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.
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