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bldginsp
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# Posted: 24 Feb 2014 08:43am
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I'd like some wisdom from you experienced off-grid battery users out there. Basic question is- for a newcomer planning a system, what will be the most cost effective choice to make in batteries?
If you buy more batteries so you run them down less, do you spend less money in the long run cause they last longer?
What type(s) or brands of batteries do you think are best? Worst?
Advantages/disadvantages to different voltage configurations?
Other ideas?
Thanks
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Dillio187
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# Posted: 24 Feb 2014 12:39pm
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There is definitely a sweet spot when it comes to buying a bigger bank and drawing down less, or having fewer batteries with a deeper discharge. I try to take mine down no more than 30% (to 70% SOC) but I want my batteries to last as long as possible.
You also don't want to have a bunch of parallel battery strings, so that comes into play as well.
The best batteries are probably the dedicated solar system batteries like Trojan and Rolls/Surrette. Worst? car batteries.
I good middle ground are 6V golf cart batteries. It's what I'm currently using and they are a good bang for the buck. My next system I will probably spend the money for bigger batteries.
I've heard of some people using fork lift batteries too, just good luck moving them!
Advantages to different voltage configs..higher voltage systems require smaller wires and are more efficient, but there isn't as much choice in inverters and appliances that will run direct from the bank. 12V has so many options there that many people go with a 12V system. The problem with this is if you only want 1 parallel battery string, you are limited in your choice of batteries.
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bldginsp
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# Posted: 24 Feb 2014 01:10pm
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Thanks Dillio
Why do you not want to have a bunch of parallel battery strings, and why are you limited in choice of batteries if you have only one string?
Dumb questions, I guess, but I'm new to this. Never see this where everyone is on the utility.
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Dillio187
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# Posted: 24 Feb 2014 02:34pm - Edited by: Dillio187
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when you start running parallel battery strings, they have different impedance no matter how hard you try to make them the same. This leads to one string charging and discharging at different rates than another, so you basically stress one set more than the other. This is due to manufacturing differences in the batteries themselves, or the cabling used to connect them.
I've seen people run parallel banks before that have had luck with them, but I'd bet money on most of them having batteries taking more load than others.
I myself had a bank setup like this and used a clamp-on ammeter to prove my hunch. With a 30 amp draw on the bank, one set of batteries was supplying 18 amps, and the other 12 amps, with identical length supply cabling to a buss-bar system. The bank supplying 18 amps always needed more water fills than the other bank. I know why now.
The next day I ordered a 24V inverter and wired my 4 golf cart batteries into a 24V bank. That was more than 2 years ago and those same batteries are doing just fine now, and they always take the same amount of water too!
The reason you are limited in batteries is because you have to wire the batteries in series to achieve your desired voltage. If you want more than a 225ah 24V bank, you have to buy more expensive batteries, because in a series bank you add the voltage, not the capacity. If I wired 8 golf cart batteries in series, I would have a 225ah, 48V bank. If I wired 8 golf cart batteries into 12V banks, I would have a 900ah, 12V bank.
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MtnDon
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# Posted: 24 Feb 2014 02:36pm - Edited by: MtnDon
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The ideal battery bank will be a series only string of 2 volt cells. (a lead acid 6 volt battery has three cells in series.) There are 2 volt individual cells available; look at Rolls / Surette for an example..
Next to ideal would be a series string of batteries, like 6 volt golf cart, L-16, etc.
Both help avoid issues that can develop because of differences in internal resistance of batteries or cells. When in parallel these differences can cause uneven discharge and recharge of the cell/battery with higher resistance. Over years that can lead to early failures.
Another advantage to series only is there are fewer cells to maintain.
FYI, Trojan has some 2 volt L-16 batteries, not cells. Three cells within one case but all in parallel instead of series. In turn each two volt battery is connected in series. They are not as ideal as "real" 2 volt cells but a good compromise on price / quality over the 2 volt Rolls / Surette.
The common choice of golf cart batteries is a compromise many times, a compromise made because of the higher initial cost of higher quality batteries. Another reason some folks (me) choose golf cart batteries has to do with weight. The typical L16 weighs almost 120 lbs, a GC2 about 67 lbs. I did not want to face a 120 lb battery when I am 70. Keep weight in mind when planning where the batteries will be placed in use.
For anything but the simplest small system, IMO, forget 12 VDC systems. There are, again IMO, enough of a variety of other hardware to choose from in 24 VDC, or even 48 VDC. You might not be able to find cheap bargain basement inverters but mostly I do not think they are a great deal anyways, quality wise. There is a good selection of good to best quality inverters.
Twenty four volt system also have an advantage, as a rule, over 12 volt systems when it comes to charge controllers. CC often can cover multiple voltage systems; 12 to 24 volt is common, better CC can cover 12 volt to 48 volt, some to 60 volt systems. The advantage comes with the 24 volt system CC often being able to handle more watts (about 2x) than when used on a 12 volt system. That is because of the amperage handling capacity. Look at the specs on some CC and you should be able to see what I mean. When it comes to choosing a CC here are many variables that need to be accounted for. But I'm making thread drift aren't I?
Battery brands to choose? Have a look at a site like AZ Wind-Sun. They only sell good brands. Also check out thesolarbiz.com; they've been around in the battery business for over 4 decades. I know several people who have bought from them. They can arrange for local pickups in most places in the country and that can save a lot of freight dollars.
Personally I believe a 24 VDC system is an excellent choice for medium sized systems.
Before looking at what batteries there are you should know how much power your system will likely use. Then you can calculate battery capacity and see what will work best for your budget. In general it is best to plan and build what you need rather than starting small and increasing the system size at a later date.
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sparky1
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# Posted: 24 Feb 2014 04:16pm - Edited by: sparky1
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12 volt DC
my choice my 2 cents
since 1,000 of items are operated via 12 volts. if you go 24V DC you have narrowed your ability to be frugal.
I haven't looked at 24 volt Light Bulbs or LED's much less appliances .& I wont waste money inverting a small 12 volt system to 120 to operate things. as for my well I have a 12-24 volt water pump---it will deliver 95 PSI water if you let it.it is doing Just fine off 12 volts. I did buy a 24 volt solar panel ,soon I will add another battery to the well pump circuit & charge that group separately as 24 volts.
I have had 12 volt lighting since 2008 in my house & for the last 39 years 12 volt Campers & motorhomes.
sparky1.
PS I use 4 D group battery's $149.00 @ Tractor supply in battery Trays,
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MtnDon
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# Posted: 24 Feb 2014 06:15pm
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bldginsp, There will be a lot of difference of opinion on everything to do with off grid power. For each individual case though I do believe strongly in the statement I made at the end of my previous post... Quoting: MtnDon Before looking at what batteries there are you should know how much power your system will likely use. Then you can calculate battery capacity and see what will work best for your budget. In general it is best to plan and build what you need rather than starting small and increasing the system size at a later date.
Your use will determine what would be best, not what I use nor whatever anybody else uses. There may be more than one solution. But what you need in terms of power, what you need in terms of appliances, entertainment, pumps, etc. is what determines the suitable voltage and equipment. I'm aware that can be difficult; we went through several sizing exercises before we etched it in stone.
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Truecabin
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# Posted: 25 Feb 2014 12:26am
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Quoting: MtnDon In general it is best to plan and build what you need rather than starting small and increasing the system size at a later date.
you right 20/20 hindsight is the best kind
i think asking somebody to plan a whole system when there happy with just lights and once in awhile 120v item makes it sound like they are making a mistake to start small
i'm with sparky1 wire your cabin like an rv you can be frugal
another way is to find somebody with a system that does what you want at the cost you want and copy it
some people dont want to put an expensive system at there cabin it might get stolen so frugal is less risk
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creeky
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# Posted: 25 Feb 2014 09:25am
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The NAWS site has some good posts on batteries. The general feeling seemingly is that expensive batteries aren't as important as good battery maintenance. And battery lifetime is basically the same.
I use 120 lb batteries. I have a tractor and so moved them easily. They went into a building built for them. No worries. I use Crown 395s and they are proving excellent.
But in a small system: 12v dc with 4 6v deep cycle batteries looks to be the sweet spot. As many have discovered: Adding inverters, fans, etc is easier and cheaper at 12v.
Tho I don't own one, the morningstar pure sine inverter draws millivolts on idle and is only $260 on amazon. For that reason I would wire for and use ac. For the "overhead" of 10% you can easily get your money back from a slightly bigger battery bank/solar on fixtures alone.
and just for fun. a 220 amp hr deep cycle battery at 6 v with 4 batteries in series/parallel gives you 440 amp hrs at 12v of course. So at 12.5v nominal you have 5500 watts. With a 20% discharge for long life, that's an easy 1100 watt/hr of power per cycle (day?).
that's a lot of led lights. pump use. tv. stereo. laptop. phone.
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bldginsp
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# Posted: 26 Feb 2014 10:04pm
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MtDon- I'm seeing the logic of your statement that its best to design for what you really need, rather than expect that you can increase system size later by adding on. You may or may not be able to add to your existing batteries and come up with a workable configuration of voltage and strings, and anyway it sounds like you can't add a string of new batteries to strings of used ones. Also, if you increase size later, you will have to trash your old inverter and get a larger one. Perhaps a good comparison is- you can't make your flashlight bigger, you have to get a bigger flashlight.
It seems that there is no advantage in terms of efficiency or cost to having higher voltage batteries (or battery configuration), except the wire size from battery to inverter, probably short anyway.
And yes, you really have to make hard calcs and decisions about your use before you can design your system. But I'm just trying to understand the parameters as I get started.
Don- you have 3 strings of batteries, I'm wondering if you are experiencing great differences in discharge rates between strings?
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MtnDon
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# Posted: 26 Feb 2014 11:03pm - Edited by: MtnDon
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I do consider myself fortunate. Our parallel strings have performed well for us for 5 years now. I do equalize every other month or so; the log shows an average of almost 6 equalization charges per year. Some are instituted by me, others occur on a timed basis that the system is programmed for. The third string of batteries was not in my original plan but was added to ensure adequate performance without requiring the generator in the cold winter months and late fall, early spring months. If the cabin was occupied full time in winter I would have put more effort into a more weather proof compartment for the batteries, one where they could be maintained at a warmer temperature. However, I look at the cold as my friend also; the cold prolongs the life of a lead acid battery. Conversely folks in the low altitude Hotter than Heck desert have shortened battery lives.
I also feel like a hypocrite touting the best thing to be a single series string, but owning a triple parallel. It was the economics of the thing at the time. However if asked what a good or best system would be, I like to give what I feel is best as a first choice. After that compromises and economics enter the picture.
I still maintain that for all but the smallest of systems, 24 volts is better. Small = anything with a maximum of four 6 volt golf cart batteries in my book. I've never been impressed with many 12 VDC appliances so that argument doesn't sway me. But that is me, not everyone. The biggest reason for 24 VDC systems is the reduction in parallel strings. Our system would be crazy if it had been built as a 12 VDC system.
Quoting: bldginsp ....trying to understand the parameters as I get started.
And that should result in a better system for your use pattern.
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Dillio187
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# Posted: 27 Feb 2014 09:35am - Edited by: Dillio187
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Quoting: bldginsp It seems that there is no advantage in terms of efficiency or cost to having higher voltage batteries (or battery configuration), except the wire size from battery to inverter, probably short anyway.
there is more to it than that...
if you purchase an MPPT charge controller, you gain efficiency everywhere. Series your solar panels together to form a higher voltage array = more efficiency between the panels and charge controller. Your charge controller now only needs to down convert the voltage to 24V instead of 12V, you gain efficiency there. Same with all the pathways in the system.
Plus as Don mentioned above, charge controllers work on amperage. You can push twice the watt hours through a charge controller at 24V as you can at 12V, so you can build twice as large of a system using the same controller. it's a win-win situation.
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bldginsp
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# Posted: 27 Feb 2014 01:39pm - Edited by: bldginsp
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I see what you're saying Dillio. So if you set up your batteries for 24v, or 48v, operation of the whole system is more efficient, but then you are limited to whatever appliances are available at that voltage configuration (if using a DC system). But with an inverter to AC that doesn't matter.
Do AC inverters operate more efficiently if DC input is a higher voltage?
Lastly, how do you figure the amp hours to get with your batteries? I guess you just do amperage calcs with your appliances and guess how many amp hours you will use on a daily basis, then decide how much amp hour capacity to get batteries for. But you don't want to drain your batteries down to 20% every day, so is there a rule of thumb on how far down you can drain your batteries on a daily (or nightly) basis and still expect them to last?
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Dillio187
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# Posted: 27 Feb 2014 03:32pm - Edited by: Dillio187
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Quoting: bldginsp I see what you're saying Dillio. So if you set up your batteries for 24v, or 48v, operation of the whole system is more efficient, but then you are limited to whatever appliances are available at that voltage configuration (if using a DC system). But with an inverter to AC that doesn't matter. Do AC inverters operate more efficiently if DC input is a higher voltage?
you got it. Yes they do generally run more efficient, but you're probably splitting hairs and it more depends on the design of the inverter.
Lastly, how do you figure the amp hours to get with your batteries? I guess you just do amperage calcs with your appliances and guess how many amp hours you will use on a daily basis, then decide how much amp hour capacity to get batteries for. But you don't want to drain your batteries down to 20% every day, so is there a rule of thumb on how far down you can drain your batteries on a daily (or nightly) basis and still expect them to last?
If you know what you want to run in the cabin, buy a Kill-A-Watt meter and measure the items for a few days to a week. You can then figure the daily amp hours used by each item, and size your bank accordingly.
You never want to go below 50% SOC (state of charge) or you will drastically reduce the life of the battery. To be safe, I start my generator and charge my batteries when they drop below 70% and I don't think the sun will be out the next day, but your usage may and will probably vary.
So it's just a matter of figuring out how much power per day you want to use, and then size your bank accordingly. So if you want to draw 100ah out of your 24 volt, 225ah battery bank per day, you'd better be darn sure you can recharge that bank every day either through solar, or a generator backup because 100ah is a 56% SOC and cutting it really close.
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MtnDon
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# Posted: 27 Feb 2014 04:13pm
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I don't believe it makes any measurable difference to efficiency if an inverter is running off 12 volts or 24 or 48. There will be more of an efficiency difference within any one inverter when you compare full or near full output compared to a very light draw. That is an inverter that is only providing a small percentage of it's rated output may only be 50 % efficient while doing that small load. But when giving close to maximum rated power it may be 80-something to 90-something efficient. Maximum efficiency at or near maximum rated output (continuous duty rating, not surge).
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bldginsp
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# Posted: 27 Feb 2014 05:39pm
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Another good argument to go with DC if your needs are limited. When using only a few lights on an inverter system, if efficiency is only 50%, you are doubling the power use. With a DC system and LED lights (or whatever), you use only what the lights are actually using. Thus the system can be sized that much smaller, less battery, less panel, smaller CC. Since my place will not be full time use, i think I can get by with a single panel, single battery system. But I haven't run load calcs on my lights and ipad charger yet.
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ChuckDynasty
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# Posted: 27 Feb 2014 08:35pm
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A lot of things come into play as mentioned above. Your southern exposure is another big consideration, especially during winter months. Wife, kids if any may want more power. I'll be a part timer as well for some time and want small, simple and portable. I don't need or want much. I can cook over wood, propane, kerosene or Coleman stoves. Warm beer is ok with me too. Sawdust toilet and I'm all set. I've had big and all I crave now is small and simple. Nothing wrong with larger I just don't need it.
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Just
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# Posted: 27 Feb 2014 10:15pm
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Thet's all I have,, 1 Walmart deep cycle marine 180 amp hr's 60 watts of panel , 12 volt , 6 led's 1 water pump' radio ' 12 in flat screen tv
weekends only .. built the place in o7 first battery lasted till the fall of 2013 ,6 years so I replaced it with the same walmart 99$ battery very happy with the little power supply ," good luck"" we have a cheap 1000 watt inverter but use it very little , vac or small electric tools..
ADDED 40 WATTS OF PANEL IN 2O12
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bldginsp
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# Posted: 27 Feb 2014 10:48pm
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Just- does your 60 watt panel reliably recharge your 180 amp hour battery?
Rule of thumb I saw on az wind and sun.com was that you should have 1 to 1.5 watts of panel at 12 volts for each amp hour of battery capacity. But you are 1/4 to 1/3 of that.
How about other users- how many watts of panel do you need to keep your batteries charged?
They say at 24 volts, 2 to 3 watts per amp hour and at 48 volts 4 to 6.
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MtnDon
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# Posted: 27 Feb 2014 11:03pm
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The problem with rules of thumb is there are different size thumbs.  For a given size battery bank and similar discharges you need fewer watts of panels New Mexico than you need in New England, for example. A winter day in NM is about the same as a summer day in the NE. So lots of variables
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MtnDon
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# Posted: 27 Feb 2014 11:11pm - Edited by: MtnDon
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We have what would be considered an unbalanced system by many.
630 amp-hours at 24 volts for the batteries. 640 watts of PV. But we are in NM where sunshine is the usual order of the day.
On a normal summer day we go into float an hour or so before the sun gets too weak to produce anything meaningful. In winter we sometimes cut that much finer, but still manage a little float time most days. If we were full timing in winter I would want more PV capacity. However for the May thru September seasons the 640 watts is fine. In summer I can even equalize the batteries using the sun rather than running the generator, though I usually run the generator just to run it and help keep it working.
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Just
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# Posted: 27 Feb 2014 11:34pm - Edited by: Just
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Quoting: bldginsp 60 watt panel
our panels are in full sun for most of the day. not much use in winter.
the battery seems to be charged when we need it, the system works so well we think about it very little.
20 +40 watt panels on roof
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creeky
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# Posted: 28 Feb 2014 08:34am
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MtnDon. Where does that 50% low load efficiency statistic come from?
According to AIMS their pure sine full load efficiency is 90% and their 1/3 eff. is 95%.
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MtnDon
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# Posted: 28 Feb 2014 10:52am
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It's math. An inverter sitting there, turned on and NOT in search/sleep mode might draw 20 watts. If it is used to power one CFL of 23 watts the efficiency is about 50%. It gets worse if the CFL was replaced by an LED. And it gets better as more devices are turned on.
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bldginsp
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# Posted: 28 Feb 2014 10:53am - Edited by: bldginsp
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Just from the broadest electrical perspective, a piece of equipment operating at far below capacity will generate much less heat, and heat is what causes electrical equipment to lose capacity. That's why I was surprised when Don said an inverter used at low end of its capacity would be far less efficient. Perhaps it is a function of design.
Don posted before I did
So there is a constant draw by the unit itself, being used or not, and total efficiency is a function of total use, right?
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MtnDon
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# Posted: 28 Feb 2014 02:09pm
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I mispoke a bit... most inverters will reach peak efficiency at somewhere around 70% +_ rather than maximum. But the differences are going to be small.
And some inverters have a search or sleep mode. They use milliamps while waiting for something to be turned on. This results in power savings but does produce a brief delay in power being turned on. The better ones are programmable for delay and how much power will trigger the activation. Our Outback can be adjusted to where a small LED nightlight or a switch with a built in "on" indicator will activate the inverter. When all powered devices are turned off the inverter waits a few seconds before powering off.
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