SO YOU SEE OPERATING A 12 AMP 120 VAC RV ROOFTOP AC OFF DC BATTERIES ISNT PRACTICAL NOR WILL YOU GET MUCH RUN TIME

I have owned RV's for over 40 years and I now have one with 400 Watts of Solar Panels, Four Golf Cart Batteries (460 Amp Hrs of energy storage capacity),,,,,,,,and an Onan 4KW Genset. NO WAY I WOULD RUN THE AC OFF MY BATTERIES, I would start the Genset.

YES Billy Bob and Bubba I know the AC does not run 100% of the time, it depends on the inside and outside temps and the RV size and insulation

Yes Billy Bob if you have a big enough Inverter and enough batteries you can run an AC for a certain limited time period but that's not long nor practical before you discharge them 50% at which time you should stop.

YES IT CAN "WORK" BUT NO ITS NOT VERY PRACTICAL. A 3000 TO 4000 WATT GENSET WOULD DO A GOOD JOB AND ONLY USE MAYBE ONE GALLON PER HOUR OF GAS TO OPERATE subject to load and efficiency. Sure go ahead and do it if you please and have enough batteries but I wouldn't advise it

John T Long retired electrical engineer so no warranty. Maybe others have different opinions maybe not???

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