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  1. #1
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    Default Crazy inverter + AC idea

    Guys,

    I'm struggling to find a place to live in Santiago that provides true 24hr electricity. Some do, but then they won't allow me to take my dog (who is pretty cool, I want to keep him).

    I *must* have electricity 24 hours, and I hate running my generator. Santiago can get pretty hot at night, and I sleep much better with AC at night, but you can't really run a generator because of the noise.

    Would this work?

    If I buy a nice big inverter (say 3.5KW). I hook up 24 batteries to it, at a cost of RD$2200 each, or RD $52800 total.

    I run everything in my house off it, including the AC. I know that running AC off the inverter is terrible for batteries, so just assume that every 6 months I have to replace all 24 batteries at a 6 month cost of $52800.

    In USD (40/1) that amounts to an additional US$2640/yr, which is very affordable for me if it makes the electricity problems go away.

    No, its not the most cost efficient way to do it, a generator would be much cheaper. But I need to focus on my business, not on refueling the generator, keeping it working, etc. And, I know "you should just work without AC". But I value my time, and I work at about 20% efficiency in the heat.

    Are there any electricity experts here with comments? Could this work?

    Adrian

  2. #2
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    It will be a very interesting experiment.

    I agree about the efficiency and the heat, maybe that's why I like working at night so much.

    The best advice I ever received about inverter batteries was from Jimmy Stewart that occasionally posts on the board. He put me onto a brand of deep cycle, zero maintenance marine batteries only available at the time from Santo Domingo Motors. They cost RD$2,200 a piece 2 years ago and came with a life time guarantee. My friends at Santo Domingo Motors told me they had never received one back since they had been selling them. My friend is still using my old inverter inverter with the same batteries and never had a problem.

    Bottom line, it pays to invest in good batteries.

    Also, you could always move to Santo Domingo, we have 1 or 2 true 24hr buildings

  3. #3
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    One question you have to ask: how will I charge my batteries if there is little power?

    Another question: will I always have charged back-up batteries?

    Third point: instead of having one huge inverter, spread your risk and have at least 2.

  4. #4
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    In answer to your question, sure you can run an air conditioner from an inverter/ battery system. The inverter just has to be sized accordingly. Inverters come in all sizes. Trace Dr series (the one that many use) allows you to run two inverters in parallel from one battery bank so you can have split phase 110 for the 220 you need to run most air conditioners.

    If you have a small air conditioner, two 2.4 KW inverters would probably do the trick. You should size it so that with everything running at once, it does not exceed the maximum continuous rating of the inverter or the bypass circuit (with some inverters when the inverter is off/charging the electricity still comes through it and still has a fuse.) A safer figure would be to aim for 75% of inverter capacity with everything running. For example, if your house uses 10 amps on one 110 phase and 15 amps on the other 110 phase with everything running -don't include startup surges for motors in this figure- (1800 watts on the greater of the two), you would need a 2.4 kW inverter assuming that it could handle the startup surges of your AC fridge and appliances.

    With an air conditioner, you must consider the startup load (typically 2-3 times the rated voltage). With Trace Dr series it is not usually a problem since they are rated to handle surges roughly 3 times the continuos maximum operating load.

    Be aware that a system of this size will make some heat particularly when charging and the battery chamber has to be vented to allow the hydrogen gas created by the charging process to escape.

    Depending on the size of you air conditioner and how long it runs, you will probably find that it won't be as hard on the batteries as you might think. It is recommended generally that you buy enough batteries so that a normal discharge pulls the batteries down to only about 50% disharge. If you double the number of batteries, you more than double (nearly triple in some cases) the time your inverter can run. Also, the less you discharge you batteries in a normal cycle, the longer they will last.

    Another consideration is that the power has to be on enough to charge the system. If you have several days in a row with only say, 6 hours of power, even though your system might be capable of running 18 hours, it might still run out of juice since the batteries probably won't have been fully charged at the beginning of the cycle. Also, if you have low voltage, it can take considerably longer than normal to charge the batteries.

    One last suggestion. If you are going to have a huge battery bank, either let someone else fill them or get your self a battery filler from NAPA or Snap On in the states. It makes filling batteries much, much easier and isn't too expensive.

  5. #5
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    Default system

    look at this http://www.bergey.com/

  6. #6
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    lhtown,

    Thanks for the great info! Have you ever set something like this up?

    Some questions:

    1. How many batteries would be necessary to keep it working for 18 hours? Is 24 enough? I called Trace in the USA today, and the guy suggested 40 batteries. But he wasn't confident in his numbers and only seemed to have theoretical knowledge.

    2. If I wanted to increase the load, would the 2x2.4kW inverters run 2x12000 BTU split AC's, or 1 AC + washing machine, or 1 AC + iron? Or would I need to step up to 2x3.6kW inverters to handle that load?

    3. Is the modified sine wave issue of the DR inverters a problem for AC's? I have been told in the past by an AC company that they won't honor the warranty if I'm running it from an inverter. Would I need to go to a Trace pure sine wave inverter? (Two of these starts to get above what I can pay right now).

    I'm very interested that you suggest the battery life might be reasonable. That is very cool. How long do you think the batteries might last?

    If anyone knows anyone actually running an air conditioner successfully from an inverter, please post or PM me privately. I would very much like to talk to someone who has done this.

    Adrian

  7. #7
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    Here are a few answers. I have to assume you either know a little bit about home electricity or are a fast learner.

    1. To estimate battery run time, you need a few figures. First of all, you need an estimate of how much power you consume. This figure is NOT your peak usage, but rather your average usage. It can be hard to guess and can vary drastically from household to household. Perhaps the best way is to look at your electric bill. If you used, for instance, 1,000 KWH for the month and there were 30 days in the month, you divide 1,000 by 720 for a figure of 1.38. This would mean that in any given moment, you are using 1380 watts of power (1.38kw). Of course, this assumes that your meter gives a correct reading and that your inverter will be running everything in the house. Here is a battery life estimator from Xantrex (parent company of Trace) to help you figure how long your batteries will last.
    http://www.xantrex.com/support/howlong.asp
    If you are unable to use the above method, perhaps the next best would be to try to calculate the wattage of appliances in your home and try to guess how much usage they get per month. It can be tough to figure, but an educated guess is better than nothing.

    To find the Amp/Hrs of your battery, you can go to http://www.trojanbattery.com/ and either find the battery you are planning to use or find a similar battery. BTW, you will see that not all batteries are the same. Even though most batteries sold in the country from Trojan and Trace are similar, there are some differences and it can pay to pay attention to the technical specs.

    After you have the battery run time, calculate enough batteries so that they won't be drawn down more than 50% on a normal disharge. How many more than that you want depends on you.

    Also note that there are larger batteries than the typical golf cart batteries. They would allow you to use fewer batteries. You would just have to check availability and costs.

    2. In order to know if you can run what you want to, you will just have to run the figures for your application. Also, when you install the dual inverters, you will have to try to load balance your circuit breakers so that each inverter has roughly the same load. Full size washing machines can run on some Trace inverters but they do take a lot of juice to spin including a pretty heavy initial startup.

    If you are pushing the capacity of the 3.6, pay attention to the bypass circuit rating. When the inverter is off/charging, you electricity will still be passing through the inverters to get to your house. Even though the inverter is not running, it can still pop a fuse leaving you completely in the dark until you reset it. For the trace 3.6, I think it is 30 amps. In my opinion, it is undersized on that model. You can actually get more power from the inverter with it running than directly from the street with ithe power on. I have a Trace 3.6 and it has never shut down from overload on the inverter, but the bypass has popped once or twice.

    3. I have never known the modified sine wave to be a problem for that type of load. There may be some electronic equipment that is sensitive to it and can even be damaged. Also, you should know that appliances that are normally silent will likely make a humming noise that may be quite subdued or quite loud. Ceiling fans, some floor fans, and microwaves are all likely candidates for a "hum." I have not known refrigerators, washers and other such loads to be affected by the harmonic distortion. As far as affecting your AC, it will probably run better off the inverter than the street power.

    Also, please note that I am not recommending an inverter for your situation. I am only telling you that it can work! It might not be cost effective. Generally, for smaller backup load, inverters are cheaper than generators as well as being quieter, cleaner and generally more transparent. For larger loads, generators are generally considered to be more cost-effective. Also, a generator is easier to install and more portable than a huge inverter system but at the same time less likely to be stolen because of the sheer weight and size of the unit.

    You will also have to consider the condition of the wiring to your house. If it is very bad, it might not support running your house and at the same time charging your batteries. With two inverters it should help since the charge should be distributed more evenly between the two phases. With a one large one phase system it is problematic since basically you have everything in the house on one phase plus you are trying to charge the inverter from that same power source.

    In my house, I have a Trace 3.6 that runs everything but my 2 airconditioners and electric hot water heater including a large regrigerator, microwave, washing machine, deep freezer, computer, etc. We try not to use the iron, microwave and washer at the same time. The maid is trained to do the ironing when the power is on unless it is an emergency to cut down on inverter use. I would probably be happy with a 2.4 considering that it is a continuous usage rating and that the peak rating is far higher for startup loads. Also, the bypass circuit is 30 amps in the 2.4 as well as the 3.6. I have 16 batteries which I estimate would give me a runtime of 18-36 hours. They have only run out once since I installed them over a year ago and that was probably because I added water during one of the longest power outages of the year.

    I would have bought 12 batteries, but I thought I might later put a small 110v airconditioner on it. As it ended up, I am glad I have 16 batteries, but I probably will never put an airconditioner on it unless it is one of the portables ones like PriceSmart is selling now.

    I expect my batteries to last 3-5 years. They are over a year old and show no signs of weariness. I only have to add water about every three months since the batteries aren't worked that hard.

    One last word, you pay a premium for electrcity from your inverter. It really does take more electricity to charge it than you get from it. Efficiencies vary by model. Trace inverters are among the most efficient. Also, it takes some electricity to maintain all of the batteries in a state of readiness (float).

    BTW, there is nothing crazy about your idea. It is completely workeable, and there are valid reasons to do it. As you know, it isn't the usual route or necesarily the easiest or the most economical.

  8. #8
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    Default Inverter questions answered

    I know of only one person in this country who can give it to you straight.
    His name is Torsten, he's an electrical engineer and his phone number is 886-0623.
    Marco,

  9. #9
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    lhtown, Rocky, thank you both for your info. Marco, I will follow up with Torsten, he's the german guy right?

    We have made some sizing calculations, and I am sharing them for anyone else considering this option. Its possible there may be an error, so if anyone notices a mistake, please tell me. This was worked out in conjunction with a local electrical engineer and Trace technical support in the USA.

    Bottom line: A powerful system can supply backup electricity for 18 hours for RD$182,000 or US $4550 (@40:1).

    ----

    Max load:

    We calculated my current max load at 1.5kW (12.5A) because thats how big my current inverter is, and everything has been working fine. I'll also be adding a fridge (5A).

    We calculated 3 sets of peak usage for my apartment:

    2 x AC's peak: 36 A
    or
    1 AC + washing machine (less than an AC at peak), less than 36A
    or
    1 AC + iron: 16A+10A = 26A

    Therefore our peak is 2 AC's running together+fridge+current setup. This comes to: 36A+5A+12.5A = 53.5A.

    If we were to run the two AC's plus washing machine + iron, the system would shut down. But otherwise it should be fine.

    ---
    Batteries:
    I want the system to stay up for 18 hours.
    Battery bank capacity required = Hours X Amps total load
    = 18 x 53.5
    = 963 A/hr

    The standard trojan batteries are 225A, 6V. Our inverter will be a 24 V inverter.

    Therefore we need:
    963/225 = 4.28=5 sets of batteries providing 24 V each.
    Each battery is 6V, so they need to be 4 in parallel to get 24 volts.

    So 5 sets x 4 in parallel = 20 batteries required

    --
    Inverter:

    The A/C's are 220V, split model. But everything else in the apartment is 110V. So we need 2 x 110V inverters. The trace DR3624 provides 30A, two therefore provide 60A. lhtown has said these trace inverters will peak up to 3x their rated capacity, however we've already estimated our peak at 53.5A, so we're ok.

    ---
    Prices:

    The DR3624 is for sale at Bellon for RD$54,995. Two will cost RD$110,000.
    Batteries cost RD$3000 each. To be safe, if we add some more at 24 batteries, the cost is RD$72,000.

    Thus the total comes to US $4550 (@40:1), not including installation.

    I'd also keep my generator, just in case I need to charge the inverters or as a backup.

    But this seems like a great approach. Any comments? Have I made a mistake anywhere?

    Adrian

  10. #10
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    Adrianb,

    Your final calculation probably is not too far off although likely a bit under. Bear with my terminology, I am not an electrical professional and probably am not using stricly professional terms.

    Sorry, but if I understand correctly, I think your figures are incorrect.
    First of all, in order to size the INVERTER, you have to figure two things. 1. Maximum STARTUP current needed to get all of your appliances rolling. The initial "hit" can be up to three times the rated wattage of the appliance especially for things with compressors like refrigerators and air conditioners. As I said, with the Trace DR series, it isn't usually a problem, but should be considered. 2. Maximum OPERATING current is the amount of wattage drawn when everythig is running at once (after initial startup). If you are using Trace DR inverters, it will be the most important figure.

    Secondly, in order to size the BATTERY BANK, you must know what the AVERAGE current draw is for your household. For example, your 5 amp (seems low to me) refrigerator would probably only run 30-50% of the time. Assuming it draws 110v, its running wattage draw would be 550 watts. Its average current draw for figuring the battery bank size would be 180-275 watts. Your air conditioner might only run at night and even then would probably shut off at times. Even among the things you have running in your home on your current system, I can practically guarantee you that your average usage is far lower than the rating of your inverter.

    It seems that you used the peak rating to figure the battery life greatly shortening their life. However, I think you also made a mistake in calculating how long the batteries would last. If I were to take the peak rating of 53.5 amps (at 110 volts) to figure battery life, I would come up with a battery life of only about 3-4 hours.

    Also, make sure you understand the relationship between watts, amps and volts.
    amps x volts = watts

    there fore:

    1,000amps x 24 volts =24,000 watts or 24kilowatts
    100 amps x 240 volts = 24,000 watts or 24 kilowatts
    24,000 watts at 120 volts = 200 amps
    etc.

    A side note, since wiring is sized accordin to amperage, to push enough current though your battery cables to output 60 amps at 120v. requires some heavy duty cables or doubling of cables since the amperage will be through the roof (cabling rated for at least 300 amps). For that reason, your battery bank should
    be located within 10 feet or so of the inverter.

    Good luck.

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