power inverter/ a/c unit

Friend has an old bus turned party/tour bus, wants to put a/c in it. Was wondering if a person could put in a R/V roof top unit and power it with an inverter. What size would you need. R/V units call for 20 amps, Inverters only list watts as I am finding.

Amps x voltage = watts.

watts divided by volts = amps

I ran a small window unit on a 2500 wat with 4 deap cycle batteries and it would keep my truck cool for about 8 hours on a charge. The problem was the truck wouldnt charge that much in a day of driving. An inverter works better than a generator because they have double the wats for surge for starting the air conditioner. It takes 3500 wats in a generator to run the same airconditioner.

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???

If you had a 60 amp alternator running full time full blast (or maybe an 80 or 100 amp would do more of course) that could help offset the 120 amps the Inverter would draw to operate a 12 amp 120 volt air conditioner!!!!!!!! But like you noted, 60 to 80 amps of alternator power cant keep up with a 120 amp load. Of course, it depends on the size of the AC and the alternator etc and the AC's duty cycle and several factors.

John T

Most truck alternators are 135 amps, but of course trucks use a lot of amps while running so there is not a lot left. Plus with the engine off the truck drains the batteries significantly over night so you need to charge those 4 batteries every day too. They sell battery powered air units for trucks and those units come with a big altermator I think 500 amps.

(quoted from post at 20:36:59 07/21/15) Amps x voltage = watts.

Click to expand...

Only is the power factor is at unity with a resistor load. AC units have motors. The Volt Amps could be 1500 and the watts 1200. Still have to size for the VA .
Some generators have that in the small print.
As Ol' John T said. Buddy will be much much happier with an Inverter Generator instead of converting 12VDC to 120 AC.

Thanks Guys this is a 98 bluebird bus with 3116 cat. It has two 12v batterys. A very large looking altenater, also already has an inverter rated at 1500watt cont 3000 watt max that Some lights, a tv, and music equipment. Never thought of a gen set, don't want a lot of noise though.

A quality genset will ge very quiet, but get ready to spend some money for oe that's quiet. Honda makes some good ones. They have the inverter type that can be stacked so you buy 2, but only use one for light losds then start the other for running things likebyour airconditioner. A friend of mine has that set up and it works good and you can barely hear it inside.

This was my business for many years. I ran Blue Bird and Thomas bodies (all on Ford chassis). Blue Bird are by far the most weather tight. He will need TWO air units no question. I had limo tinting on all windows and RV gensets underneath the buses running off of the bus fuel tank. When they were sitting closed up it would make the bus very cool, but as soon as it starts to roll it is all over. They are too drafty for the units to keep up. I ran 15k units. I also had fans mounted underneath the booth seating and under the couch style seating to keep the air moving.

The nice thing about those types of charters is that they generally run at night so you don't gave sun to contend with. Oh, and tell your friend to pull all of the knobs off of the units and give the driver one to control it with. When passengers get on after being in some bar for two hours and the little Barbies think it is cold they try to mess with the thermostat. Suddenly you are rolling, everyone is cooking, and you find some stupid girl that just can't figure out how that happened.

Tell him good luck. He will work his tail off making a party bus comfortable in the summer. I wasn't sad to see the last bus leave the driveway.

Need a description of the bus before being able to answer the question. Things such as age, model, make, size, amount of insulation in body (if any at all), & engine size all matter. If you would have more than 10-15 people on board, definitely need fresh air exchange or it gets really stuffy & smelly in short time.
Further info. Spent about 7 years in the bus business. First outfit I worked at was basically school operation, but had 3 road coaches & one we called "Old Glory".
That was a school body shell that had been refitted. IHC DV550 engine, air suspension, coach seating, extra insulation, add on a/c. The a/c was self contained, had its own engine to spin the compressor & fans. Was semi-comfortable at less than 80 deg outside temp if not parked in sunshine. After about 40k miles it developed so many body rattles that the boss traded it for another road coach.
Rest of my bus time was spent in road coaches. Thick body insulation, sealed, tinted, double pane windows. A/c compressor was driven off the Detroit 8v71 or 6v92 engine. Condensor fan was electric, powered by 24 volt, 400 amp alternator. When that a/c kicked in, it would make the engine sag for a few seconds.
More than you wanted to know.
Willie

ANOTHER OPTION . . .

Forget about the Air Conditioner, Inverters, Gen Sets, etc. . . . and install a Swamp Cooler. A/C only CHILLS the air, while a Swamp Cooler Chills AND Humidifies the air. Plus, you can swap out the squirrel cage motor & water pump motor with 12 volt DC motors and run them off the vehicles existing electrical system.

I live in a 40 ft. 5th-wheel Travel Trailer that has a roof-top mounted RV A/C unit, and I haven't used it in over 10 years (and I live in the DESERT). Instead, I use a Swamp Cooler. Much cheaper to run and it cools down the entire trailer very effectively.

Doc :>)

You're welcome. Now with your new info I can say:

Again, lets do some math and engineering and not just guess.

1) That 3116 might have a HUGE alternator much bigger then an automotive 60 amp. THEREFORE if it can provide say 100 amps??? continuous into a battery bank, that's ALMOST enough to provide the 120+ amps required to power a 120 volt 12 amp rooftop Air Conditioner.

2) I doubt only one RV rooftop AC will cool a bus that big subject to temperature and insulation

3) Depending on design and quality, a 1500/3000 surge Inverter may be close to its max to power a 120 volt 12 amp AC (That's 1440 watts if all is perfect which it isn't) I WOULDNT BET MY LIFE IT WILL START AND RUN IT VERY WELL However, a 2500 watt more likely would do the job

4) Compared to the sound of a bus and a cat engine, a quality RV genset like a Cummins Onan is NOT real noisy.

John T

INDEED, the AC may pull 120+ amps out of the battery bank to run a 12 amp 120 volt AC, so the alternator needs to be big enough to power BOTH the AC PLUS the other loads. Using the engine batteries and an Inverter to run a RV rooftop AC IS GONNA TAKE ONE HUGE ALTERNATOR

John T

AC chills and DEhumidifies,
I can't think of many times
you would want to add humidity.

Best all around solution would be one of the "tripaks" that they use on big trucks to use while they are parked overnight to cool the sleepers while the driver sleeps. They provide heating, air conditioning, and even electric in place of having to run the main engine all night. Usually fairly economical and run on diesel fuel. Noise issue can be minimized by placing the unit in an insulated compartment.

A swamp cooler sounds like a good solution in your area. How well do they work in 90 percent relative humidity?