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Old 04-29-2016, 08:05 AM   #1
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Solar Experts, Please Check My Math

Using a Kill-A-Watt meter.
It says that my coffee maker draws 880 watts at 120 volts.
For sake of argument, it runs for 10 minutes.
What I want to know is how many amp hours it removes from the battery bank in that amount of time.

How many watts does it draw per minute
880 divided by 60 minutes = 14.6 watts per minute
14.6 watts per minute X 10 minutes = 146 watts
146 watts divided by 12 volts = 12.1 amp hours.

So if my math is correct my morning coffee is removing 12.1 amp hours from the battery bank.

Is this math correct? Thanks much for your time and consideration.
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Old 04-29-2016, 08:45 AM   #2
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Quote:
Originally Posted by MattTaylor View Post
Using a Kill-A-Watt meter.
It says that my coffee maker draws 880 watts at 120 volts.
For sake of argument, it runs for 10 minutes.
What I want to know is how many amp hours it removes from the battery bank in that amount of time.

How many watts does it draw per minute
880 divided by 60 minutes = 14.6 watts per minute
14.6 watts per minute X 10 minutes = 146 watts
146 watts divided by 12 volts = 12.1 amp hours.

So if my math is correct my morning coffee is removing 12.1 amp hours from the battery bank.

Is this math correct? Thanks much for your time and consideration.
Hi Matt, that is correct.

One tiny item, watts are watts - they are not voltage specific. Maybe better said, the voltage has already been factored in.

My version of the same equation:
880 watts / 12.5 (volts) = 70.4 amp-hours. 70.4 X 16.7% = about 12 amps.
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Old 04-29-2016, 09:03 AM   #3
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I don't mean to pick in you or sound preachy, but... this goes towards one of my pet peeves - using "units" incorrectly. One of the most common errors is using "amps per hour" as an equivalence for "amp hours", which just confuses the underlying principles.

Similarly, your first step was to calculate "watts per minute". Watts is a measure of the rate of power consumption, so it is meaningless to talk about watts per minute (unless you are talking about a change in the rate of power consumption, which does not apply to your question). It's also meaningless to talk about the total power consumed by the coffee maker in watts, as you did in your third step. This was one of the first lessons I learned in introductory physics in the mid 1970s - get the units right, and the correct answer will follow.

Off the soap box, and to your question.

Watts = Volts * Amps, therefore, Amps = Watts / Volts. For your case, Amps = 880/120 = 7.3 amps.

10 minutes = 1/6 hours.

Your coffee maker draws 7.3 amps for 1/6 hours, or 1.2 amp hours.

You used both 120 volts and 12 volts in your calculation, which explains the "slippery decimal". I don't know which is correct (120 or 12 volts), but I suspect that 120 volts is correct since most coffee makers run on 120 volts.

But more importantly, you used the units incorrectly in your calculation, which allowed you to arrive at (nearly) the correct value only by coincidence.

Hope this helps.
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Old 04-29-2016, 09:11 AM   #4
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I don't mean to pick in you or sound preachy, but... this goes towards one of my pet peeves - using "units" incorrectly. One of the most common errors is using "amps per hour" as an equivalence for "amp hours", which just confuses the underlying principles.

Similarly, your first step was to calculate "watts per minute". Watts is a measure of the rate of power consumption, so it is meaningless to talk about watts per minute (unless you are talking about a change in the rate of power consumption, which does not apply to your question). It's also meaningless to talk about the total power consumed by the coffee maker in watts, as you did in your third step. This was one of the first lessons I learned in introductory physics in the mid 1970s - get the units right, and the correct answer will follow.

Off the soap box, and to your question.

Watts = Volts * Amps, therefore, Amps = Watts / Volts. For your case, Amps = 880/120 = 7.3 amps.

10 minutes = 1/6 hours.

Your coffee maker draws 7.3 amps for 1/6 hours, or 1.2 amp hours.

You used both 120 volts and 12 volts in your calculation, which explains the "slippery decimal". I don't know which is correct (120 or 12 volts), but I suspect that 120 volts is correct since most coffee makers run on 120 volts.

But more importantly, you used the units incorrectly in your calculation, which allowed you to arrive at (nearly) the correct value only by coincidence.

Hope this helps.
Nice speech jacksonmacd. However; you entirely missed the actual point of the question (comprehension). Might try giving it another read.
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Old 04-29-2016, 09:14 AM   #5
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Watts = Volts * Amps, therefore, Amps = Watts / Volts. For your case, Amps = 880/120 = 7.3 amps.

10 minutes = 1/6 hours.

Your coffee maker draws 7.3 amps for 1/6 hours, or 1.2 amp hours.
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Old 04-29-2016, 09:29 AM   #6
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I believe the question is as follows, perhaps it is me that is mis-understanding.

Quote:
Originally Posted by MattTaylor View Post
What I want to know is how many amp hours it removes from the battery bank in that amount of time.
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Old 04-29-2016, 09:37 AM   #7
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and it was for 10 min. right

Quote:
Originally Posted by JFNM View Post
I believe the question is as follows, perhaps it is me that is mis-understanding.
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Old 04-29-2016, 09:39 AM   #8
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Noooo!

I'm sure by the time I finish writing this, someone else will have clarified things, but here goes.

Watts=power=Amps x Voltage
Watts/Voltage=Amps
If his coffee maker runs for an hour it uses 880W/120V=7.3Amp-HOUR
@120V, the coffee maker uses (7.3amp-hours/(60min/hour)) x 10min=1.2 amp-hours
However, the OP wanted to know the 12V battery 'consumption', hence his equation, which was correct. The only difference between the first two posts is one used 12V and the other 12.5V as that is closer to the actual battery voltage (though it would be more conservative to use the 12V as the current (amps) will increase as the battery wears down.)



However, I would add that the inverter from 12V to 120V isn't 100% efficient, so I would divide the 12V amp-hours by .9 to be conservative.
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Old 04-29-2016, 10:09 AM   #9
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OP,
To better answer your question the following information would be needed;

Does your coffee pot cycle the power on/off/vary during it's 10min estimated usage vs constant power draw?

Most Kill-a-watt meters have the capability to tell you total power (Amp Hrs ) used since last reset (time based) in addition to watts at a voltage. You may want to see if your version does as it would be much easier and more accurate for you.

Then need to know what efficiency your inverter changes 12vdc into 120vac and if its a PSW or MSW type? (assuming you are running a 12vdc nominal battery bank system vs 18,24 or higher)

This may be in your manual. Usually 80-95% ish. If your inverter is Pure sine wave vs Modified sine wave then the Kill-A-Watt meter would not be affected in its ability to read correct power usage. If MSW then all bets are off.

Remember the amount of power used to bring a battery bank back TO a state of charge will always be more than what was used FROM the battery to get to that same state of charge.

Adding a battery monitoring system could better and easier answer your question but of course at an expense.

As you can see there are many variables that come into play in answering your question but for estimating hopefully the previous posts gave you some help. XX Amp-Hrs @ 120v is DIFFERENT than XX Amp-Hrs @ 12v. My best estimate for you at this time is 1.2AHrs@120 x 10AHrs@12 / .9 inverter eff = 13.3 AHrs from your 12 volt battery bank.
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Old 04-29-2016, 10:16 AM   #10
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What I did when I was pondering the same question was to look at the"amps used" on my Trimetric 2030 before I started the coffee pot, and then again when the coffee pot was done, and if I remember correctly, my batteries were down about 12 amp hours from where they were before I started the coffee pot.
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Old 04-29-2016, 10:39 AM   #11
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You guys forgot the peukert effect.

Batteries are rated at 20 AH. The inverter is pulling 70+ amps.

Don't you need to know how many parallel batteries the inverter is drawing off ?
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Old 04-29-2016, 10:51 AM   #12
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Matt, clearly you need to return to school and get your EE to calculate such things. Even then, it is probably unknowable as you need to ALWAYS know the exact state of you batteries, every electrical connection in the system, and every inch of wire (one gets hit/damaged, the resistance is going up so voltage loss will be greater....).

Or... since you are probably attempting to do a simple energy budget/audit you could use a nice round number like 15 amp-hours for one "cycle" of you coffee maker. Then, once you get your system installed, you can observe it for a few weeks and fine tune your number to match what is actually being consumed (which will be close to your calculation, not orders of magnitude different).

Four years back in college or a reasonable estimate based on some reasonable information and a some reasonable assumptions. I dunno, tough choice.... I guess it's no wonder that people are scared of their electrical system - heaven forbid a DIY solar charging project.
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Old 04-29-2016, 10:51 AM   #13
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Holy cow fellas, lol!! Generally speaking I try not to cause a ruckus this early in the morning

After posting this question I had the bright idea to do some testing to see if my 12.1 answer was at least close, so here's what I did.

I removed all incoming charge from the panels, so the bank was getting no charge at all. Turned off the 12 volt system so the coach was draw nothing from the bank. Turned on the inverter, plugged in the Kill-A-Watt, then the Mr. Coffee. I made a full pot of coffee and timed it with my iPhone. Here's what I found.

Kill-A-Watt readings
Volts 119.5
Amps 7.41
Watts 886

Time to Brew = 11:36.02

Trimetric 2030-A Amp Hour reading
Start = -59.0
Finish = -76.1

That's a difference of more than my calculated value and I can't account for the extra 5'ish amp hours taken out of the battery bank. Can that be attributed to line losses or inefficiencies? I know when I power on the inverter it draws a little, then there is vampire power going on with the TV and Microwave plugged in. So ...
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Old 04-29-2016, 10:53 AM   #14
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Good one Twin...

Forgot to ask about the;

Ah capacity of each battery
How many batteries in system
Battery composition material
Ambient Temp of said batteries
System wire sizes and termination style
North or South of the Equator
What elevation will it be used at
Water hardness used in the Coffee Maker
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