Wednesday, March 21, 2012

Frugal Air Conditioning for Offgrid and RV Living

Right about now is when air conditioning begins to feel like a necessity, at least in hotter areas. But what to do if you're offgrid or camp fulltime in an RV and are on a tight budget? A few solar panels aren't going to power air conditioners. An offgrid diesel generator will, but running it fulltime is expensive, even on diy biodiesel. Of course, there are a lot of things you can do to reduce your need for a/c, such as ventilation, shading, a reflective roof on your domicile (mobile or not), routing any heat produced by appliances (including a refrigerator) outdoors etc. and we should definitely be doing all those things, but it's nice to be able to sleep in air conditioning. So I am going to cover just that, while spending the minimum amount of money.

First of all, you need to minimize the area you need to cool. If it's so hot and humid that you have trouble resting, you don't necessarily have to cool an entire house. Ventilate it, yes. Make it as cool as possible. But the main thing we are concerned about is a sleeping area, and perhaps a cool area to spend the hottest part of the day. So in a house, you should close off the bedroom and concentrate on cooling it. Of course if you are vandwelling that is a moot point, because you are already in a small space.

Second, you will need a power source, including a generator, battery bank and inverter. Fortunately, inverters have dropped in price enough that one capable of running the smallest air conditioner is no longer very expensive. A 2500 watt inverter can be had for under $500 nowadays, and that will be sufficient to run a 5000 btu window unit a/c while also running a couple of efficient lights, a TV/DVD combo or computer, and even a small microwave oven as long as you make sure the compressor in the a/c doesn't cycle on while the microwave is running. Now, 500 bucks may not sound very cheap but it is, compared to what a 2500 watt inverter cost just a few years ago! BTW, a modified sine wave inverter will work just fine; you don't need to spring for the high-dollar pure sine wave inverters.

On to the battery bank. This is the biggie; the one you can't scrimp on too much. You're probably not gonna want to run your genny overnight, so that means ya gotta have enough battery capacity to carry the load overnight. A small a/c is gonna draw
about 6 amps of 115 volt "wall power" so that means about 60 amps of 12 volt battery power going into your inverter. BTW, that can surge to triple the running amperage on startup, which is why you need such a big inverter. Now, 60 amps times say 8
hours is 480 amp/hours battery capacity needed to carry the load overnight. For comparison, a 12 volt marine deep-cycle battery has a capacity of maybe 100 amp/hours so you would need about 5 of those puppies. But wait; you can't run those
batteries all the way down every night or you would have to replace them very soon! Besides, what if you want to sleep late or you would like to run a coffeemaker in the morning, before having to mess with that generator?
You really should only run your battery bank down to 50% on a regular basis; 80% is even better but we ARE on a budget here.

While we're at it, let's get better batteries than the typical marine battery. The best deal for offgrid folks on a budget is the golf cart battery. Trojan T105 is the industry standard here; they are much better than marine batteries while costing
about the same. The T105 is 6 volts, like other golf cart batteries, so will have to be paired in series for 12 volts. This battery is 220 amp/hours; two of them in series will still be 220 amp/hours but at 12 volts. Four pairs connected in parallel
will be 880 amp/hours, now we're getting somewhere! This battery bank still won't be too heavy for your house-on-wheels to carry (about 500 pounds), nor too expensive, nor too bulky to find a place to store it.
But it's a big enough battery bank to shut down that genny at a reasonable hour, stay up watching TV for quite awhile, run the a/c overnight, and still run the coffeemaker and maybe even the microwave in the morning, without beating down the
batteries too badly.

Now, what kind of air conditioner do I need? A NEW one, for one thing. I have measured the amperage draw of old vs. new, and the new air conditioners definitely draw less current for a given btu rating than do the old ones. Second, a SMALL one. The cheap, 5000
btu, manual control unit is exactly what you need for offgrid living. If it isn't doing the job, you need to make the area you are cooling smaller, insulate it better, or both. If you have a big enough generator and are living in a house, by all means buy a second, larger air conditioner to cool the whole house during the day while the genny is running, if you want to and can afford it. But for overnight running, you need the minimum amp draw of a small unit. Oh, and by the way, don't bother with the "low" setting on that little air conditioner: I measured the amp draw, and it doesn't make it use any less power.

That brings us to the (next to) last item on the list, and believe it or not, the most flexible: the generator. You DON'T need a honkin' 20 kw genny for this usage! In fact, you can get by minimally on one of those 2-cycle, 1000 watt portable
jobs you can buy for just over $100; in fact, I have added a link to one of those if you are interested. Actually, one of those makes a good backup even if you have a bigger genny. A genny that small probably won't directly run the air conditioner, but what we're gonna do here is buy the biggest battery charger you can find (which is the last item on the list); the standard manual 40 amp continuous charger that all auto parts stores and most big-box stores carry will be the minimum. Not the one that says 40 or 50 amp engine start; if this one has an engine start function it will be 200 amps or so, but we're not gonna use it in that mode. You will need this regardless of which generator you buy, unless you buy one of the high-dollar inverters that have a built-in 100 amp or so charger.
That 40 amp battery charger will be just about a 50-75% load on one of the 1000-1200 watt generators; an efficient range to run any generator in. That means you won't be able to run much else at the same time. Maybe a small refrigerator. If you expect to be able to run the a/c and other loads while charging your batteries, you will need at least a 3000 watt generator. It is highly unlikely that you will need more than 5000 watts.
If you can get a diesel generator, by all means do so. You will probably want to eventually anyway, if you stick with the offgrid lifestyle for long. Diesels are indeed more fuel-efficient. But don't be fooled by what some folks say about the efficiency of a gasoline genny; for this
type of use one of the small ones will run 3-5 hours per gallon of gas. You will probably be using about 2 gallons per day
during the hot season.


Anonymous said...

Great Article. How long does this 880amp battery take to charge at 40amps after a night of running the A/C?

Tracy said...

Thanks. As mentioned, you will only be using about 50% or less of the battery capacity. The A/C will cycle on and off, so figure 300 ah from the battery for an 8-10 hour run. That means you will be bulk charging for 10 hours or so, plus absorption charging for awhile longer. Basically, count on running it from first thing in the morning 'til just before bedtime.
That's why I said you can get by minimally. A bigger generator and better, higher-rate charger, or simply a second small generator and 40 amp charger, with the DC outputs paralleled, would work much better.
Of course, getting by with a small gas generator or two is a short-term solution. Eventually the off-grid home really needs a serious diesel generator and some PV and/or wind power.
And when I say serious diesel generator, I don't mean huge; just capable of handling extended runtimes.

Rich said...

"BTW, a modified sine wave inverter will work just fine; you don't need to spring for the high-dollar pure sine wave inverters."..

Have you heard of anyone running a Sanyo Mini-split AC/Heatpump using a MSW (modified sine wave) inverter?

I have a nice 120vac 2.5 kw "stackable' MSW inverter (Ebay) and I'm thinking of another one, but in 230vac for my Sanyo.

Tracy said...

Not specifically. But if the inverter will handle the starting surge, I see no reason why it wouldn't work, with a sufficiently-sized battery bank.
Of course you do so at your own risk. Motors tend to run hotter on MSW than on pure SW, so it is possible that a less-than-robust compressor could fail when running on such an inverter. It would probably be even more likely to fail from being powered by a portable generator, though.
Don't get me wrong, I'm not saying the Sanyo is not robust; just that as the owner, you have to weigh the risks and make the call.

sgrin said...

So if I had a 5kw generator, a 100A charge controller, a 2500w inverter,and 8 trojan t105 6v batteries, How long would it take to charge those batteries? Assuming I only use 300 ah a day as you suggest. 3 hours? I have been reading all kinds of things on the internet only to get confused by so much conflicting info.
I like your blog very much!

Tracy said...

Assuming that your generator and charger combo is actually capable of delivering a 100 amperes to your battery bank for the duration of the bulk charge, it will take at least 4 hours; because no battery is 100% efficient. That still won't fully recharge it; it will need an absorption charge at a lower rate.
What this basically means is that you can fire up the genny when you get up in the morning, and bulk charge while you cook and eat breakfast and do whatever else you need before heading out for the day. If you will be staying around, go ahead and let it complete the bulk charge and maybe even absorption charge for a couple of hours or as long as you need the output of the generator. If you can't hang around that long and don't want to leave it running unattended, just bulk charge for as long as you can. When you return in the evening you will want to run the generator anyway to power the A/C and other stuff, so you charge it for a few more hours.
Bottom line: between bulk and absorption charge, you will need to run the genny for at least 6 hours to fully recover from a 300 ah draw-down. You can go a few days of only bulk-charging for 3 or 4 hours per day, but you will need to run it for a full cycle at least once per week, and preferably twice.
I know that's clear as mud, but battery charging is more complex than it appears, especially when you throw a generator into the mix.

Anonymous said...

How about adding or replacing your alternator with a high output 250amp one?

Tracy said...

High-output alternators are useful devices, but they are horribly inefficient and thus a poor choice for any kind of long-term electricity generation. I used one for awhile to recharge a battery bank that I ran an air conditioner and some other stuff on, and the engine (a Lister-clone diesel) used twice the amount of fuel as the AC generator head I replaced it with. Same engine, everything else the same. The loss wasn't in the inverter either, because it was a Trace industrial inverter that I tested at over 90% efficiency.

Reuben Alvesson said...

Hi I need to run an AC for a one room Minihouse (about 30m3 volume to cool). Question: What size of A/c do I need ? 5000 btu is enough ?

Tracy said...

Yes, 5,000 btu should be enough for 30 cubic meters if well insulated.

Newbie Vandweller said...

Hi, I'm trying to make a list of things I need to become a van dweller and run a solar setup to power a small refrigerator nonstop, a portable air conditioner small enough to cool a van, maybe a hotplate, and to charge my phone. I'm guessing that I might need a 3000w inverter for all of this. Can you help me choose and size up my other needs. I'm thinking 2 12v Vmax marine batteries w 155aH, 4 100w flexible solar panels (for stealth reasons and a 30a or 40a mppt solar controller. I don't have a clue really though.

Tracy said...

I think you are on the right track in terms of how much power you will need vs. what you can fit, carry and use in your van. The primary power user will be your refrigerator. A small compressor type refrigerator will use pretty close to 1,000 watt/hours per day. A 400 watt solar array (like 4 of these panels: ) should cover that with a reasonable excess for powering your other loads. This charge controller has good reviews and will probably meet your needs:
The batteries you are looking at are a reasonable choice. The battery bank is really the heart of your system. If possible, increasing tha capacity and using a pair of 6V batteries in series (instead of 12V in parallel) would be a worthwhile upgrade, especially for the times that you need to use the air conditioner. Series connection is more efficient in terms of energy storage and battery life than parallel connection. To that end, I would urge you to consider a pair of L16s, like these:
There are a couple of things you didn't mention. First is a means of charging your battery bank while you drive. You need a battery isolator to send charging current from the alternator to your battery bank any time the engine is running. Not only does that add to the charge when you drive; it also provides an emergency backup. It's no fun to wake up to a dead battery bank because you had to run the air conditioner all night. With a battery isolator, just start the van's engine and you can begin recharging the batteries.
Now to the inverter. Yes, 3,000 watts should be sufficient for your needs. Buy the best inverter you can, though. I recommend Xantrex. I have an old inverter they made back when their name was Trace. I have traveled with it in a van camper with it, lived offgrid in the wilderness with it, and still have the same inverter nearly 20 years later. You can't beat that kind of quality.
A very important feature a high-quality inverter has that you won't find in a cheaper inverter is a built-in battery charger and shore power connection. The way it works is, you add an RV power cable (like this for campgrounds and you also need an adapter for 15 amp so you can plug in other places ) so you can connect to shore power (friend/family home, established campground, etc.)and just like that, you are running your loads and also charging your batteries. There will be times when your batteries are low and you need a full charge. It is very good for battery life to occasionally give them a full charge and maybe even an equalize charge, and especially in the hot summer it's nice to sometimes be able to run the air conditioner all night while doing that, and then have a nice hot shower in the morning. Campgrounds are good for that. Here is the inverter I would be looking at.It is very similar to the one I have:
Yes it is expensive, but I have wasted money on cheaper inverters and in my experience, a really good inverter/charger is well worth the money.
I hope this has been helpful.