Smallest and simplest setup for off-grid A/C use?

I know this question has been asked to death and debated to death. But that makes it harder to find up to date, authoritative answers. The technology and prices keep changing.

Going by what is available off the shelf now, what is the simplest set-up I need to run a small air-conditioning unit in a small trailer?

The trailer is 17.5 feet, but the box is only 13. It is well insulated. We currently have a 7200 BTU roof-top A/C. It more than keeps everything cool. In fact, it does fine running on low. We never run it on medium or high. We could easily get away with having a 5000 BTU A/C, or smaller if we could find one. But finding anything smaller than 5000BTU (2500 - 3000) is difficult if not impossible.

I have seen those mod kits you can buy and install that lower the initial power-draw for firing up the A/C compressor, so if I use one of those, does that make a real difference in how small/cheap my solar setup can be?

How many watts worth of panels would I need?

How big (watts?) worth of inverter would I need?

How big of a battery bank in amp hours would I need?

This is all assuming that the A/C would be used heavily on some days and I would have a generator available for backup if the sky is cloudy or weather is bad.

Related question : what is the smallest (in terms of watts/amps) gas generator I would need to run a small A/C unit?

Sure GalacticStone - very simple stuff.

How many amp-hours (at 120 VAC) does you air condition consume over the course of one typical day of use?

From there, simple math.

Without knowing what AC unit we are discussing nor how much you use it each day, I found the specs for a Coleman 9200 BTU (http://www.campingworld.com/shopping/item/coleman-mach-8-cub-air-conditioner-arctic-white-shroud/72717). It specs 1550 watts while running at 100 degrees outside temp.  I have no idea if it cycles (duty cycle) or how much power it consumes on average so all of this is just a guess based on very little information.  The actual measured power consumption from your AC unit would make a big difference.

1550 watts = 124 amps DC (1550/12.5 volts)
Run AC 8 hours 
8 hours x 124 amps DC = 992 Ah/day DC

If you had a typical lead-acid battery bank, you would need a 2000 Ah bank to run the AC for the 8 hours (to avoid going below 50 percent state of charge).  Assuming 230 Ah 6 volt GC2 batteries, you would need twenty of them. This would weigh something around 1600 pounds.

You would need at least a 2000 watt inverter to run your 1500 watt load.  However; that is cutting it close (startup load and all that) so something a little larger is probably a better bet (like 2500 watt).

I have no experience with the things that smooth/handle the startup voltage so can't answer that.

You would then need to replace the 1000 Ah of power used from the battery bank each day, that is 12,500 watts.  Assuming 6 hours of good sun, you would need 2000 watts of solar (12,500 watts / 6 hours).  Using the large, higher voltage, residential panels rated something around 400 watts, you would need five of them.  You would need an MPPT solar charge controller that can handle that kind of power - probably two or three networked together.

I have no knowledge of such large systems but I do know that Lithium batteries would offer a DISTINCT advantage in this scenario (size, weight, and storage capacity) as would a higher voltage battery bank (say 48 volt).


A generator to run a 1550 watt load would be a 1550 watt generator.  However; the startup load has to be considered so possibly more like a 2000 or 2500 watt generator.

Edit: Peukert's law probably throws all the above calculations out the window. I'm not sure if pulling a constant 125 amps out of a bank of twenty batteries would be an issue or not.  All just theoretical, for the fun of it!!  ;D

Running the a/c is simple - power it with a pure sine inverter of suitable size, probably 2000 watts. Running it for very long, however, requires a huge battery bank to supply that inverter. I doubt if a trailer that size has either the space or the weight capacity to power even a small a/c for more than a few minutes. Maybe 30 min if you are lucky.

You may be talking about a Coleman Mini-Mach, which was a 7200 btu unit.It has been replaced with a 9200 btu model,  which draws a steady 11.7 amps @ 120v in moderate temperatures and a whopping 13 amps at desert temperatures. Since the battery draw by an inverter is a bit more than 10x the output amps, that means you are drawing over 117 amps from the batteries continuously. Ten Group 27 deep cycles, weighing about 650 lbs, would last maybe 4 hours, but I suspect the voltage would drop too low for the inverter to function well before the 4 hours was reached. ANd I don't see any way a small trailer could carry that many batteries.

You can start and run a 5000 BTU window air conditioner with a cheap 1200W generator.  I bought three window units to keep my house cool while remodeling the house.  They worked so well that ten years later I am still using them.  The newer models use R410 refrigerant and only last about three years.  That is still less expense than the annualized cost of a heat pump.  But that's another story.

There are a few youtube videos of off-gridders running 5000 BTU AC's from inverters, batteries and solar panels.  The problem with this is that you generally need AC about three hours after the sun goes down.  This will take a toll on your batteries.

I am going to pull the 13KBTU unit from my 5W and adapt a 5000 BTU window unit as a roof mount.  This is impractical unless you are skilled with and have access to fabrication equipment.  Lacking these, you can still put one through a window or cutout.  It won't look good though.

I've tested at least four of these window units and all of them run about 480 watts.

The start assist device you wrote about is probably just a larger capacitor.  It helps protect the compressor if you have voltage drop during startup but it will not reduce your running load.

Running your AC on low or medium also will not appreciably reduce your load requirements.  This only changes your fan speed.  All the cheap AC fan motors I have tested (and Dometic RV units meet this criteria) draw nearly as much fan current on low as they do on high.  On my Dometic the difference is less than 50 watts. 

RV units are designed for durability, not efficiency.  Window units are designed to efficiently lose their refrigerant in three years.

Not trusting memory, I just checked the Dometic fan load. The difference between high and low is about 38 watts.

A 5000 BTU unit will cool a 17" trailer in any climate.

Gary RV Roamer said:

Running the a/c is simple - power it with a pure sine inverter of suitable size, probably 2000 watts. Running it for very long, however, requires a huge battery bank to supply that inverter. I doubt if a trailer that size has either the space or the weight capacity to power even a small a/c for more than a few minutes. Maybe 30 min if you are lucky.

You may be talking about a Coleman Mini-Mach, which was a 7200 btu unit.It has been replaced with a 9200 btu model,  which draws a steady 11.7 amps @ 120v in moderate temperatures and a whopping 13 amps at desert temperatures. Since the battery draw by an inverter is a bit more than 10x the output amps, that means you are drawing over 117 amps from the batteries continuously. Ten Group 27 deep cycles, weighing about 650 lbs, would last maybe 4 hours, but I suspect the voltage would drop too low for the inverter to function well before the 4 hours was reached. ANd I don't see any way a small trailer could carry that many batteries.

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Gary I'm confused, where did you find 10 times the draw for the inverter, this is the information I have been looking for, so I if run a 4amp load the total draw from the battery will be 40amps? That would give me 2.6Hrs on a 214 amp GC! battery bank, this don't seem correct. Where did I go wrong?

JFN said:

Gary I'm confused, where did you find 10 times the draw for the inverter, this is the information I have been looking for, so I if run a 4amp load the total draw from the battery will be 40amps? That would give me 2.6Hrs on a 214 amp GC! battery bank, this don't seem correct. Where did I go wrong?

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4A X 120V = 480 watts
480 watts ? 12V = 40A

Adds up fast. If I remember correctly my load meter shows about a 160A draw on the batteries when I'm running the microwave on the inverter.

Thank You, does this cover the inverter power requirement?

JFN said:

Thank You, does this cover the inverter power requirement?

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There are others with expertise that can better answer that question. This, like Gary's, is just a quick way to get an idea re power draw.

Based on what I know, it's simply not feasible to run AC on batteries. I think most boondockers avoid weather which requires AC.

Hi Folks,

It would mainly run at night. I guess I would be better off just using a generator. I want something small and quiet. I hear the Hondas are best for that.

What about a small A/C like this one?

http://www.climaterightair.com/products/climateright-models/climateright-cr2500ach-dog-house-heater-and-air-conditioner-with-free-all-weather-canvas-cover.html

Unit Weight (lbs.) 54.7000
Cooling BTU 2500
Heating BTU 2800
Volts 115V
Cooling Watts 480W
Cooling Amps (Operating) 4.3A

Or this - http://minihvac.com/product/unique-mini-air-conditioner/

Another small one about to go into production : https://www.kickstarter.com/projects/1253665084/zero-breeze-the-worlds-coolest-portable-air-condit/description

Gary I'm confused, where did you find 10 times the draw for the inverter, this is the information I have been looking for, so I if run a 4amp load the total draw from the battery will be 40amps? That would give me 2.6Hrs on a 214 amp GC! battery bank, this don't seem correct. Where did I go wrong?

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Simple arithmetic, as Sun2Retire explained. Power (measure in watts) = volts x amps and the power needed is the same whether it comes from battery or shore line. It's actually worse than that, since the inverter isn't 100% efficient. For estimating purposes it's better to assume that the battery amps will be closer to 11x than 10x.

The rest of the battery amp problem is that the amp-hour capacity figures are based on what is called the 20-hour amp rate. That's a very low amp draw (often about 4 amps)  over a 20 hour time period. A battery has a much lower amp-hour capacity when the discharge amp rate is high, and in this case the amp rate is sky-high!

And finally, you can safely use only about 50% of the battery amp-hours without causing permanent loss of capacity in the battery. Heavy duty deep cycles can maybe allow 60% discharge without much impact, while typical marine/RV batteries should not go beyond 50%.

I seen enegerstar rated 5000 BTU windows unit run on .harbor freight 900watt generators.    In fact I put a 5000btu windows into the back of my trailer.  And at night it cools the hole trailer. 27 ft. This allows me to get a 12 to 14 hr run on my 3500 generator vs 7-8 if my main rooftop AC unit is running out .....

If you are parked in the desert  or anywhere you have soft ground you can dig a hole just big enough so the top of the generator is at ground level when you put it in the hole.  This is (or was) SOP in army field operations and will make the cheapest generator quieter than any Honda.

The 5000 BTU AC runs fine on a 50 foot, 12 AWG extension cord.  You might want to have a lanyard cable made to lock the generator to something, to keep it from walking off at 4AM.

If you require something less than 5000 BTU you might look for a surplus industrial electrical enclosure cooler.  I saw some on craigslist ( or ebay, can't remember) a few months ago that were new take-offs for $100.  These are built to run forever because a failure can shut down an entire production line.  Since they use R134 refrigerant they won't have evaporator coil failures as is common with R410 window units.

Found this video that may give you some ideas:
http://www.gonewiththewynns.com/off-grid-solar-rv-air-conditioning

Cheers

whiteva said:

Found this video that may give you some ideas:
http://www.gonewiththewynns.com/off-grid-solar-rv-air-conditioning

Cheers

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