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Old 02-23-2020, 08:46 AM   #1
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Question 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, 09:56 AM   #2
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Originally Posted by Losangeles View Post
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!

Let me give it a shot.

First, how many amp hours is 3 minutes? 3/60 = .05 amp hours (Ah)

Second, how many amps is 1200 watts at 120 volts? 1200 watt / 120 volt = 10 amps. As this is the amps @120v, convert to amps @ 12v. 120 vac / 12 vdc x 10 amp = 100 amps

Now, how may amp hours? 100 amp x .05 Ah = 5 amp hours.

This assumes no loss from the inverter for the conversion of 12 volts to 120 volts which is not realistic as there is always some loss. But it gets you into the ballpark.

**********************
This only works if you have lithium batteries as lead acid batteries are affected by Peukert’s law.

"A common mistake is made when it is assumed that a 200 AH lead acid battery will provide 200 amps for 1 Hour. It won’t. In fact, a battery of this type may only provide about 40 minutes of continuous 200 amp service at best.

This is due to a well known characteristic associated with lead acid batteries. Specifically the capacity will decrease as the rate of discharge increases.

In other words the relationship between battery capacity (how much energy is available) and the rate of discharge is not a linear one.

The phenomenon being described here is known as Peukert’s law."
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Old 02-23-2020, 10:00 AM   #3
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Originally Posted by Losangeles View Post
...

May I ask what how many amp hours would I use if I use a 1,200 watt electric kettle for 3 minutes?
...
Watts = Volts times Amps. Rearrange the terms so Amps = Watts divided by Volts. Assume your lithiums are stable at 12.8. Now - 1200 divided by 12.8 = 93.75 amps for 3 minutes.

3 minutes is 0.05 hours. 93.75 times 0.05 hours = 4.69 amp-hours.

But take those numbers with a grain or 6 of salt. Losses in wires, losses in converting 12V to 120V, assumptions regarding battery voltage, etc., etc. Plus, the numbers to the right of the decimal point are pure fluff. A better guess is probably 7 to 8 amp-hours.

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Old 02-23-2020, 10:02 AM   #4
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...
Now, how may amp hours? 100 x .05 = 50 amp hours.
...




Now, how may amp hours? 100 x .05 = 5 amp hours.
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Old 02-23-2020, 10:03 AM   #5
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Tom's approach is OK except for a math error at the end. 100 X .05 = 5 amp hours, not 50.
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Old 02-23-2020, 10:08 AM   #6
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Tom's approach is OK except for a math error at the end. 100 X .05 = 5 amp hours, not 50.

We have to give Tom some slack - too early on Sunday to worry about decimal places.
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Old 02-23-2020, 10:11 AM   #7
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Thanks so much all - 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. (maybe about 8 amp hours or so)

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

John
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Old 02-23-2020, 10:11 AM   #8
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Originally Posted by tdf-texas View Post
...

First, how many amp hours is 3 minutes? 3/60 = .05 amp hours (Ah)
...
And if we really want to be hard on the poor guy:


3 minutes divided by 60 minutes is a ratio. No Amp units to the right of the equal sign. (And perhaps no units at all? Math majors - please comment ).
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Old 02-23-2020, 10:13 AM   #9
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So if the math is correct this seems to suggest using propane will save around 5 amp hours.

Just for tea!
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Old 02-23-2020, 10:18 AM   #10
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So if the math is correct this seems to suggest using propane will save around 5 amp hours.

Just for tea!
Coffee.
Helps the brain with this math...
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Old 02-23-2020, 10:36 AM   #11
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Now, how may amp hours? 100 x .05 = 5 amp hours.
I posted and then caught it right away - but before I could correct, everyone saw the error.

I really should do the posts in notepad, review, then post - but... too much trouble.
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Old 02-23-2020, 12:54 PM   #12
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Does anyone have any ideas of how that kettle would compare to running the microwave for 3 minutes?


That might be a nice comparison.
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Old 02-23-2020, 02:47 PM   #13
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Does anyone have any ideas of how that kettle would compare to running the microwave for 3 minutes?
...
A 1200 watt microwave vs. a 1200 watt kettle for 3 minutes each: Trivial differences at best.
The microwave might be a tiny bit more efficient since the kettle would retain some of the heat after the water is poured out. But to measure that difference would require many cups of coffee.
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Old 02-23-2020, 10:43 PM   #14
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Originally Posted by tdf-texas View Post
First, how many amp hours is 3 minutes? 3/60 = .05 amp hours (Ah)
But there is no current in that, so instead:
First, how many hours is 3 minutes? 3/60 = .05 hours (h)
... then continue as Tom posted:
Quote:
Originally Posted by tdf-texas View Post
Now, how may amp hours? 100 amp x .05 Ah = 5 amp hours.
... which becomes
Now, how may amp hours? 100 amp x .05 h = 5 amp hours.
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Old 02-24-2020, 11:50 AM   #15
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Originally Posted by alanmalk View Post
A 1200 watt microwave vs. a 1200 watt kettle for 3 minutes each: Trivial differences at best.
The microwave might be a tiny bit more efficient since the kettle would retain some of the heat after the water is poured out. But to measure that difference would require many cups of coffee.
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Originally Posted by alanmalk View Post
A 1200 watt microwave vs. a 1200 watt kettle for 3 minutes each: Trivial differences at best.
But microwave ovens are rated in output power (of microwave energy), and the input of electrical power to produce that is much higher. What would normally be called a "1200 watt" microwave will require at least 1500 watts of electricity.

An electric kettle 100% efficiently turns electrical energy into heat, and if the element is submerged nearly all of that heat goes into the water. A microwave oven is much less efficient in producing heat in the stuff being heated, but at least all of the microwave energy does go into whatever is in the oven.

The best thing about a microwave oven for heating water is that you can heat exactly the amount of water that you want. In contrast, a kettle usually has a minimum fill level.
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Old 02-24-2020, 12:45 PM   #16
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....
An electric kettle 100% efficiently turns electrical energy into heat, and if the element is submerged nearly all of that heat goes into the water. A microwave oven is much less efficient in producing heat in the stuff being heated, but at least all of the microwave energy does go into whatever is in the oven.
...
Conventional wisdom would suggest that you are correct. But absolute proof would require some measurements, and the production of many cups of coffee - as I said earlier...
--
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Old 02-24-2020, 12:56 PM   #17
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.... The best thing about a microwave oven for heating water is that you can heat exactly the amount of water that you want. In contrast, a kettle usually has a minimum fill level.
At Canadian Tire for $13 I got a nice, smaller electric kettle.... just so happens that 0.5 L (the marked minimum fill level) is *exactly* enough to make a big mug of tea, in my favorite mug, and 1.0 L (the kettle's max) is exactly the right amount for 2 of these mugs if my wife wants tea as well.

so pre-marked, and efficient as no energy wasted... and can leave unattended as i wander about in the morning. Brilliant. (as i dont want to leave a kettle on a propane stove inside the RV unattended)

so life is good, for once.

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