Watt to Ah

[JustMurph]

Catalystcat makes a really good point. Using amp hours when trying to explain the mathematics really confuses things.

A watt is a measure of the rate at which power flows. It is a rate equivalent to 1 joule of energy in 1 second.

Watt hours is a measure of a quantity of energy. We use watt-hours because it's a bit more practical than expressing it in joules. To go from a rate to a quantity, we need to add a time component. A watt-hour is equivalent to the quantity of energy that will pass through a circuit if 1 watt flows for 1 hour.

Here's the bit that's probably confusing; this quantity of energy can be used over any period of time. If you used it in half an hour, then the flow rate must be 2W. If you used it over 2 hours, then the flow rate is 0.5W. Does that make sense?

So, to go back to your original problem.

200W over 24 hrs = 200W x 24hrs = 4800Wh
To get to Ah we just divide by the voltage = 400Ah.

[Kelkara]

Originally Posted by Bowdrie

1 watt = 1 Coulomb = 6.24E18 electrons per second.
Right?

No.
One watt is not one Coulomb, it is one Amp (Coulomb per second) at one Volt.
6.24E18 electrons per second is not one Coulomb, it is one Amp.

Charge (Amp-hours, or Coulombs) - is like a cheque book
Current (Amps) - how many cheques get delivered to you every hour
Voltage: (Volts) - how much each cheque is made out for
Power (Watts) - how much money arrives each hour.
Energy (Watt-hours, or Joules) - your bank balance.

When considering batteries on boats, we usually use a timeframe of hours and not seconds. So a more useful measure of charge is Amp-hours, and a more useful measure of energy is Watt-hours. So the terms Coulomb (=Amp-second) and Joule (=Watt-second) are hardly ever used.

The confusion starts when people measure the energy in a battery in "amps" which is a shorthand for "amp-hours" hoping that context will make it clear that they didn't actually mean "amps". But "amp-hours" is not the stored energy it is the charge, the energy is charge times voltage or "Watt-hours". But we know the voltage is 12V, which makes amp-hours (or those shorthand "amps") a proxy for energy. Also since we know we have 12V, then power which is amps times volts (Watts) is always proportional to the amps, so we can also use amps as a proxy for power too ...

Given the above, the word "amps" gets used for current (Amps), charge (Amp-hours), energy (Watt-hours) and power (Watts). Context should make it clear to someone who knows what they're talking about, but has the potential to confuse someone who is learning.

[GordMay]

Originally Posted by catalystcat

The second part of your statement is true - If you are burning 30W over an hour, you will be using 30Wh of energy in that hour.

The first part of your statement "W=Wh" is incorrect and seems to be the root of a lot of this misunderstanding. That would be like saying 60mph = 60mi. It's off by so much that it is not even wrong. (Apologies to Feinstein's Lectures)...

I suspect you are referring to “Feynman's* lectures on physics”:
➥ https://www.feynmanlectures.caltech.edu/info/index.html

Including:
“Time and Distance” ➥ https://www.feynmanlectures.caltech.edu/I_05.html
“Motion” ➥ https://www.feynmanlectures.caltech.edu/I_08.html

The Feynman Lectures on Physics was based on a two-year introductory physics course, that Richard Feynman taught at Caltech, from 1961 to 1963. It was published in three volumes, during the years 1963 to 1965, and used as the introductory physics textbook, at Caltech, for nearly two decades.

* Richard Feynman ➥ https://www.feynmanlectures.caltech.edu/I_89.html

I suspect you are confusing Feynman’s “Right way to do it”, with Wolfgang Pauli’s “It’s not even wrong”.

"Alternate Way to Handle Electrodynamics" ~ by Richard Feynman, December 1963
➥ https://www.feynmanlectures.caltech.edu/info/index.html

In about the middle of the 2nd year of the FLP lectures, Feynman started to complain that he was disappointed, that he had been unable to be more original. He explained that he thought he had now found the "right way to do it" - unfortunately too late. He said that he would start with the vector and scalar potentials, then everything would be much simpler, and more transparent.

"(This isn't right) it’s not even wrong" ~ Wolfgang Pauli
➥ https://www.theguardian.com/science/...ep/19/ideas.g2

Otherwise, thanks for an interesting, and informative [learned] contribution.

[Bowdrie]

Originally Posted by Kelkara

No.
One watt is not one Coulomb, it is one Amp (Coulomb per second) at one Volt.
6.24E18 electrons per second is not one Coulomb, it is one Amp.

It's ok, I can accept that.
I posted the whole thing about counting electrons as a lighthearted joke.
Your next several paragraphs are just stuff I've known for 60 years.
You know, from back in the day when a high school shop class actually taught you something worthwhile.

[catalystcat]

Thanks Gord. I shouldn't post in a hurry or mix up my sources.
Feynman's lectures are great. Everyone should watch them a bunch of times.

But, I shouldn't get his quotes mixed up with those of Pauli, especially after having been the victim of that quote about a dozen times in the past decade. You think I would have that source nailed down...

[Baronkrak]

Originally Posted by SailingHarmonie

I certainly can tell the difference between AWA and TWA, ...

Then you should able to tell the difference between True and Apparent electrical power.

Sailing is a precise science and so is electricity.

[Baronkrak]

Originally Posted by Xavierp

MY POINT!!!!!
I am NOT trying to convert watt HOURS to amp hours.
I am trying to understand how to go from Watt to Amp h.

Watt is the unit of Power in an AC system.
Ah is the unit of Energy in a DC system
Find the True AC power using the formula W = V * A * pf
W = VA, multiply DC power by time and you get Ah

[JustMurph]

Originally Posted by Baronkrak

Watt is the unit of Power in an AC system.
Ah is the unit of Energy in a DC system
Find the True AC power using the formula W = V * A * pf
W = VA, multiply DC power by time and you get Ah

LOL. Okay then.

[SailRN]

The posts from CatylystCat, Kelkara, and JustMurph are excellent explanations!
Thanks!


Quote:
Originally Posted by Bowdrie
1 watt = 1 Coulomb = 6.24E18 electrons per second.
Right?

No.
One watt is not one Coulomb, it is one Amp (Coulomb per second) at one Volt.
6.24E18 electrons per second is not one Coulomb, it is one Amp.

Charge (Amp-hours, or Coulombs) - is like a cheque book
Current (Amps) - how many cheques get delivered to you every hour
Voltage: (Volts) - how much each cheque is made out for
Power (Watts) - how much money arrives each hour.
Energy (Watt-hours, or Joules) - your bank balance.

[wholybee]

Pages of explanations, and I don't think any really get down to the simplicity of it all.

First, to prevent possible confusion while reading various pages:
in formulas, "I" refers to current, and "E" refers to voltage. So sometimes you see I and A used interchangeably, and sometimes you see E and V used interchangeably. I mention this first because if you google ohms law, the first hit is E = I x R. And of course that formula can be rearranged algebraically any way you need to.

The second thing you need to know is how units work. First, units that are derived from a proper name (a person) are always capitalized. So, Ampere, Voltage, and Wattage, are capitalized, but hours are not. And when two factors of different units are multiplied, the resulting unit is just the combination. So, if you multiple Amps by hours, the result is Ah. And if you multiply Watts by hours the result is Wh. (I will touch on the W vs VA argument at the end of this).

So, we know ohms law is:
E = I x R

Power is defined as:
P (in Watts) = E x I

And because we now know how units work:
Ah = A x h
Wh = W x h


From those, any conversion you need is basic algebra on those formulas. For example, to convert Wh to Ah:

Wh = W x h - formula for Wh
Wh = V x A x h - insert formula for W
VAh = V x A x h - also on the units side
VAh/V = V x A x h / V - divide both sides by V
Ah =A x h
Thus, to convert Wh to Ah, divide both sides by the voltage.

Now, to touch on the arguing of W vs VA. This is an advanced topic you really don't need to worry about unless you are an EE, and several in this thread don't seem understand even though they are sure they are correct. (and at least one does understand it) But it goes something like this. (you can safely ignore the below, and everything else about that argument in this thread)

In a DC circuit, it is very simple. Watts equals V x A. So the units W and VA will be exactly the same, and no one ever uses the unit VA in a DC circuit. In an AC circuit, with and inductive load (like a motor) the Voltage and Amperage will be out of phase. What this means is that any given instant, lets say when the voltage is at its maximum voltage in the AC cycle, the current will not be at it's maximum. The current will reach its maximum at some other point, when voltage is not at its maximum. This creates a situation where if you multiply V x A, you don't actually get W. So and additional term, the power factor, is added to get the actual power in Watts.

The unit VA is only ever used while discussing AC, and is a way to know that the discussion is about AC and not about DC. You will most commonly see it in ratings for Uninterrupted Power Supplies and sometimes inverters, where they will be rated by VA and not in W. Rating a UPS for 1500VA is a clue that it can't actually support 1500W. But what it can support will depend on the load, is it inductive, or purely resistive.

Since the original question was framed as being about a 12V DC system, bringing VA into the conversation was incorrect, because the unit VA is not used in DC.

[GordMay]

EE Circuit Analysis - Complex Power (Real & Reactive Power)
Video ➥ https://youtu.be/QELpZ8Rr9rA

This tutorial delves into complex power theory in circuit analysis, a fundamental concept crucial for electrical engineering students, professionals, and enthusiasts seeking to understand power behavior in AC circuits.
Complex power theory combines both real and reactive power components, providing a comprehensive framework for analyzing power in systems where the phase difference between voltage and current leads to power factor considerations.
The video begins with an introduction to AC circuit principles, laying the groundwork for understanding complex power. It explains the distinction between real power, which performs actual work, and reactive power, which creates and sustains electric and magnetic fields.
Through detailed explanations and visual demonstrations, viewers will learn how to calculate complex power.
The tutorial covers the significance of the power triangle, power factor, and how these concepts apply to circuit analysis and design. Additionally, the video addresses common challenges in complex power calculations and provides tips for effectively solving circuit problems.
By the end of this tutorial, viewers will have a solid grasp of complex power theory, enhancing their ability to analyze and optimize AC circuits for better efficiency and performance.

[Shaneesprit]

Originally Posted by Cheechako

If really drawing that much I think you will be in trouble in several hours or less. A battery is 50% discharged at about 12.05 volts. 40% left at 11.95 V. That's pretty much "empty" for normal use. SOme refrigerators shut down at low voltage.

So does my Raymarine Auto Pilot, which is fun as you can’t diagnose the issue as it’s switched off Grrrr.

Battery Testers all suggested my batteries were good, but a load test with Volts and Amps proved otherwise

[Icarus]

Originally Posted by Xavierp

400Ah is 1/2 of my 800Ah battery bank

let's say the appliance is on all day. How long would it take to empty the bank?
2 hours or 2 days?

Depends whets plugged in and for how long..a refrigerator system draws between 6A to 10Ahr and length of running time...Inverters, lights, silly things like an electric cooktop, water pumps, alarms, radio transmissions and so on
You really need to sit down and write down your electrical usage ...and then tally it up against possible charges..solar, wind ...

[boatpoker]

Originally Posted by Wotname

Hi Xavierp, it looks like you are getting the help you needed to understand the It is kind of disrespectful to the famous English inventor James Watt to use incorrect terms for power (e.g. like volt-amp)

Its even more disrespectful to refer to a Scotsman as an Englishman

[Wotname]

Originally Posted by boatpoker

Its even more disrespectful to refer to a Scotsman as an Englishman

Mea Culpa!