Many people get confused when discussing Amps, Watts, Amp hours and Watt hours. Frequently the confusion is the result of incorrect use of these terms by some forum posters. Hopefully the following will clarify rather than further

fog the issue.

First - A bit of background

SI UNITS

When we talk about a quantity or a measurement, we talk about a number of units. To avoid confusion between people, it is essential that everyone is "talking the same language". There is an internationally recognised set of Units called The International System of Units or SI which enables this.

In the SI System, there are two fundamental types of measuring units: Basic and Derived

Basic Units are simple things like length,mass and time.(the specific units for these are the meter, kilogram and second). There are several other such base units including the Ampere (for

electric current) and the Kelvin (for temperature)

Derived Units are more complex measurements which are formed by multiplying or dividing two or more different Base Units such as meters per second.

For convenience, we often describe quantities in terms of non SI units such as horse power, gallons, nautical miles, knots or gallons per hour. Any of these units can be converted to their equivalent SI representation by multiplying or dividing them by an appropriate number.

(Example: You can convert nautical miles to the SI unit "meter" by multiplying by 1852 and you can convert knots (nautical miles per hour) to meters per second by multiplying by 0.514444.

10 nautical miles = 18,520 meters and 4 knots = 2.05776 meters per second

The Kelvin is a special case, it is exactly the same size as a degree Celsius, but with a different start point, so you can convert Kelvin to degrees Celsius just by adding a number and when calculating temperature changes, you can use the two interchangeably)

DIMENSIONS

The number of basic units used in a measurement is called the "dimensions" of that unit. So a meter or a nautical mile is a 1 dimensional unit (meters) and speed in meters per second or knots is a two dimensional unit (meters and seconds).

Note that these do not have to all be different units: acceleration for example has 3 dimensions even though it only uses two different basic units, since it uses one unit twice (meters per second per second)

Even more complex units such as the Volt can still ultimately be expressed in basic units. (The volt is actually a 7 dimensional unit which can be expressed as meters x meters * kg / secs / secs / secs / Amps

)

A plain number by itself, without any specified unit is called "dimensionless" (i.e. numbers like dozen or 42).

NAMES AND ABBREVIATIONS

Unit names are frequently abbreviated. It is a convention that where the name of a unit comes from the name of a person, the name or abbreviation is capitalised, all other units are not.

So we have Ampere, Volt, Watt and their abbreviations Amp or just A, Volt or just V and Watt or W, but meters (or m), seconds (sec or s), kilograms (kg), gallons (gal or g) etc

As mentioned previously derived units comprise two or more units which are multiplied or divided by each other. A great deal of confusing arises where these operations are not expressed properly.

**When two units are multiplied together, the resultant unit name is written as the basic unit names without any intervening mathematical or other symbol.**
(i.e The commonly used unit of "energy" stored or used in an

electrical system should always be written as simply Ampere hour (or Amp Hour, Ah).

**When one unit is divided by another , the two words are separated by a slash (divide sign "/") or the word "per" (or just p)**
(ie gallons/hour, gallons per hour or gph).

FINALLY - AMPS, WATTS, VOLTS AND TIME

There are a number of different aspects of electricity that we are concerned with:

Current Flow, Force, Power and Energy

The basic SI unit of

electrical "current flow" is the Ampere (Amp or A). It is the measure of the rate at which

current is flowing through a specific point in a circuit at a specific point in time. It is equivalent to the rate at which elementary

electric "charges" are passing the point (charges per second)

The derived unit of electric "force" is the Volt (V). It is a measure of the "potential difference" between two points in an electric circuit.

The derived unit of

electrical "power" is the Watt (W) It is a measure of the amount of energy flowing (Amps) through a point where this is a certain potential difference (Volts) across the point. So: Watts = Volts * Amps and Amps = Watts / Volts

(Example: A

solar panel generating 48 Watts at a point in time will generate a current of 4 Amps at 12 Volts or 2 Amps at 24 Volts.)

Note that Amps, Watts and Volts are all measures of different aspects of electrical force at a

single point in time.

The final aspect of electricity above is "Energy" and this is where time finally rears its

head. It is the amount of "work" that a given amount of electricity can do such as how long it can provide the required power (Watts) to a specific piece of electrical

equipment and is expressed in Watt seconds.

Rather than Watt seconds, it is more practical to talk about Watt hours (1 Watt hour is equal to 3600 Watt seconds)

Also, if we are working with a nominal 12 Volt system, it is often easier to work in Amp hours where the voltage is assumed to be a constant 12V. Since Watts = Amps x Volts, 1 Amp Hour is, in this situation, the same amount of energy as 12 Watt hours (if we are working with a 24V system, 1 Amp hour is the same as 24 Watt hours)

THE BOTTOM LINE.

If we are working in Amps rather than Watts, then:

The "capacity" of a

battery, the amount of power used or needing to be replaced is a number of Amp Hours or Ah.

It is not a number of Amps, Amps per hour or Amps/hr which as we see above are totally different things.

There is no such thing as a 100 Amp

battery. (Some

batteries are rated in CCA or Cold Cranking Amps - but that is not an indication of its capacity, it is an indication of the maximum

**rate** at which energy can safely be drawn from it )

There is no reasonable practical meaning of Amps/hr or Amps per hour (which is actually a measure of how

**fas**t a current is

**increasing or decreasing**.)

If we all use the correct units and abbreviations, we can avoid confusion and misunderstanding in our forum discussions. If we use them incorrectly, there is endless room for misunderstanding (and massive thread drift

)

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