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Electrical units/characteristics - What is V, A, W, Ah, Wh?
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Electrical units / Electrical characteristics of a battery
- What is V (Volt)?
- What is A/I (Ampere/Current)?
- What is W/P (Watt/Power)?
- What is Ah (Amperehour)?
- What is Wh (Watthour)?
- What is capacity and energy?
- What is their relation to each other?

These are some of the basic units in physics when it comes to electricity:

W: is the common symbol for the unit Watt. Watt is the unit of the Wattage. The word wattage isn't commonly used, it is usually called Power. In math calculations, it is referred to as "P"
V: is the common symbol for the unit volt. Volt is the unit of the voltage. Usual voltages for DC systems are 12V, 24V and 48V. The voltage dictates which devices can be used in any given system.
A: is the common symbol for the unit ampere, often shortened to "amp" in the english speaking community. Ampere is the unit of the amperage. The word amperage isn't commonly used, it is usually called current. In math calculations, it is referred to as "I".

These three units are related as power is voltage times current, W = V*A, more commonly described as P = U*I. P, U and I being common symbols for power, voltage and current (as opposed to W, V and A being the common symbols for their units, this isn't the same).
Also, these three physical quantites are instantaneous. That means they have no timely relation on their own. When applied over time and when this time is measured a relation of these units to time is created.
The voltage is an important property of a battery, so is the maximum amount of current it can deliver.

Wh: is the common symbol for the unit Watt-hour. A Watt-hour is the combination of power in watts used over time in hours or any other unit of time. However, hours are commonly used; there is also Watt-Second, Watt-Minute, etc. Watt-hour is the unit of energy consumed.
Ah: is the common symbol for the unit ampere-hour. An ampere-hour is the combination of current in amperes used over time in hours or any other unit of time. However, hours are commonly used; there is also Amp-Second, Amp-Minute, etc. Ampere-hour is the unit of capacity, or potential energy stored.

Energy is commonly expressed in watt-hours and used to measure the usage of power, often in kilowatt-hours (kWh). 1 kWh = 1000 Wh. It is also used to describe how much energy a battery can store. 
Capacity is commonly expressed in amp-hours and used to describe how much storage is available. This figure can be used to compare batteries of the same voltage.

The addition of a time factor doesn't effect the relation of these units. Just like power = voltage * current, energy (power over time) = voltage * capacity (current over time). Wh = V*Ah.

The unit scale is like most metric units, for example with the "m" prefix for milli (a thousandth). A milliwatt (mW) is therefore a thousandth of a watt, 1mW = .001W (1000mW = 1W). It works the same way with the other units. mA / milliAmp, mV millVolt

It also works the other way, the prefix k for kilo (thousand) is especially common for watt-hours. 1000Wh = 1kWh. KV kilo-Volt, KA kilo-Amp

Please note that the unit symbols are case sensitive and so are the prefixes as well. A milliamp is 1mA, not 1ma; neither 1MA nor 1Ma. And a milliamp-hour is 1mAh, there is no other valid spelling. The lower case "m" is especially important as the upper case "M" is mega. Between "m" (a thousandth, 10^-3) and M (million, 10^6) are several orders of magnitude.
So 1mA is not equal to 1MA. Whereas, if you touched these currents, the former you will most likely not feel, the latter will surely kill you.

Example of prefixes using watt as an example: 
1000mW = 1W
1000W = 1kW

Below m there is µ for micro (also just a plain "u" is used as well), above k there is M for mega, but these and all consecutive prefixes are usually out of scope for what we are doing here.





Quote:DISCLAIMER: I am by no means an expert on any one or all of these fields/questions/topics. The results of this FAQ is a collaboration of multiple different members to come up with a common list of questions that would be asked and we have tried to answer. I was the member who was chosen to post the FAQ. If you have question that goes beyond the FAQ, please post your questions in the relevant section pertaining to your inquiry. Thank you and have a nice day
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