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Electrical Units of Measure
What are amps, watts, volts and ohms?
­The­ three most basic units in electricity are voltage (V), current (I, uppercase "i") and resistance (r). Voltage is measured in volts, current is measured in amps and resistance is measured in ohms.

A neat analogy to help understand these terms is a system of plumbing pipes. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.

There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal to the voltage divided by the resistance.

Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden.

What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.

Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.

Electrical power is measured in watts. In an electrical system power (P) is equal to the voltage multiplied by the current.

The water analogy still applies. Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. You can increase the power generated by the waterwheel in two ways. If you increase the pressure of the water coming out of the hose, it hits the waterwheel with a lot more force and the wheel turns faster, generating more power. If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it.

Electrical Efficiency

In an electrical system, increasing either the current or the voltage will result in higher power. Let's say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation above, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.

You know that P = 100 W, and V = 6 V. So you can rearrange the equation to solve for I and substitute in the numbers.

What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?

So this system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first equation:

Now you can substitute the equation for V into the other equation:

What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages.

This improvement in efficiency is what is driving the automobile industry to adopt a higher voltage standard. Carmakers are moving toward a 42-volt electrical system from the current 12-volt electrical systems. The electrical demand on cars has been steadily increasing since the first cars were made. The first cars didn't even have electrical headlights; they used oil lanterns. Today cars have thousands of electrical circuits, and future cars will demand even more power. The change to 42 volts will help cars meet the greater electrical demand placed on them without having to increase the size of wires and generators to handle the greater current.

Electrical Units of Measure

The standard SI units used for the measurement of voltage, current and resistance are the Volt [ V ], Ampere [ A ] and Ohms [ Ω ] respectively. Sometimes in electrical or electronic circuits and systems it is necessary to use multiples or sub-multiples (fractions) of these standard units when the quantities being measured are very large or very small. The following table gives a list of some of the standard units used in electrical formulas and component values.

Standard Electrical Units

ParameterSymbolMeasuring
Unit		Description
VoltageVoltV or EUnit of Electrical Potential
V = I × R
CurrentAmpereI or iUnit of Electrical Current
I = V ÷ R
ResistanceOhmR or ΩUnit of DC Resistance
R = V ÷ I
ConductanceSiemenG or ℧Reciprocal of Resistance
G = 1 ÷ R
CapacitanceFaradCUnit of Capacitance
C = Q ÷ V
ChargeCoulombQUnit of Electrical Charge
Q = C × V
InductanceHenryL or HUnit of Inductance
VL = -L(di/dt)
PowerWattsWUnit of Power
P = V × I
ImpedanceOhmZUnit of AC Resistance
Z2 = R2 + X2
FrequencyHertzHzUnit of Frequency
ƒ = 1 ÷ T

Multiples and Sub-multiples

There is a huge range of values encountered in electrical and electronic engineering between a maximum value and a minimum value of a standard electrical unit. For example, resistance can be lower than 0.01Ω's or higher than 1,000,000Ω's. By using multiples and submultiple's of the standard unit we can avoid having to write too many zero's to define the position of the decimal point. The table below gives their names and abbreviations.

PrefixSymbolMultiplierPower of Ten
TerraT1,000,000,000,0001012
GigaG1,000,000,000109
MegaM1,000,000106
kilok1,000103
nonenone1100
centic1/10010-2
millim1/1,00010-3
microµ1/1,000,00010-6
nanon1/1,000,000,00010-9
picop1/1,000,000,000,00010-12

So to display the units or multiples of units for either Resistance, Current or Voltage we would use as an example:

  • 1kV = 1 kilo-volt  -  which is equal to 1,000 Volts.
  •  
  • 1mA = 1 milli-amp  -  which is equal to one thousandths (1/1000) of an Ampere.
  •  
  • 47kΩ = 47 kilo-ohms  -  which is equal to 47 thousand Ohms.
  •  
  • 100uF = 100 micro-farads  -  which is equal to 100 millionths (1/1,000,000) of a Farad.
  •  
  • 1kW = 1 kilo-watt  -  which is equal to 1,000 Watts.
  •  
  • 1MHz = 1 mega-hertz  -  which is equal to one million Hertz.

To convert from one prefix to another it is necessary to either multiply or divide by the difference between the two values. For example, convert 1MHz into kHz.

Well we know from above that 1MHz is equal to one million (1,000,000) hertz and that 1kHz is equal to one thousand (1,000) hertz, so one 1MHz is one thousand times bigger than 1kHz. Then to convert Mega-hertz into Kilo-hertz we need to multiply mega-hertz by one thousand, as 1MHz is equal to 1000 kHz. Likewise, if we needed to convert kilo-hertz into mega-hertz we would need to divide by one thousand. A much simpler and quicker method would be to move the decimal point either left or right depending upon whether you need to multiply or divide.

As well as the "Standard" electrical units of measure shown above, other units are also used in electrical engineering to denote other values and quantities such as:

•  Wh − The Watt-Hour, The amount of electrical energy consumed in the circuit by a load of one watt drawing power for one hour, eg a Light Bulb. It is commonly used in the form of kWh (Kilowatt-hour) which is 1,000 watt-hours or MWh (Megawatt-hour) which is 1,000,000 watt-hours.
 
•  dB − The Decibel, The decibel is a one tenth unit of the Bel (symbol B) and is used to represent gain either in voltage, current or power. It is a logarithmic unit expressed in dB and is commonly used to represent the ratio of input to output in amplifier, audio circuits or loudspeaker systems.
For example, the dB ratio of an input voltage (Vin) to an output voltage (Vout) is expressed as 20log10 (Vout/Vin). The value in dB can be either positive (20dB) representing gain or negative (-20dB) representing loss with unity, ie input = output expressed as 0dB.
 
•  θ − Phase Angle, The Phase Angle is the difference in degrees between the voltage waveform and the current waveform having the same periodic time. It is a time difference or time shift and depending upon the circuit element can have a "leading" or "lagging" value. The phase angle of a waveform is measured in degrees or radians.
 
•  ω − Angular Frequency, Another unit which is mainly used in a.c. circuits to represent the Phasor Relationship between two or more waveforms is called Angular Frequency, symbol ω. This is a rotational unit of angular frequency 2πƒ with units in radians per second, rads/s. The complete revolution of one cycle is 360 degrees or 2π, therefore, half a revolution is given as 180 degrees or π rad.
 
•  τ − Time Constant, The Time Constant of an impedance circuit or linear first-order system is the time it takes for the output to reach 63.7% of its maximum or minimum output value when subjected to a Step Response input. It is a measure of reaction time.