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This calculator seems to be calculating inverter size wrong.  RSS feed

 
Alan Estes
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Screen shot attached.

At first the size shown was super high.

I found that the "Size of Inverter" label was supposed to say "W" instead of "KW".

With that fixed, the calculation seems to be way too low.

My understanding is that most "experts" would recommend 2KW minimum for a single freezer.

Opinions, please on this, and if anything else within the calculator seems wrong, would appreciate knowing.

Thank you Gents, Alan

Filename: Untitled.tiff
Description: Calculator
File size: 236 Kbytes
 
Cj Sloane
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Hi Alan. I'm reluctant to download your image - maybe you could upload it or post a link to it.

But to answer your question - 2KW seems awfully high to run a freezer. Your inverter needs to be sized large enough to handle the starting surge of whatever you're running but freezers tend to be between 100-200 W I believe.

I have a 120V well pump which is my biggest load. When running it draws 1.2KW probably 10x what a freezer would draw. I think my inverter is 2.5 KW which will run the whole house, including the pump but I make sure I've got great voltage & good sun before turning the pump on without the generator running.

ps Not a gent
 
Alan Estes
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I didn't realize that the site would require you to download my file, so for those who do not want to do that-

Load-
2 Kenmore 21042 Freezers - 2kW
Lights (10W X 5 X 6 hours) - 300W
Total- 2.3kW/day

Battery Bank-
3 6v (in series) = 18v, 370ah= 6.6kw
2 of the above in parallel = 18v, 740ah = 13.3kw
20% = 2.6/Day

Inverter size the calculator say's that I need- 263.7W
 
Cj Sloane
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Alan Estes wrote:
Load-
2 Kenmore 21042 Freezers - 2kW
Lights (10W X 5 X 6 hours) - 300W
Total- 2.3kW/day...
Inverter size the calculator say's that I need- 263.7W


That seems kind of reasonable though I wonder about the set up. Is it like a shed with just freezers? I do know one big freezer is more efficient than 2 smaller ones - plus you don't have to worry about the surge of both coming on at the same time.

So, yeah, the 2kW is the total usage for the day but you need to size it for actual draw. The 2kW is more for calculating your storage. I think I'm reading your battery figure wrong because I never heard of a 18v set up. 12 or 24v is normal.
 
Alan Estes
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They are 21cu freezers and one is going to be modified to run warmer, for a refrigerator.

Was wondering about that 18v also and was wondering why the calculator would suggest that battery configuration.

What would you guys do?
 
Cj Sloane
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Our system is 12V.

It's not just about battery config - the inverter has to be the right V too (matching battery V). Pretty sure they are 12 only 24 volts. Maybe you could find one 48.

Is there a link to your calculator?

The only hitch is if both freezers turn on at the same exact time. You would have to possibly double the size of your inverter or come up with some way to avoid that scenario. You could probably save enough money on equipment to pay an electrician to rig up some way to avoid that.

PS
Have you ever used a chest freezer as a fridge? I tried it for a while & hated it.
 
Alan Estes
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That calculator is a spreadsheet that can be downloaded here-
https://app.box.com/s/4uyvtxfjozzkky4b9kab

The reason I did not initially question the 18v calculation is because I assumed that you could get an 18v inverter, or a 12v that would handle 18v.
So far, have found neither.

So, what kind of problems did you have with the refrigerator?

I think the person I learned about it from, explained how it could be done, but maybe had not actually done one.

Alan
 
Cj Sloane
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Try the calculators here.

The refrigerator did what it was supposed, I just hated bending down, shuffling food around to find things. Imagine a regular upright refrigerator with no shelves so every time you wanted something at the bottom you had to take everything out, put it to the side, get what you wanted, then put everything back. There are plenty of people who don't mind this, but I do.
 
bob day
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I think there is some problem with actual nomenclature here, and kilowatts and kilowatt hrs are not interchangeable

if you are reading the freezer rating off an energy sticker, that is kwh probably

the motor or back of the refrigerator will give the actual draw of the appliance at any given moment in time which more directly should size the inverter

and the cumulative load measured in hrs for the system is a more secondary consideration, more for sizing the battery bank than the inverter.

so a 2kw rating for an inverter is a measurement somewhat independent of the battery voltage since the other part of that equation is the amps --higher input voltage =lower amp input for same watt output

use a higher input voltage for your inverter and you can reduce wire size since the drawn amps determine that, and wire size can be a very costly element in the higher draw appliances, so a 48 volt input starts to look appealing, especially if your bats and inverter don't live very close to each other.

So lets say your motor draws 10 amps at 120v, that's 1,200 watts, add in other things that might be running and a 2 kw inverter sounds about right

but the real key here is your freezer motor draw, allow that most inverters will accommodate a higher surge , so make sure your inverter size isn't the hyped up surge voltage but the running voltage

to give you some idea, a regular home power saw is going to use 15 amps (1800)watts, a 120 watt lightbulb takes 1 amp (assuming 120v ac) your freezer is probably going to be somewhere in between, but 10 amps is probably close
 
Cj Sloane
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bob day wrote:
to give you some idea, a regular home power saw is going to use 15 amps (1800)watts, a 120 watt lightbulb takes 1 amp (assuming 120v ac) your freezer is probably going to be somewhere in between, but 10 amps is probably close


A freezer is much closer to 1 amp. As I mentioned above, my inverter shows 1.5 kW draw using my well pump (about 11 amps). I have a smallish (12 c ft) AC refrigerator/freezer which draws only .1 kW and the same is true when I use a freezer in summer.

These are actual figures from someone off grid, right now.
 
bob day
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yes, i'm off the grid too, and since i don't use an electric refrigerator (yet), i really don't know what the motor size will be, maybe i'm thinking of the older refrigerators that always seemed to have fairly substantial motors, but what i said about sizing the inverter from the actual motor draw and not the energy sticker is still the real key to this question.

so Alan all i would say is check the motors on your freezers and see if you can find an amp or watt rating somewhere add everything together and have a little extra on top

on a side note, i have just scratched the surface on refrigerators powered with batteries - and i'm impressed by the low draw you say yours has, sounds like a big light bulb more than an electric compressor motor, so i'd really be interested in make and model of the frig you are using, and any other pertinent details you might care to share. the 20$/mo i spend on propane is a nuisance and a real fossil fuel drain.

I could (and have ) lived without a frig, but i do enjoy those frozen smoothies and hate being chained to a propane tank
 
Alan Estes
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My idea was to put together as cheap as possible a system to power the essentials only, and add to it later.

It looks like I'm going to have to spend more and do an 8 battery 24v system.

The freezers I'm buying are upright, and $600 each.
http://www.kenmore.com/kenmore-21-cu-ft-upright-freezer-white/p-04621042000P

My biggest problem now is choosing an inverter (24v, 2.5kw).

They seem to go for $350 (china) to way more than that and all claim to be perfect.

Anybody know of someone who has tested different units and gives an unbiased opinion?

Alan
 
Cj Sloane
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Alan Estes wrote:
The freezers I'm buying are upright, and $600 each.
http://www.kenmore.com/kenmore-21-cu-ft-upright-freezer-white/p-04621042000P


In that case, I doubt there is any benefit to converting it to a refrigerator - the conversions only make sense for a chest freezer because they don't make chest refrigerators.

If you're running 2 big upright freezers, I doubt a $350 is going to cut it. My inverter was more like $1800.

 
Alan Estes
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The guy who suggested it, seemed to imply that a freezer ran in a way that was different than a refrigerator, and used less power, but provided no real information.

My thought was that maybe a freezer doesn't bring the temperature down as fast, but know absolutely nothing about the process of either.

So what criteria did you use when looking at inverter specs?

Alan
 
Cj Sloane
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Bob, the one I have is the Kenmore 10.7 cu. ft. Top-Freezer Refrigerator w/ Humidity-Controlled Crisper. I found a sticker inside which says "full load is 6.5 amps" close to what you said but that is the surge which must be very short because I've never seen it draw anything close to that.

It is cold enough to keep ice cream frozen, something my sunfrost couldn't really accomplish.
 
Alan Estes
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Found a refrigeration expert who said that a refrigerator uses more power even though it does not cool as much because it is a refrigerator and freezer in one which make things more complicated.

Alan
 
Cj Sloane
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Alan Estes wrote:
So what criteria did you use when looking at inverter specs?
Alan


Pure sine wave. Built in battery charger (from generator) and AC transfer switch. Automatic generator turn on/off is awesome but my current generator isn't the right kind. Here's a link to the current version of the one I have. They have a 24v version. Soooo much beefier than the <$500 models.

 
Alan Estes
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Ok, looking at that makes things a lot clearer.

There was much more time spent in design, rather than just throwing something together that works.

It also is designed to hopefully last much longer.

When I look at that it reminds me of the 50hz to 60hz phase converter we had to use at the radio station I was assigned to in Germany.

Alan
 
bob day
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That sounds awesome, like i could go into sears and buy it pretty easy--how is it holding up, how old is it? anything you've seen that you might rather have bought?
 
Alan Estes
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I haven't bought one yet, just a plan.

Alan
 
Cj Sloane
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Alan Estes wrote:Found a refrigeration expert who said that a refrigerator uses more power even though it does not cool as much because it is a refrigerator and freezer in one which make things more complicated.
Alan


But they do sell stand alone refrigerators.

I used to be very opposed to auto-defrost in freezers but apparently they make up for the power they spend. This could be true because my chest freezer keeps developing ice along the top/door and that can't make a good seal.
 
Cj Sloane
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bob day wrote:That sounds awesome, like i could go into sears and buy it pretty easy--how is it holding up, how old is it? anything you've seen that you might rather have bought?


Almost 9 months and it's holding up fine. I do need to top up the batteries every day - we've had a lot of snow - get's sunnier in February.

I think the 11 cu ft is the sweet spot between size - power consumption - & price. Smaller doesn't use much less power but bigger would strain things. I need to get used to it taking longer for food to defrost in the fridge.

I did have to make a shelf for the freezer, though. I'll take a pic, it's easier than explaining. - stand by!
 
Alan Estes
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My plan is to buy two of the same so that in an extreme long-term situation I could keep at least one of them running, with the parts from the other.

Will probably also buy the spare parts that seem to go out most often, if the price isn't too high.

Alan
 
Cj Sloane
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There is a spot for a freezer shelf but it didn't come with the unit. To purchase one was $80!!! I just happen to have a piece of wood the right size. It is much easier to see & access what I have with that shelf. I did opt for the fancy stainless steel version to match my other appliances.

Between the meat in the freezer and the meat in the fridge I guess that's like a months worth of meat.
 
Alan Estes
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I live halfway between Springfield, MO and Little Rock and the most money saving thing here is to fill the big freezer with beef and deer every fall.

Alan
 
Cj Sloane
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I keep a big chest freezer at the basement where I work. I just have to bring some home with me once and a while. I give them $100/year as rent and it saves me an awful lot of power!
 
bob day
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I've heard the same thing about smaller size refrigs using the same power.
when you say top off the bats, as in start the gen and charge them a bit?

do you have an idea what that ends up in gas each month? are you running a propane gen or gasoline

does your inverter also act as a charger, or just a switch?

i'm running extra cheap, no pure sine waves and if i thought i could find a good renewable heat source i might refine my propane frig for a better seal and controls--right now it's probably upwards of 20 years old and everything is mostly shot except that if you light it it runs and gets cold, but no adjustments so in the cold everything freezes and in the heat it barely freezes in the freezer.

your recounting the price of a quality inverter reminds me i need more than just bats and the price of the inverter will probably cost more than the frig

i did see some modules that you could put together on ebay that looked fairly good (from china) and build your own psw inverter at a fraction of the normal cost, but i'm a little hesitant to get into that learning curve, i've got so many other things going on that are so much more fun (and some that aren't (the ACP).

anyway, thanks for the thoughts, maybe some day......
 
Alan Estes
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Ok this is what I have come up with, for my situation and location.
If you see problems with it, PLEASE, let me know, (nicely).

Tactical wood gas, Big Dragon Gasifier Kit $1000
(I have 80 acres, 7 miles from a small town, with lots of trees).

Homebuilt LP generator using a small engine and 24v alternator. $70
Between myself and family we have more small engines than we know what to do with.

Trojan L16E-AC 6v 370Ah batteries $207 each
4 (in series) = 24v, 370ah= 8.8kw
2 of the above in parallel = 24v, 740ah = 17.7kw
Using only top 20% = 3.5kw available per Day

Samlex SA 2000W 24V Pure Sine Wave Inverter $780
(Later if I can afford it, an Outback and keep this for backup)

hot water heater is already using LP
House heat is wood.
Stove is electric, but the guy who sells the Gasifier also sells a 2 burner stove.
Wife is from Colombia, SA and think that clothes dryers are a silly luxury.
Can live without AC by spending summer time in the basement.
Water is from spring/cistern so the pump uses very little power.

Gasifier 1000
24v Alternator 70
Batteries 1656
Inverter 780
Total 3506
 
bob day
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There are do it yourself gasifiers all over youtube if you're trying to save money, it's something i've thought about but have never done anything with.

and i use my RMH as a water heater backup, and some do it yourself solar hot water most of the summer. Using a gasifier to heat water seems a bit redundant/ inefficient, although it might be a good backup system for summer when the clouds block the solar and you'd rather not fire up the RMH.

but really, we're all just out there experimenting and figuring out how our technology can serve us best. So have a ball, remember, if you're not having fun you've got the design wrong.
 
Alan Estes
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I looked at a lot of plans and ideas for gasifiers, but am really impressed with that one.

http://www.tacticalwoodgas.com/#!online-store/cmfr/!/Big-Dragon-Full-Kit-With-metal-filter/p/36895175/category=9623788


 
Cj Sloane
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Alan Estes wrote:
Gasifier 1000
24v Alternator 70
Batteries 1656
Inverter 780
Total 3506


I don't understand the Gasifier/Alternator but if you go that route, start a thread. I think you need a battery charger - with the Outback, it's built in. Your wiring / connectors need to be made/purchased. Also, consider getting some solar panels. If your generator dies when it's cold, you could maybe keep food frozen but in the summer you'd be sunk. Consider getting enough solar panels that on a full day during the summer you wouldn't have to run the generator. If not, maybe a cheap backup generator would tide you over.
 
Cj Sloane
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bob day wrote:
when you say top off the bats, as in start the gen and charge them a bit?

do you have an idea what that ends up in gas each month? are you running a propane gen or gasoline

does your inverter also act as a charger, or just a switch?


We have a gas generator, we used to have a propane one but gas is easier. This time of year it could run for an hour or two / day. In the summer we don't have to run it unless there are several cloudy days or we want to run the pump at night.

The Outback inverter, which is kind of pricey, acts as a charger and I believe has a transfer switch (all loads run directly off it???). With some generators you can set it to auto-start the generator for various conditions you can program. Nice to turn it off & on from inside when it's 20 below.
 
bob day
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sorry for the delayed response, seems i missed the email alert for some reason, might be being flooded with emails from this pipeline nonsense.

Anyway, There are times when Money seems to be a real solution to some of the issues we face when we go off grid with our power.

The type of multi purpose inverter you have sounds like it "should" be standard issue for all sorts of reasons, ease of use, economical use of materials, resilience, am i missing any?

unfortunately i often get caught in the trap buying substandard, cheap, and in the long run end up spending more.

Anyway, thanks for all the good things to think about, Bob
 
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