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Old 05-16-2018, 08:27 PM   #1
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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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Old 05-16-2018, 08:40 PM   #2
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Watts divided by volts = amps
1000 / 122 = 8.2 amps
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Old 05-16-2018, 08:47 PM   #3
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Seeking help calculating amp usage

Quote:
Originally Posted by HarveyP View Post
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?


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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Old 05-16-2018, 09:04 PM   #4
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Quote:
Originally Posted by HarveyP View Post
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
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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Old 05-17-2018, 09:16 AM   #5
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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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Old 05-17-2018, 09:35 AM   #6
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Originally Posted by Harrysait View Post
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.
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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Old 05-17-2018, 09:36 AM   #7
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Quote:
Originally Posted by HarveyP View Post
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
Just switch the Kill-a-Watt meter to amps!
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Old 05-18-2018, 12:20 PM   #8
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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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Old 05-19-2018, 08:45 AM   #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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Old 05-19-2018, 05:36 PM   #10
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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
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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Old 05-20-2018, 11:18 AM   #11
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Just switch the Kill-a-Watt meter to amps!
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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Old 05-20-2018, 11:45 AM   #12
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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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Old 05-20-2018, 02:44 PM   #13
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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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Old 05-20-2018, 03:17 PM   #14
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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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