From a post in the electric bill reduction thread:
Oh boy. Just like in my solar panel sojourn, I have a lot to learn. I've never given a thought to A/C before, not much call for it in Fairbanks, Alaska where I lived for the past 50+ years. So far my research into A/C is like when I tried to learn Unix: everything I learn serves only to expand the horizons of my ignorance.

Off the top of my head I can think of seven general areas of ignorance that I need to address.

1) What general style of A/C is appropriate? There are (a) portable units, (b) window style units, (c) split systems, (d) central A/C, and probably a few others I am not aware of. I am leaning towards (c), the split system, something along the lines of LMU240HE , not necessarily that exact model but of that type.

2) I am not sure how to size the unit. I know there are rules of thumb, X BTU capacity for Y room area, but there are a lot of other factors involved. Here is what I am trying to cool: about 800 square feet, with floor to ceiling glass in three of the four walls. The building itself is completely un-shaded until about 7pm when the sun finally goes behind a big jacaranda tree. The North wall is solid brick and never sees direct sun. The East and West walls have gauzy white translucent curtains. The South wall faces the sun during the day, has no covering, but my awnings do not allow any direct sun to reach the inside of the house; during the day it is not possible to actually see the sun. There will be two A/C outlets (heat pumps), one in the bedroom, the other in the kitchen/living room. The two rooms are of comparable size. There will rarely be more than two people in the house. For 10 months of the year there is no need for A/C, but April and May can be uncomfortable, with outside temperatures in the high 80's, low 90's. Apparently some of these split systems also operate in heating mode. [How does that work?] That might be useful for about 10-15 days out of the year, but not essential. So, given that information, how big a unit would be appropriate? It is my understanding that an over-sized unit working easily will use less electricity than an undersized unit working too hard. Better to err by too much than by too little.

3) Juggling the variables of efficiency, up-front cost, long-term operating expense, and quality can be mind-boggling. Apparently the magic number is the SEER (Seasonal Energy Efficiency Rating) and the US Department of Energy has mandated a minimum rating of 13 for new units sold in the US. Many of units in the 12K--24K BTU range (including the one linked above) seem to be at this minimum rating. Is it cost effective to pursue higher ratings, considering up front costs, operating costs, and longevity?

4) How can I get a handle on my actual electricity usage costs? Yes, dividing the rated BTUs of the unit by the SEER rating will give me nominal operating wattage, but how do I factor in the time variable? For example, my refrigerator uses 50 kWh per month. It is "on" 24 hours a day, so theoretically it is using about 68 watts. But of course the compressor only runs about a quarter of the time, so it really uses about 250 watts, the rest of the time it uses nothing. So if I have an 18,000 BTU, SEER-rated "13", air conditioner, how much of the time is it actually using its 1385 watts? [BTU/SEER] Does it cycle on and off like my refrigerator? If it is on 8 hours a day will I be using 11 kWh a day, or some lesser number? How do I figure this?

5) How do I go about choosing a specific make and model? I see brands like Amvent, Sea Breeze, Klimaire, Turbo Air, Pridiom, Federer, Mr Slim, Global Industrial, Friedrich, Solius, Frigidaire, Napoleon, etc, and that's just for the heat pumps. There's another list of manufacturers for the compressors. Here in Mexico, it's likely to be Manuel's Tacos and Air Conditioners. All I can do, I guess, is rely on recommendations from people I trust. (Hint, hint.) Does anyone here have specific recommendations based on the above information?

6) Power requirements. I will have 220V power available on the roof where the compressor will be installed, I will only have 110V inside the house. Does the inside unit also run on 220V?

7) Installation. From what I have read, this is NOT a DIY process. Special tools are required (vacuum pump, pressure gauge, torque wrench) and fanatical devotion to cleanliness to avoid contaminating the refrigerant. How do I find a competent installer?

Sigh... maybe it would be easier for me just to move back to Alaska.

tanstaafl.

I am guessing you don't have ducts so central would be out unless you want to spend a ton putting those in if you even could.

Window and portable units suck because they are loud and don't really cover a large area. I think a split system would be the best if you can't have central. There are multi zone split systems too so you would only need one compressor.

I looked at the install manual for a couple of split systems and it looks like the indoor unit gets power from the outside unit.

http://www.alpinehomeair.com/related/Friedrich%20M18DYF%20Installation%20Manual.pdf

Split units are what are primarily used here in Belize. Added benefit of being much harder to steal than portable or window units.

I have a portable unit and it's kinda a pain. You've still gotta find somewhere to vent it and find a way to get rid of the water. It was free for me though, and works great other than that, so I really have no complaints.


Here in Belize, I usually find out who can competently work on stuff by asking other expats who've been here a while. Word gets around. Plus, you may make some new friends in the process.

Split is the way to go.

A "heat pump" is the entire system, not just the inside part. They work by pumping heat from one place to another. Typically air cons pump heat from inside to out, but since the components inside and out are about the same, many systems can run in reverse, pumping heat from outside to in. This is way more efficient than a resistive heater.

The cool thing about split systems is you can have heads in all the rooms, and use them only when you need them.

The split system I had in Singapore used separate power to run the fans on the inside head. The one I'm planning to install here is also that way, but 220v for the inside units. In Taiwan the inside heads are 110v.

In my opinion, you should buy a system from one of the big asian brands, as this is all they use over there and the quality is good. A couple of the brands in your list I recognize as crap.

This is what I'm installing:

http://www.rsd.net/search/item.php?item_no=FUJI+ASU12RMLQ

http://www.rsd.net/search/item.php?item_no=FUJI+AOU36RML1

As far as sizing it, use the online calculators. Do your best, then double it. I wish I had done that for my house. It's a 1947 vintage with absolutely no insulation (or moisture barrier), and there's days in the summer that the house will heat up to 85°f inside with the central air going full blast. I can only guess that the online calculators were assuming a far better insulated house than I have.

The cool thing about the split systems is that rooms will drop off the air con system as they reach temp.

If you're looking to keep the air con off the electric bill:

http://www.gasairconditioning.org/

Those seem slanted more towards the industrial side than residential, the smallest apparently being about double the size I will need. (Further investigation suggests I should be looking at a system around 18-24K BTU.)

As far as staying off the electric bill... all I need to do is add solar panels (PVs) to reduce my billable kWh to below a 12-month average of 250 kWh per month, and my electricity costs will drop from a current projected $133 per month (12-month average) to $7 (not a misprint: seven dollars!) per month. Adding A/C changes the picture to somewhat less dramatic figures of $159 and $12, but still quite acceptable. [This assumes 1850 watts 8 hours per day for 2 months of the year for the A/C] The reason for the extreme reductions is twofold: the solar panels are kicking in 222 kWh per month (62% of my total consumption) and more importantly they are getting me out of the punitive DAC (Domestica Alta Consumo) rate structure. I will pay 6 cents per kWh for any electricity usage beyond the 222 kWh from the PVs, whereas without the PV's I would pay more than 36 cents for every single kWh used.

The downside to all this is that I face an up-front cost of $9000+ USD for the solar installation. My electric bill savings will amortize that in 5.2 years, and after that it's all gravy.

The PVs are to augment the existing electric service, not replace it. There are no batteries for storage, anything like that. If the power fails, I am in the dark like my neighbors. [Hmmm... if the power failed in the daytime, could I throw a switch to separate myself from the grid and use the 1400 watts from the PVs to keep important things like computers and refrigerators running?] The most important purpose is to get me away from the DAC rate.

tanstaafl.

If it's any help, I have one of these:

http://www.toyotomiusa.com/products/portableairconditioners/TAD-T32JW.mv

It's 12K btu, and adequate to cool a 10' x 15' room when it's 100°f outside. That's the room that gets the afternoon sun, and I've mentioned the almost total lack of insulation. It's also the room I work in, so we tend to just cool the one.

From my experience, even given that it doesn't get as hot there as here, the 24K btu would probably be minimum for you. Definitely don't go less. I think I'd go for the 36K unit and not have any regrets later.

I threw that gas thing out there more for an interesting read. I worked in an office once that had one, it was way cheaper to run than an electric air con, at that time.

I have a portable unit that I bought when the compressor in my air conditioner broke and it was going to take a week to fix (ended up taking like three weeks). Now I use it for working in the garage during the summer. I love the thing and the hassle of finding the vent and emptying the water is far surpassed by how well it works.

My portable uses the exhaust heat to evaporate the water and sends it out the outside discharge vent. I put really long vent pipes on it, but my room reminds me of the movie "Brazil".

Grid-tie inverters will disconnect themselves from the grid automatically when the power fails. This is to keep electrical workers from being harmed while working on the lines.

That said, you should still be able to do this. It's called islanding. Remember that your PV panels will only generate peak power for a relatively small window of time (say...from 10am to 3pm or so), before and after which the power output drops drastically. Once the power output drops below a certain level, the inverter will shut down and your power will turn off.

They probably won't start a fridge even at peak power. You'd need a bigger "inverter" and batteries.

Yeah. Without batteries, are you *sure* you're really going to get the payback you think you are going to get?

Those panels probably cannot power the AC at night when you might need it before going to bed..

It doesn't matter what the panels doing at night or any other time of day. All that matters is the net power usage (amount generated - amount used). During peak times, he'll likely be feeding power back into the grid which will help pay for A/C during non-peak times.

Batteries don't matter at all...the grid is your "battery". You basically "store" the excess electricity you generate in the grid until you need it** Obviously, the disadvantage is that if the grid goes down during a period when you're not producing peak power, you're done (unless you just use a standard UPS).

Adding batteries to a grid-tie system is a great idea, but it adds a lot more cost and maintenance. Maintenance is almost non-existent on a battery-less system. The more batteries you add, the more difficult they are to maintain, because the batteries have to work together. If one is not operating as well as the rest, it can reduce the efficiency of the whole bank. This can make a tremendous difference in how well your system performs.

**This may vary from country to country and even by utility company. In the US, the utilities buy back the power at retail, vs. wholesale, so you get exactly 1kwh back for each kwh you generate.

One big battery should be enough to start a fridge, if you're worried about keeping the fridge working. I'm sure that 1400 watts from the pvs is absolute max and won't supply the starup draw.

Yes.

I am in no way trying to replace my electric company, nor really even supplement it. I am just supplying some of my own electricity so that I don't have to buy it all from CFE. Doing so gets me away from a very punitive rate structure imposed on "heavy" users. I think most people on this bbs would have difficulty avoiding that so-called heavy use group. If your electric bill is running you more than $30 a month (at a typical rate of $0.12 USD per kWh) that would put you into "the dreaded DAC rate" (tm) here and you would pay at least $112 USD per month.

My planned solar panels should provide a bit over 220 kWh per month, enough to not only take me out of DAC but put me well into the "basico" subsidized rate. Here's the math:

8 solar panels each rated at 180 watts at maximum output, 1440 watts total.
Annual Insolation Value in Ajijic, Mexico is 6.2, meaning I get the equivalent of 6.2 hours of maximum output during the 12-15 hours of daylight.
System efficiency (adjust for losses through heat, inverter losses, etc.) runs about 81.5%.
Days per month average 30.5. (Very slightly less on non-leap-years )

(8 * 180 * 6.2 * .815)/1000 = 222 kWh per month.

Cross-check: PVs are about 13% efficient converting sunlight into electricity. On average the earth receives about 1,000 watts of energy per square meter, some places more, some places less. Figure about 1100 here in Ajijic. My solar panels are .8 x 1.6 meters, total area for eight panels is 10.24 M^2. At 13%, they would generate, ummm, let's see...

(8 panels * .8W * 1.6L * 1100 Watts/m^2 * 13% Conversion * 6.2 Insolation * 81.5% Efficiency * 30.5 Days)/1000W/kWh = 226 kWh per month. Close enough.

No, but CFE electric company can. Power outages are rare and usually of short duration, so that isn't a concern. As I said, the primary goal of the PVs is to have to pay CFE less than 150 kWh per month on a 12-month average. I will go beyond that 150 kWh one or two months out of the year because of the A/C, so I will have to pay CFE 14 cents per kWh for the first 100 kWh over 150, and 36 cents per kWh for anything beyond that. I will have one or two bills in the $100 range each year, the rest of the time in the $10-$12 range. This is low enough annual consumption, even with air conditioning for two months of the year, to keep me well away from the DAC rate, with projected annual savings (compared to DAC) of $1765.

tanstaafl.

Did you factor cloudy days into your equations? They can make a massive difference in the amount of solar energy you collect.

You're assuming that the energy you sell to the power company will be deducted from your usage rate. Maybe you've already checked, but I wouldn't count on it.

What happens if the way they do it is that they see that you consume, on average, 200kWh, and also provide, on average, 75kWh? Your assumption is that they would just bill you for 125kWh. I would be surprised if what they do isn't charge you for 200kWh and then pay you for 75kWh.

Of course, there are still a lot of variables here. You'll still be directly using some (if not most) of the power your PV panels generate, especially since you're looking to use it to supplement AC consumption, which is also going to be used more during the day.

But, again, you are likely going to be selling power to the power company, and you need to see whether they are just going to use that to deduct from your consumption, or if they are going to pay you separately for it.

Yes.

There are perhaps 10 days a year in which there are clouds in the sky, and practically never is the sky completely overcast. Ajijic is high desert. The last drop of rain fell here on September 30, 2010. The rainy season is due to start in a week, but even then, like Camelot, it only rains at night. Really.

tanstaafl.

Not exactly.

I will pay for every kWh that my meter shows when they come around to read it. The PVs mean that 222 kWh that I consume every month do NOT go through the meter. In the event that I consume fewer than 222 kWh (like, if I were visiting Alaska for a month and turned off all my circuit breakers) my meter would actually turn backwards and I would build up a credit that would go towards offsetting future electric bills. Under no circumstances will they actually pay me cash money.

The time of day when I consume the power is irrelevant. Only the total amount consumed during the month, minus the amount that is provided by the PVs during the month, matters.

tanstaafl.

Good! So the grid is your battery.

Back on the aircon topic, split systems are the way to go, just make sure they are of the 'inverter' type that is vastly more efficient having a soft-start feature and also variable running speeds for the compressor rather than the old 'on-off all or nothing' type. I think most split systems are of the inverter type these days anyway.

Panasonic, Fujitsu or Daikin are all good brands here (we have Panasonic) and yes, the interior unit is powered from the exterior unit.

Another plus for split systems is that they are quieter since the compressor is further away.