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Forum Index : Solar : Power consumption of inverter fed fridge
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| Mirriulah Newbie  Joined: 07/04/2017 Location: AustraliaPosts: 1 |
G'day folks I am looking at a 240v freezer to fridge conversion for my cabin near Lightning Ridge... I would like to know how to understand the power consumed from my battery when: Fridge is 240v @ .46amps Battery is 12v by 175a/hr Running the fridge on mains power, my watt meter says it is consuming 270watts over a 20 hour period. Can someone help me understand what that would mean to my 12v battery. Thanks in anticipation |
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| Warpspeed Guru  Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Fridge 240v x 0.46A = 110.4 watts. That sounds about right while the fridge is running, but there are a few other things you need to take into account. First is the fridge will not be running constantly, but will cycle on and off depending on ambient room temperature, and how frequently someone opens the door. The fridge will probably have a defrost cycle with an electric heater that can easily be twice the normal running power. The rating plate may say something like 250 watts maximum. But that will only occur infrequently during a fairly short defrost cycle, maybe once every 24 hours. The killer will be the startup power requirement for the motor which can easily be ten or twenty times the running current, but only for typically about one second. I suggest you buy a power monitor that can record peak current if yours does not already do that, these are cheap and on e-bay. That will tell you everything you need to know. The two most important readings will be peak current, and Kwh measured over a reasonable period like 24 Hours. This is what I bought : http://www.ebay.com.au/itm/240V-LCD-Energy-Meter-Watt-Voltage-Volt-Meter-Monitor-Analyzer-Power-Factor-AU-/112024914508? hash=item1a15348a4c:g:RCsAAOSwzJ5XW8KE Your watts over 20 hours needs to be increased according to inverter efficiency and then divided by 12v to turn watt hours into amp hours. The battery looks o/k. But the inverter will need to be sized to be able to handle motor starting current EASILY. If it struggles it will probably not live long. So peak starting current needs to be known to realistically size the inverter. Cheers, Tony. |
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Tim_the_bloke Senior Member  Joined: 15/11/2009 Location: AustraliaPosts: 105 |
Yes. I suspect trying to run a 240V fridge is what wrecked my inverter. I suggest you consider purchasing a 12V fridge. Get the biggest you can afford. It will pull much much less power than running a 240v fridge through an inverter. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I suppose it depends on the cost of a brand new 12v fridge versus the cost of a suitably large inverter. A 12v fridge should be more efficient, but then again a 240v freezer with some really serious insulation may actually do better if the door is not in constant use. Only way to know for sure is to keep making measurements. Published data cannot be relied upon because the way it was tested may have nothing to do with the way you plan to use yours. There can be some real surprises when you start taking real measurements. I just discovered my F&P washing machine that draws 555 watts takes 83 watt hours to do one full heavy duty wash (45 minutes). I thought tat pretty reasonable. When switched off, it draws 3.6 watts, BUT that 3.6 watts over 24 hours uses 86 watt hours. More than doing a full load of washing EVERY single day. So I now switching it off at the power point, and it is well worth doing. That is if you know about the problem. So I think the trick (if it is a trick) it to do a very serious energy audit of every single electrical item you own, and decide if there is a simple low cost way to do it better. Quite often there is, and the requirements of batteries and inverters can be greatly reduced without any significant change in life style. Cheers, Tony. |
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| M Del Senior Member Joined: 09/04/2012 Location: AustraliaPosts: 155 |
Will this be a chest freezer to chest fridge conversion? If so, everything Tony said is applicable but the door opening time. as well as the above in different words Main thing to watch is starting current and having an inverter capable of it. Note this can be inside the inverters short term surge rating, but using the full surge rating every time the fridge starts will quickly kill the inverter. ie inverter has a constant load rating of 500 watts and 1000 watt surge rating, fridge uses 88watts with a surge of 880watts. You really need a 1000 watt inverter with a 20000 watt surge rating for this fridge. Plus add up all possible starting currents for everything powered by the inverter as one day everything will start at one time and overload your system. Think of it like cutting wood measure, draw line, remeasure, think, recheck measurements and then cut. Mark |
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Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Another good reason to turn off your F&P washing machine at the wall is to stop corrosion of switches and electronics. I have repaired a few thousand of them, people who turn them off at the wall get a much longer life. Leaving it on at the wall results in high voltage DC running all the time. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
[Quote=Warpspeed]Fridge 240v x 0.46A = 110.4 watts. That sounds about right while the fridge is running, but there are a few other things you need to take into account.[/quote] Loosely speaking my 60 litre Engel uses about 40 Watts when the compressors running. [Quote] I suppose it depends on the cost of a brand new 12v fridge versus the cost of a suitably large inverter. A 12v fridge should be more efficient, but then again a 240v freezer with some really serious insulation may actually do better if the door is not in constant use.[/quote] Other thing missing from the equation is the capacity of the 240V Freezer. Even 140L freezers, about the smallest I've seen, are huge compared to my Engel, & probably much more serious in terms of insulation as you say. Assume for a moment it's 180 litres, that's 3 of my Engels & 120 watts; Not looking too bad in that instance. [Quote]There can be some real surprises when you start taking real measurements. I just discovered my F&P washing machine that draws 555 watts takes 83 watt hours to do one full heavy duty wash (45 minutes). I thought tat pretty reasonable. When switched off, it draws 3.6 watts, BUT that 3.6 watts over 24 hours uses 86 watt hours.[/quote] Hmmm, is that calc right? 83 Watt hours for a load of washing? 555W x 0.75 hours would give 416 Watt hours? Phil. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Its not a calculation, it was the reading straight off a digital watt hour meter. This watt hour meter records peak minimum and maximum power, as well as maximum peak inrush current and accumulated watt hours. The maximum recorded power 555 watts was during the initial acceleration during spin dry on the fast spin dry speed selection, it only occurs very briefly. The average power during the whole 45 minutes duration of a "heavy duty wash" selection is surprisingly low. There can be other front panel selections like "delicates" or "wool" and medium and slow spin dry rates could be selected which probably use even less power. Pretty impressed with it really. One of these should be able to work off only about a 600 watt inverter if necessary. Cheers, Tony. |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Really didn't think they'd be THAT efficient.... But I'm not one to argue without checking facts.... Found this page that pretty much confirms their low power usage. You've inspired me. I just connected that very same watt meter to my old Whirlpool for a full load on normal cycle. It's currently drawing around the 700 watt mark on the wash cycle. It's over 10 years old & getting near it's replacement date. And interestingly, as my wife brings work washing home every day, (Salon Towels etc), we average about 24 loads a week. (And Yes, we do have a well watered yard)... Phil |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Phil, I was pretty amazed too. But the numbers do not lie. My tumble dryer just died, the drive belt around the drum shredded, and I have always looked down on this greedy 2Kw power hog as being my enemy. What I have always wanted is a natural gas heated tumble dryer. Rinnai make one, but the reviews of this are highly unfavorable, and it costs well over a grand, which puts it well beyond any practical reach. While I had my dryer in pieces, a sudden idea occurred to me. I have a spare Rinnai gas space heater that I could couple up to a standard tumble dryer and for very little outlay have something really useful. So I am out in the back shed right now making it happen. The Rinnai space heater has a top mounted air blower that blows downwards with the hot air blowing out of a heat exchanger at the bottom. If I move the air blower to the bottom, which is a very easy thing to do, the hot air will then come out of the top. If I cut a hole in the base of the tumble dryer it sits neatly on top of the Rinnai gas heater. Air flow and air temperature is just about the same for both too. I have natural gas here, but for off grid, those Rinnai heaters can also be made to run off propane with a simple gas jet change. A 2Kw heating element running for an hour is some pretty serious power. For off grid, a large propane bottle should dry all your clothes all winter requiring minimal inverter or battery power. I am a long way off getting this running, but have it all sitting in the garage and it not only looks possible, but pretty easy to do. I have already moved the blower, which was a simple job, and even the original wires reach. Cheers, Tony. |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi mirriulah, it should be pretty simple to work out 270 watts in 20 hours means an average of 13.5 watts per hour if your cabin is powered by panels you will need at least 3 times that (41 watts) to charge the battery and run a fridge during the day so that the battery can supply power to the fridge at night. the battery is rated at 175 ah times 12 volt so that is a total capacity of 2100 watts this rating is for a full discharge over 20 hours if we discharge it over 12 hours we get a little less. Also depending on the battery chemistry you use, the total amount of discharges you do dictates the life of the battery. so for some wet lead acid batteries we try and use them most of the time between 35-50% discharge so that will be 700 to 1000 watts available EVERY NIGHT. There are some overheads and inefficiencies with running an inverter, I will take a stab and say the 13.5 watt per hour might turn into 20 and there could be another 20 from the inverter itself so the total will be 20+20 times 12-14 hours with little or no sun it will use 400 watts from a 700 to 1000 watt battery bank. Providing the inverter can start the fridge compressor. The freezer to fridge conversion (or this and there are others) means that you will get either a chest freezer or an upright with drawers and take advantage of the better insulation and stopping the cold air from falling out every time the door is opened. An extra thermostat needs to be fitted to raise the temperature of the freezer to above freezing and a sump or drain needs to be fitted because condensation will build up inside the freezer. Freezers with an external condenser work the best, you can add more insulation (my freezer works better when I leave my folded laundry on top) so the difference is in the run time it will drop from 15 minutes per hour to maybe 3 minutes every 2 hours. so the equation now is 70 watts per day, that's 35 watts at night plus the 200 watts for the inverter the new total is 235 watts per night. Saving of 165 watts. A good 12 volt fridge will use between 200- 500 watts a night so it is line ball on power usage, BUT 12v/24v fridges can be expensive to buy but you dont need an inverter, its all swings and roundabouts with this stuff. I'm confused, no wait... maybe I'm not... |
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| Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498 |
Hanging your clothes under cover and putting a fan on them will dry better than just hanging them. There are only 10 types of people in the world: those who understand binary, and those who don't. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Its certainly a very difficult call. Another thing to think about is the inverter no load idling power. Even when the motor is not running the inverter will be chewing through "some" power, which will probably turn out to be fairly significant. Cheers, Tony. |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
So two loads later.... the first load was a bit light; Work & school clothes etc, probably didn't warrant a full machine of water. 41:30 Total cycle time & 254 Watt hours. Second load was a heavy load of towels; chocka block. But the Watt meter lost the plot & didn't monitor properly. Still about 670 was average wattage in the first load, while it was more like 870's in the second. Maybe I need to be less harsh on the FP machines; just see top many in for repairs at the washing machine mans shop. Phil. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Another unsuspected energy vampire is the ordinary 240 volt electric wall clock. These are typically about 12 to 15 watts. Say 0.3 Kwh per day. I had two of those. Something that uses a single AA battery and runs for a whole year does exactly the same thing. Its well worth plugging every electrical load into your power meter and writing it all down. I managed to HALVE my daily power consumption. The astonishing thing is, its not the big short term loads like a microwave oven or power tools that consume most of the power, but multiple piddly little things that only bleed a very few miserable watts, but run 24 hours per day. Cheers, Tony. |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Going back about 3 years our domestic / home office power bill was peaking at about $7000.00 PA. Two 9kvA Air Cons, (output, not input); with 5 heads, wood heating for winter off peak hot water plus instant gas in the Ensuite. 2 fridges & a freezer. Last two years I've kept the PA consumption around the $4000 mark. Biggest saving is in the heating/cooling. Keeping more thermal inertia going & stealing heat from the ceiling space in winter & planning on stealing that cool air from under the house next summer to displace the + 50°C stuff that normally sits in the roof. PIR's on lights in a few work areas; and just one remaining Teenager that constantly forgets to turn things off..... Too late on a Saturday night to calculate the saving of a new washing machine over 24 loads a week over most weeks of the year. To-morrow maybe. Cheers. |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I counted that at 200 watt overnight, I think that is more than enough for a small inverter. I'm confused, no wait... maybe I'm not... |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
I was surprised, I had a 2400w continuous mod square wave for camping, when idling it drew about 400mA whenever it was switched on with no loads It unfortunately went bang on trip when somebody tripped over the leads to the battery and it took full panel output Interestingly the replacement a 8kW 12vdc inverter (again mod square wave) uses almost the same- about 450mA I never would actually use it as a 8kW, but it certainly has the overhead to start any motor (I even used it to run my welder at a campsite- ran 3x120AHr batts flat in under 5 minutes, but welded up a broken drawbar on a trailer fine) It regularly ran a 120l fridge with no issues, along with several computers, lights, 52"tv and assorted plugpacks in my van even my toploader runs on it Its a huge beast, but it was on special and the 2400w wasnt available from my supplier, and it only cost about $40 more than the 2400w and I needed it in a hurry so I got the metal suitcase lol |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
btw Yahoo2, I think you mean Watt Hrs (Whr or more commonly used is kWhr) Watts is the instantaneous usage eg I am using 200W over an hour, I will use 200Whr over 24 hrs I am still only using 200W I have used 4800Whr (4.8kWhr) (note that when using kWr's you have to note both the draw and the time involved) so as an eg, `overnight' might mean from 9pm to 6am thats 9 hrs using the 200w draw, thats 1800Whr or 1.8kWhr changing either the current drawn or the time its measured over will both change the kWhr figure (which is why most use 24hr and kW for the mains) |
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| Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814 |
To further complicate things is the duty cycle eg 200w lightbulb will have a dutycycle of 100% so hook it up to our meter and we will get after 24hrs our 4.8kWhr reading on the meter but say its a fridge (a compressor fridge like a house, not a peltier fridge) If you sit and listen to it- it turns on and off maybe (as eg) it turns on for a minute then doesnt run again for another 10 min now (I'm ignoring startup spikes etc) it runs 6min every hr after 24hr- our 200w fridge isnt using 4.8kWhr like our 200w lightbulb in fact our 200w fridge is only using 0.48kWhr huh??? they both use 200w, both running for 24hr, yet the energy used isnt the same???? its why we must use the correct units and (when required) the timescale involved |
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