Author Topic: roof ac//inverter  (Read 5928 times)

Offline Busnut83

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roof ac//inverter
« on: December 05, 2006, 09:03:57 PM »
can an inverter run a roof ac??? what size inverter???? how many batteries???

Offline Buffalo SpaceShip

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Re: roof ac//inverter
« Reply #1 on: December 05, 2006, 10:26:37 PM »
Assuming you mean, running an A/C off of an inverter whilst "running down the road", with the alternator putting the amps right back in... yes, you can do that with most 2000+w inverters. We regularly run our front rooftop unit off of our 2500w unit while underway. A big 24v alt and 4000w inverter could easily power TWO rooftop units, I'd think. And even a modest batt bank could handle the surges of these systems.

But, if you're talking about running an A/C unit off of an inverter while parked/ engine off... well, you'd need a huge battery bank and still wouldn't get much runtime. Such a draw would likely fair even worse than the amp-hour ratings math would tell you.

Brian B.
Brian Brown
4108-216 w/ V730
Longmont, CO

Offline TomC

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Re: roof ac//inverter
« Reply #2 on: December 05, 2006, 10:30:38 PM »
Yes it can.  Typically you need at least a 2500 watt inverter (to be able to start the compressor).  If the A/C pulls 1650 watts at 120vac, then at 12v it will be pulling 137 amps.  Figure 85% effeciency and you're looking at closer to 160amp draw.  If you consider a typical size 8D deep cycle at 220 amp hours and figuring you don't want to run the batteries down beyond 50% (for maximum battery life), you'll need 12 batteries for 8 hours.  Or the math- 12 batteries times 220= 2640.  Divide that by half for 50% discharge and you have 1320 amp hours.  Divide 1320 by 160 amps per hour and you have 8.25 hours.  Considering that 12 8D's (150lb each) will weigh 1800lb, in my opinion, it is better just to run the generator, and use just two 8D's for your miscellaneous lighting and lite duty inverter work.  I believe the inverter should be considered a second power source for things like coffee maker, microwave, TV, Stereo, DVD, small heater,etc.  Bigger loads like the A/C (which is the biggest loads) should be taken care of by the generator.  Good Luck, TomC
Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.

Offline pvcces

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Re: roof ac//inverter
« Reply #3 on: December 05, 2006, 10:44:33 PM »
Busnut83, you're only providing some of the information needed to give you an answer. From what you asked, here is what I can tell you.

The inverter has to be able the start the AC compressor; they vary by a factor of at least two.

Heating and cooling is a no-no on batteries because it is too inefficient of a method. Nevertheless, some people do it.

A full sized roof top AC will need, at a minimum, a 2500 VA inverter to start the AC. Better to go with 4000 VA for reliability and efficiency. The bus generator can power the inverter in this case. The SW4024 can run two ACs, just barely.

Six golf cart batteries will only cover about 2 hours if full charged; only about 1 1/3 hours while cycling. This means that job is better left to shore power or a generator.

The batteries will weigh 390 pounds; an auxiliary generator might use around 1/2 gal. per hour running two ACs.

A comparison of stored energy: a couple ounces of propane can displace a 65 pound battery.

Good luck figuring out what you would like to do.

Tom Caffrey PD4106-2576
Ketchikan, Alaska

Offline buswarrior

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Re: roof ac//inverter
« Reply #4 on: December 05, 2006, 11:23:45 PM »
And another variable to lower that big number would be the actual run time of the AC compressor.

I know of a coach that was able to keep the bed room cool overnight using 8 golf cart batteries without pulling them below 50%.

Windows out, insulated, and already cooled down prior to enjoying the relative silence of a ducted roof air on low and no generator.

The AC wasn't cycling on too much in the dark.

So, if you want, you may enjoy a quieter night without carrying too much extra weight, as long as you get your math right and build the coach right.

happy coaching!
Frozen North, Greater Toronto Area

Offline NewbeeMC9

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Re: roof ac//inverter
« Reply #5 on: December 06, 2006, 03:35:09 AM »
I carry a couple of the cheapo oscillating fans that don't use much draw, and the batterries will run them all night easily, quietly.  However I'm not sure how it would work in the desert.  I have loud colemans that don't have a low speed either.
It's all fun and games til someone gets hurt. ;)

Offline Ross

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Re: roof ac//inverter
« Reply #6 on: December 06, 2006, 07:31:15 AM »

I know of a coach that was able to keep the bed room cool overnight using 8 golf cart batteries without pulling them below 50%.

I can run one AC all night and not go below 50%.  That's on five Interstate 1850's (1000AH) and a Xantrex RS3000.  Bill Glenn (space shuttle bus) can go all night and have 70% left.  I think he also has an RS3000 with six Interstate L16's.  With the right inverter and the right battery bank, it is easy to go all night with one AC.  It's nice when traveling because the batterys get charged while driving all day, then at night you have all you need without running the genset.