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Old 02-23-2020, 07:48 AM   #1
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Question Amp hours used: 1,200 watt electric kettle, on for 3 minutes?

Amp hours used: 1,200 watt electric kettle, on for 3 minutes?

Hi all

I'm not great with watts / time / amp hours used....

May I ask what how many amp hours would I use if I use a 1,200 watt electric kettle for 3 minutes?

I ask as I'll have 200 amp hours of Lithium batteries, and I want to know how much I'm drawing from them to use an electric kettle to boil water, instead of propane.

thanks!

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Old 02-23-2020, 08:23 AM   #2
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1200 watts / 12.8 Voltage = 93.75 Amps

93.75 Amps + 10% (conversion cost) could be up to 20%
103.125 = 93.85 + 9.375

If you ran that load for 1 hour you would use 103.125 amps hours, so divide by 60 for mins.
103.125 / 60 = 1.71875‬ , so for 3 mins of use you would use 1.71875 * 3 or 5.15625‬ amps hours. For a 200 amp hour lithium bank (80% discharge) you would use roughly 3.2% of your battery capacity.

=greg
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Old 02-23-2020, 08:36 AM   #3
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The formula solving for amps is:
Amps= watts/volts

Your 1200w kettle is really rated at 1200w per hour and when looking at battery ratings for capacity in amps it is also considered at amps per hour. Typically the battery rating eg 100 amps is amps drawn over a 20 Hour period. So that battery is really looking at a rating that would draw 5 amps per hour for 20 hours, or about .1 amps per minute.

That kettle at 120VAC would use 10amps (1200watts/120Volts AC) if used for an hour.
If used for 3 mins the hour of time (Amp Hours) needs to be adjusted to minutes. So, 3mins / 60 mins x 10 amps = .5 amps at 120 volts.

Now to look at your 12 VDC battery draw - if 100% efficient conversion would be
1200w/12v = 100 amps per hour x 3/60mins = 5 amps in 3 mins. This is a heavy load.

Some things that will increase this:
>Your inverter is not 100% efficient
>Your wiring gauges (and length) need to be calculated to handle the load
>What the impact to your battery bank voltage will be when hitting it with a big load (100 Amps) for even a short period of time. Battery voltage might drop below 11.5v depending on the size of the bank. To offset this voltage drop (due to a 3 min @ 100 Amp draw) you can start your engine and supplement the voltage via the alternator.

Hope that helps w calcs.

My two cents - heating water with electricity in an RV (running on batteries) is a HEAVY draw and not the most ideal method. I hope you have other needs for a heavy draw setup other than heating water for 3 mins. Very expensive cups of hot drinks.
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Old 02-23-2020, 08:43 AM   #4
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Quote:
Originally Posted by Scalf77 View Post
1200 watts / 12.8 Voltage = 93.75 Amps

93.75 Amps + 10% (conversion cost) could be up to 20%
103.125 = 93.85 + 9.375

If you ran that load for 1 hour you would use 103.125 amps hours, so divide by 60 for mins.
103.125 / 60 = 1.71875‬ , so for 3 mins of use you would use 1.71875 * 3 or 5.15625‬ amps hours. For a 200 amp hour lithium bank (80% discharge) you would use roughly 3.2% of your battery capacity.

=greg
thanks so much greg. awesome.
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Old 02-23-2020, 08:48 AM   #5
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Quote:
Originally Posted by 1der View Post
The formula solving for amps is:
Amps= watts/volts

Your 1200w kettle is really rated at 1200w per hour and when looking at battery ratings for capacity in amps it is also considered at amps per hour. Typically the battery rating eg 100 amps is amps drawn over a 20 Hour period. So that battery is really looking at a rating that would draw 5 amps per hour for 20 hours, or about .1 amps per minute.

That kettle at 120VAC would use 10amps (1200watts/120Volts AC) if used for an hour.
If used for 3 mins the hour of time (Amp Hours) needs to be adjusted to minutes. So, 3mins / 60 mins x 10 amps = .5 amps at 120 volts.

Now to look at your 12 VDC battery draw - if 100% efficient conversion would be
1200w/12v = 100 amps per hour x 3/60mins = 5 amps in 3 mins. This is a heavy load.

Some things that will increase this:
>Your inverter is not 100% efficient
>Your wiring gauges (and length) need to be calculated to handle the load
>What the impact to your battery bank voltage will be when hitting it with a big load (100 Amps) for even a short period of time. Battery voltage might drop below 11.5v depending on the size of the bank. To offset this voltage drop (due to a 3 min @ 100 Amp draw) you can start your engine and supplement the voltage via the alternator.

Hope that helps w calcs.

My two cents - heating water with electricity in an RV (running on batteries) is a HEAVY draw and not the most ideal method. I hope you have other needs for a heavy draw setup other than heating water for 3 mins. Very expensive cups of hot drinks.
Thanks so much Ray - super helpful.

The good news is that in my rig I'm running a Truma Combi, so super efficient for using propane for hot air and the occasional hot shower.

but if I want a quick hot beverage, once a day, good to know it will use a relatively small amount of my batteries.

and I have 720 watts solar on the roof, and so that will refill the batteries quite quickly. Victron system, so it can handle the loads.

thanks again
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Old 02-23-2020, 08:48 AM   #6
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Quote:
Originally Posted by 1der View Post
The formula solving for amps is:
Amps= watts/volts

Your 1200w kettle is really rated at 1200w per hour
NO IT IS NOT.


Amperes are already "per hour", or more accurately "per time" - they are coulombs per second, where the coulomb is a specific number of electrons.


A kettle takes 1200 Watts. Period.


That means that it will consume 1200 Joules of energy per second. Or 1200 Watt seconds (same thing). Or 1200Wsec / 3600 (sec/hr) = 1/3 of a Watt Hour.



At 120V, that is 1200W/120V = 10 amps. Not "amps per second" - just amps.


At 12V, with conversion losses, that's about 110 amps.


For three minutes, or 1/20 hour, that is 110 amps * 1/20 hour = 5.5 amp-hours at 12V.
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Old 02-23-2020, 09:03 AM   #7
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Quote:
Originally Posted by Wowbagger View Post
NO IT IS NOT.


Amperes are already "per hour", or more accurately "per time" - they are coulombs per second, where the coulomb is a specific number of electrons.
You are correct Wowbagger. I think the rest of my explanation is OKAY though. Perhaps you can double check that and explain in a BIT more DETAIL.
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Old 02-23-2020, 09:28 AM   #8
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Proof that there is more than one way to get the same data.

Here are some snap shots of a coffee pot use and some other devices used. This is on my 4D battery system, and at the time I was using a tripplite modified sine wave inverter.






-greg
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Old 02-23-2020, 09:46 AM   #9
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Greg, that is awesome charting.

I know when we use our 900w Microwave it drops the 480ah battery bank voltage quickly from 12.7 to 11.9 and That is running through large gauge cable and pretty short runs to minimize voltage drops.

Your chart shows voltage drop - what are the values it drops to? At what voltage (11.5?? ) does voltage drop become an issue?
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Old 02-23-2020, 10:23 AM   #10
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Ray, theses are pretty old charts, I just posted pictures that were in my album. I will see if I can dig up the original data. In this data collection the wire size from my battery to my inverter was 2/0. My runs are long, one side of the van to the other. Also it depends where I actually had the voltage monitoring connection at. In any advent you are going to see voltage drop, with large loads. My worry for voltage drop is when it gets close to the inverter low voltage lock out, or the low voltage disconnect I have for the battery, which I usually have higher than the inverter set point. Nothing worse than hitting those in the middle of cooking or waiting for coffee.

-greg
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