How many watts does a typical 13,000 BTU A/C draw?

Mex-Busnut · July 13, 2011, 10:54:41 AM

I am looking at alternatives to a generator, and found this inverter:

http://www.harborfreight.com/5000-watt-continuous-10000-watt-peak-power-inverter-96706.html

I assume I would need a bank of 4 batteries or so to feed it? I want to be able to go 24 hours unplugged.

Thanks in advance!

robertglines1 · July 13, 2011, 11:15:36 AM

Would think you would be wishing to much there. For example a very efficient one saying drawing 12 amps would probably run a few hrs at best on a massive battery bank. Other more informed with figures will chime in . It has been discussed here to some extent. I listened and decided to go with running my gen set instead of the enormous cost of equipment required to accomplish a minimal amount of power off grid. Try search function. I think you would be looking at a pure sine inverter and at least a 10 battery bank @ a 3 to 5 grand investment. Just rough figures. Those who have done it will chime in and correct me.. I decided to run gen set and buy fuel myself. Bob. By the way ck out the amp on the mini -split run amp aprox 9

bevans6 · July 13, 2011, 11:20:03 AM

There should be a permanent inverter thread! For a lot of reasons that's not a great choice for your application, chief being that it's not designed to be permanently wired into anything, just has four plugs on the front, and that it's modified sine wave which is not well suited to powering induction motors in air conditioners. Pure sine wave is a better option at around twice the price.

A typical 13000 btu AC will draw 18 to 20 amps starting, and around 12 amps running on 120 VAC. That is 2160 - 2400 watts starting, and 1440 watts running. Assume no more than 85% efficiency (inverter loss and MSW incompatability), so you would be drawing call it 1700 watts whenever the AC is on, or 142 amps at nominal 12 volts. Assume no more than 50% state of charge draw-down on your batteries, 24 hours of use would require 6,816 amp hours of rated battery capacity. That's around 27 typical 8D batteries. At 150 lbs per battery that's two tons of batteries.

There is a reason no one runs air conditioners from batteries...

edit:
I bought an inverter pretty much the same as this one, except they changed the model number.

http://www.samlexamerica.com/products/productdescription.asp?ProductsID=19023

It is 24 volt, which matches the alternator output of my bus (yours may be different) and reduces the supply current and cable requirement by half compared to 12 volt, is 3000 watt pure sine wave, and is designed for direct permanent connection to the AC distribution panel. They do have other models now with built in switching and chargers, that weren't available when I bought mine.

Brian

robertglines1 · July 13, 2011, 11:23:54 AM

amps X volt = watts so 120 volt X 10 amps = 1200 watts so if a roof top pulls 15 amps X 120 volt=1800watt continuous : probably up to 2200watts on start up. Hope that helps. Bob saw bevans posted also will let mine stand- sure his is more accurate.

Mex-Busnut · July 13, 2011, 11:27:41 AM

Thanks for all the great info!

So a decent 8,500 watt generator would handle the 2 rooftop A/C units?

By decent: I mean NOT a Chinese version.

bevans6 · July 13, 2011, 11:28:54 AM

It should handle that load just fine, but I am not a generator guy...

Brian

robertglines1 · July 13, 2011, 11:35:46 AM

8500 continuous --yes not surge a trick by manufactures to make a gen set seam bigger. 8500 divided by 120 = amps

jjrbus · July 13, 2011, 11:47:51 AM

I started with this type of inverter from TSC. I thought it would be ok. First they just plain lie about the rated output. 2nd how do you charge the batteries? It can be a PIA.

Next I went to a decent Tripp lite inverter, good quality but had a single stage battery charger. Took forever to charge the batteries.

I ended up with a trace DR3624,modified sine wave, hardwired in, 3 stage battery charger, served me well for years and well worth the money. Probably paid for itself in the battery's it saved me from destroying.

With 4 6V golf cart type batteries would go 24 hours easy. TV, small house type refrigerator, microwave, coffee maker, lights, computer. NO AC, one roof air would run the batteries down in about an hour or less!
HTH JIm

thomasinnv · July 13, 2011, 04:54:03 PM

My generator is 7500 watt and it will carry all 3 of my 13,500 roof units. I can even run the micro. Me thinks the engine and alt might actually have a higher capacity but just tagged for a lower output as some units are.

Mex-Busnut · July 13, 2011, 05:49:47 PM

Quote from: thomasinnv on July 13, 2011, 04:54:03 PM
My generator is 7500 watt and it will carry all 3 of my 13,500 roof units. I can even run the micro.

What brand/model, Brother Thomas?

thomasinnv · July 13, 2011, 06:38:31 PM

It is a guardian generac QP7500. it comes in it's own box. some people don't like them but i have had 2 of them before the one in the bus and one onan and they all performed every bit as well as the onan, and cost less. it was a squeeze getting it in though, i put it in the otr a/c compartment and it is literally the same width as the opening. took a little jockeying to get it in. east coast generator is who i bought all my generacs from.

Ps. the fuel usage specs are quoted a little on the heavy side in my experience. My full load fuel usage is close to what they state for half load usage. pretty efficient for the output i think. Not as good as deisel obviously.

buswarrior · July 13, 2011, 07:08:06 PM

Air Conditioning on the batteries can be done... but...

A busnut who I have lost track of, had the rear bedroom in an MC9 heavily insulated and with the door closed, well isolated from the rest of the coach.

If the coach is already cooled down, during the night, with no sun load, the bedroom AC should cycle to keep that bedroom coolin most places they planned to be, so not a completely flat out continuous draw.

His design criteria was 50% battery draw for the night, and included a fair bit of experimentation along the way to arrive at his practical consumption estimates.

Last I saw him a decade ago, there were 8 golf cart batteries on a custom pull out tray in the spare tire compartment, with the space to put another 8 when he shifted to full-timing.

The justification?

SILENCE.

For some, priceless.

Do your math and research, then do it your way.

happy coaching!
buswarrior

pvcces · July 13, 2011, 10:30:30 PM

If you do all your loads and capacities in KWH, your battery needs will be easy to arrive at.

A golf cart battery can produce about 1 1/3 KWH, which means that you can use about 2/3 KWH before recharging, if you want your batteries to last. And, you will only get that much out of a golf cart battery if you discharge it over 20 hours.

A good air conditioner will use about 1 KWH of power per ton of capacity. That's 12,000 BTU per hour.

Two rooftop units might run up to 2 1/2 tons or 30,000 BTU. That means that you would be able to get about one hour of useful battery out of 4 golf carts. At 8 batteries, you would get two hours run time. That battery bank would weigh 520 pounds.

Using L16 batteries, you can get close to twice the power of golf carts with very little increase in battery footprint. If this idea interests you, be sure to check on what they cost. A busnut friend of ours replaced his set of four L16s recently for $1300.

A golf cart battery can be replaced by less than 3 ounces of propane. Compare that to 65 pounds for the battery.

A lower cost way to accomplish what you want is to buy an inverter generator such as a Honda or Yamaha 3000. They can start and run one air conditioner, if it's not real hard to start, and they are quiet enough to be used a lot of the time. The main drawback to this solution is that they should set in the open when running, and not be used in a compartment.

The rule that I go by is to avoid cooling or heating on batteries as much as possible. Then, a solar setup can carry you a surprising amount of time.

Good luck on your quest.

Tom Caffrey

TomC · July 14, 2011, 06:21:07 AM

I can appreciate you trying to find your items on a budget. But a lawn mower engine run generator (loud) and cheap inverters from Harbor Freight-they may work for awhile, but they will fail under hard use and you'll eventually want to buy both a quality generator and a quality inverter/charger like the Magnum 2800. You'll need to re charge the batteries, and the automatic inverters are mindless. I've had the same Trace 2,500watt MSW inverter/charger since 1994. My personal opinion-save your money and buy the best quality product you can when you can, rather then buying a everything at once that is el cheapo-you'll be much happier in the long run with much less frustration. Good Luck, TomC

Busted Knuckle · July 14, 2011, 06:37:01 AM

TomC,
While I know an agree with your no generator too quiet philosophy.
And also agree with buying quality the 1st time.

Please keep in mind that he is doing this build on a very tight schedule and budget in order to have it on the road in time to deliver his daughter to college in Wisconsin this Fall.

We all know that money saved by doing it right the first time is the way to go, but sometimes $ ain't as important as other things such as the the time spent using it to take his daughter off to college in it.

BK