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05-16-2018, 09:27 PM
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#1
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Senior Member
Join Date: Apr 2005
Posts: 323
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Seeking help calculating amp usage
Hi all. We are planning to install a residential fridge. I have it plugged into a Kill-a-Watt meter now. In 48 hours and 13 minutes, it used 1.06 KWH, getting 122 volts AC on shore power. Can anyone here help me calculate amps used? Perhaps amps used in 24 hours? I know there is a further calculation that would tell me amps drawn from my battery bank when running through an inverter that is probably 85% efficient. The math is beyond me, but I know this forum is blessed with some very savvy users. Thanks, HarveyP
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94 Holiday Rambler Endeavor LE
Cummins/Allison
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05-16-2018, 09:40 PM
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#2
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Senior Member
Monaco Owners Club
Join Date: Feb 2008
Location: Western Montana on the Divide
Posts: 1,394
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Watts divided by volts = amps
1000 / 122 = 8.2 amps
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Bob Retired Army Traveling alone now except for Charlie the Beagle.
2008 Camelot 40 PDQ 4 slides ISL400 towing a 2016 Dodge Ram 1500 4x4 Quadcab
Western MT in summer, AZ, NV in winter
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05-16-2018, 09:47 PM
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#3
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Community Moderator
Newmar Owners Club
Join Date: Apr 2014
Location: Floyds Knobs, In.
Posts: 5,181
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Seeking help calculating amp usage
Quote:
Originally Posted by HarveyP
I have it plugged into a Kill-a-Watt meter now. In 48 hours and 13 minutes, it used 1.06 KWH, getting 122 volts AC on shore power. Can anyone here help me calculate amps used?
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If your numbers are correct and my math is correct.
1.06kwh in 48 hours @ 122volts = around 22watt/hour
22/122 = .183 amps @ 122 volts constant current for 1 hour.
This would be the average current over 48 hours.
Actual current will be much higher at times. Your inverter will need to be sized to handle these peak current draws.
On the label of the refrigerator you should have an estimated annual usage. What does it say?
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Steve
2002 Newmar Mountain Aire 4095
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05-16-2018, 10:04 PM
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#4
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Senior Member
Newmar Owners Club Freightliner Owners Club
Join Date: Aug 2015
Posts: 2,263
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Quote:
Originally Posted by HarveyP
Hi all. We are planning to install a residential fridge. I have it plugged into a Kill-a-Watt meter now. In 48 hours and 13 minutes, it used 1.06 KWH, getting 122 volts AC on shore power. Can anyone here help me calculate amps used? Perhaps amps used in 24 hours? I know there is a further calculation that would tell me amps drawn from my battery bank when running through an inverter that is probably 85% efficient. The math is beyond me, but I know this forum is blessed with some very savvy users. Thanks, HarveyP
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Please double check the units of measure...
Using 1.06 kWh. A kilowatt is 1000 watts. 1.06kwh x 1000 w/kw = 1060 watt hrs.
Watt hrs / voltage = aH...since you will be using a 12v system,..say...
1060 wH / 12.6 = 84.12 aH
84.12 aH / 48.2 hrs = 1.745 a/hr
1.75 a/hr x 24 = 41.88 amps per 24 hr period
If you say the Inverter is 85% efficient..
41.88 / .85 = 49.27 amps per 24hr period.
Pretty efficient if you ask me...
My coach averages about 11 amps per hour with fridge and all the other static dc loads...So, a 1.75 amp per hour average drain for the fridge is pretty low.
Hope that helps.
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Charlie & Ronni
2016 Ventana 4037
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05-17-2018, 10:16 AM
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#5
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Senior Member
Workhorse Chassis Owner
Join Date: Nov 2014
Posts: 201
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If you are looking at a small, 10.5 cf, like Whirlpool, you will be using around 80 AH per day, add other usage and it will be around 100 AH per day. If you have 4 GC batteries that takes you down to 50% state of charge.
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Harry
Tempe, Az
2004 Trek
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05-17-2018, 10:35 AM
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#6
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Senior Member/RVM #90
Monaco Owners Club
Join Date: Nov 2002
Location: Columbus, MS
Posts: 34,760
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Quote:
Originally Posted by Harrysait
If you are looking at a small, 10.5 cf, like Whirlpool, you will be using around 80 AH per day, add other usage and it will be around 100 AH per day. If you have 4 GC batteries that takes you down to 50% state of charge.
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Hi Harry!  Welcome to IRV2! We're sure glad you joined the gang!
Noticed you are new and wanted to say hello!
Good luck, happy trails, and God bless! 
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Joe & Annette
2002 Monaco Windsor 40PBT, 2013 Honda CRV AWD
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05-17-2018, 10:36 AM
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#7
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Senior Member/RVM #90
Monaco Owners Club
Join Date: Nov 2002
Location: Columbus, MS
Posts: 34,760
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Quote:
Originally Posted by HarveyP
Hi all. We are planning to install a residential fridge. I have it plugged into a Kill-a-Watt meter now. In 48 hours and 13 minutes, it used 1.06 KWH, getting 122 volts AC on shore power. Can anyone here help me calculate amps used? Perhaps amps used in 24 hours? I know there is a further calculation that would tell me amps drawn from my battery bank when running through an inverter that is probably 85% efficient. The math is beyond me, but I know this forum is blessed with some very savvy users. Thanks, HarveyP
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Just switch the Kill-a-Watt meter to amps! 
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Joe & Annette
2002 Monaco Windsor 40PBT, 2013 Honda CRV AWD
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05-18-2018, 01:20 PM
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#8
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Senior Member
Join Date: Apr 2005
Posts: 323
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Thanks to all of you. I haven't started the install yet, but feel hopeful that 4 golf cart batteries will be sufficient. Thanks again, HarveyP
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94 Holiday Rambler Endeavor LE
Cummins/Allison
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05-19-2018, 09:45 AM
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#9
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Junior Member
Join Date: Jan 2018
Location: Newark Valley, New York
Posts: 9
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Here's a link to an OHM's law calculator. Easy to use.......
Ohms Law Calculator
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05-19-2018, 06:36 PM
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#10
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Senior Member
Join Date: Aug 2012
Posts: 717
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Quote:
Originally Posted by HarveyP
Hi all. We are planning to install a residential fridge. I have it plugged into a Kill-a-Watt meter now. In 48 hours and 13 minutes, it used 1.06 KWH
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Just curious--what kind of refrigerator is this? I'd never argue with what a kill-a-watt says (and wish everybody would use one), but the residential refrigerators that a lot of people put in RVs are "household" ones that use have ratings of about 1.3 kwh/day, and yours is using less than half that. Is yours real small?
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05-20-2018, 12:18 PM
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#11
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Senior Member
Join Date: Aug 2012
Posts: 717
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Quote:
Originally Posted by MSHappyCampers
Just switch the Kill-a-Watt meter to amps!  
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No can do.
While the kill-a-watt has a readout for instantaneous usage in either amps or watts, the only readout it has for usage over a period of time (which is what the OP wants) is in kilowatt hours.
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05-20-2018, 12:45 PM
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#12
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Senior Member
Ford Super Duty Owner
Join Date: Dec 2010
Posts: 1,008
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OK, so you say you used 1.06 kWH in 2 days. That would mean 530 watt hours per day (1060/2). So, at 12 volts that would be 45 AH per day (530/12). Not too bad. How big was the reefer, my 22 cuft uses around 192 AH per day.
So, if you have 2 100 AH 12 volt deep cycle batteries, it would take you from 100% down to a little more than 50% in 24 hours without generator, shore of solar. Don't have your specs but I am sure that a 1000 or 1500 watt inverter and at least 2 batteries would do you well.
That should have an Energy Star rating around 195 or so.
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Scott Brownstein
Palm Island, Florida
2015 Georgetown 335DS
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05-20-2018, 03:44 PM
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#13
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Moderator Emeritus
Join Date: Jan 2000
Location: Silver Springs, FL. USA
Posts: 19,625
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I suspect the amps you really want, however, are the battery amps x the number of hours. The amps @ 12v or roughly 11x the amps drawn @ 120v (conversion plus a 10% inverter overhead ).
Still, the KWH is the same whether measured at the battery or the fridge. 1.06 KWH = 1060 watt-hours, and 1060 watt-hours divided by 12v means 83 amp-hours drawn from the batteries if no shore power available. Add another 10% to that for inverter losses.
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Gary Brinck
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC
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05-20-2018, 04:17 PM
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#14
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Senior Member
Ford Super Duty Owner
Join Date: Dec 2010
Posts: 1,008
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ahh...but that is 83 AH in TWO DAYs. If you know the energy star rating, tell me what it is. Mine is 564 and it averages, 8 amps or so, including the inverter losses and everything else. You can do the math. Energy Star is not a good absolute but it is really good at predicting the differences between units when we have data for one of them.
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Scott Brownstein
Palm Island, Florida
2015 Georgetown 335DS
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