Devices connected to an electrical power source, like a wall outlet or battery, consume electrical power. However, some devices might not use all electrical energy to do actual work. These devices use some of the electrical power for reactive purposes.
A good example would be an induction motor that uses a portion of the electrical power supplied to create a magnetic field. When dealing with electrical devices, you want to understand the electrical power doing the actual work and the total electrical power supplied.
We’ll explore the difference between Watts and Volt-Amperes (VA) in electrical systems.
Evolution of Electrical Measurement
Georg Ohm and James Watt are among the early scientists who laid the groundwork for modern electrical measurement. Georg Ohm made an important discovery that helps us understand electrical circuits.
He discovered that more electrical current flows through a circuit if you increase electrical potential between two points on the conductor, provided nothing resists the flow of electrons. His discovery led to the formulation of the Ohm’s Law: V = I × R
V stands for volts, while R stands for resistance. Early scientists adopted the Italian word "intensità" (intensity) to signify the intensity of current flowing through a circuit. As a result, I became a representation of the current in Ohm’s Law.
On the other hand, James Watt made steam engines convert as much energy from steam into mechanical work. His innovations resulted in Watts becoming a new unit to measure work done by a steam engine.
Today, Watts is a unit used to measure the electrical power doing actual work. Scientists came up with units that would be accepted globally to measure electrical systems.
Introduction To Watts and Volt-Amperes
James Watts made a separate condenser that condensed steam at a lower temperature than the engine cylinders, thus increasing engine power output. His steam engine innovation revealed the real power of a steam engine, leading scientists to adopt the Watt (W) as a unit measuring real power consumed by an electrical device connected to a power source.
However, an electrical device might consume more electrical power than it uses to do actual work. Scientists needed to measure the total electrical energy consumption regardless of the energy used to do actual work. Enter Volt-Amperes (VA), the unit measuring the total electrical power supplied to a device connected to a power source.
Watt (W) is a unit of electrical power measuring actual work done, while Volt-Amperes (VA) is apparent power - total electrical power supplied to a device by a power source.
Watts (W) in Detail
This unit of electrical measurement shows how much of the electrical power supplied does actual work. If you connect an electric motor to a power source, it will use a portion of the power supplied to do mechanical work (spin fast) that drives a load.
A portion of electrical power supplied establishes and sustains a magnetic field that creates the rotational motion. Aside from electric motors, induction heaters and welding machines use a portion of the total electric power supplied to do actual work. True power measures what actually does the work.
Characteristics of Watts
When you assess electrical systems, Watt is a unit of electrical measurement you’ll often meet. You’ll understand that it signifies a portion of the electrical power input that does useful work like driving a load or producing light. What’s more, Watt has the following characteristics:
Measures True Power
Watt measures the real power a device connected to a power source consumes when doing useful work. In the case of an electric motor, it’s the energy consumed to rotate the motor shaft and possibly drive an electric vehicle.
Represents Consistent Energy Flow
Watts shows the rate of doing work. Since doing work requires electrical energy, we can say that Watts is the energy required to do a particular work per unit time. An electrical energy has to be consistently supplied for a device to do useful work. Therefore, Watts represent the electrical energy consistently flowing from a power source to a device.
Shows Efficiency
A system that uses most of the supplied energy to do work is more efficient. Since Watt is a unit showing electrical power doing useful work, it shows the efficiency of a device connected to a power source.
Relationship to Work Done
If you connect a device to a power source, the Watt rating shows you the amount of electrical power it consumes to do useful work. But power refers to the amount of work done per unit time. Power could mean work done per minute or hour. It could also mean work done per second.
Work is only done if a force moves an object over some distance. And power means the distance an object with a force acting upon it has covered per unit of time. In the context of devices connected to a power source, work done is the useful task the device performs.
It could be a light bulb producing light or an electric motor spinning fast to move a load connected to its shaft. Power is the rate at which the device performs useful tasks. Think of Watt as a unit showing you the rate at which a device connected to a power source does useful work.
Apparent Power (Volt-Amperes (VA))
Apparent power is the total electrical power supplied to a circuit or a device connected to a wall outlet or power source. This electrical power is measured in Volt-Amperes (VA). As you’ve already learnt, some devices don’t use the entire electrical power supplied to do useful work.
An induction motor is a prime example, consuming only 78% of the apparent power to do useful work. It’s crucial for this device to use reactive power to establish and maintain a magnetic field that aids in spinning its shaft. Also, reactive power helps with voltage regulation, among other useful things.
A transformer is another electrical device that uses 85% of the apparent power to do useful work. It’s important to remember that apparent power is different from true power (or real power).
Apparent power includes reactive power, which is the electrical power used to sustain some components in a device. A device with an electric motor, transformer, and capacitor needs a portion of the total power supplied to it to establish and maintain a magnetic or electric field that keeps some components working.
That means many devices use a percentage of the apparent power to do useful work. The higher the percentage, the higher the efficiency. In other words, devices have what we call a power factor. It’s the ratio of real power to apparent power.
Characteristics of Apparent Power
Includes Real and Reactive Power
Apparent power is the sum of real power (Watt) and reactive power measured in Volt-Ampere Reactive (VAR).
Accounts for Resistance
Additionally, apparent power supplies electrical energy that counters resistance and reactance.
Power Factor Relationship
Apparent power helps us understand the efficiency of a device since we can compare the Wattage with the apparent power.
Calculations of Apparent Power and Watt
Calculating the total electrical power supplied to a circuit is straightforward:
Apparent Power = Volts × Amps
If an electrical outlet supplies 120 Volts and 15 Amps, the total electrical power supplied to devices connected to it is 1800 Volt-Amperes (or 1800 VA).
Real Power consumed by the devices is however less than what the electrical outlet provides. Real power takes into account the power factor of the connected device. Assuming the device has a power factor of 0.8, its real power (Watt) is;
Apparent Power × Power Factor
1800 VA × 0.8 = 1440 Watts (or 1440 W)
Practical Implications
Apparent power (VA) and real power (W) have practical implications on appliances and devices. Also, they help us choose the right generators and UPS systems to keep essential devices online in a power outage.
Appliances and Devices
Volt-Amperes helps us know the exact electrical power needed to power all appliances and devices in a building and the wiring that can handle the electrical load without melting.
What’s more, VA accounts for the power factor, allowing us to know the efficiency of all devices to ensure they draw enough electrical power for their operation.
Watts, on the other hand, helps us know the exact portion of the apparent power (VA) consumed by the devices to do useful work. At the same time, the Wattage rating allows us to compare the energy efficiency of appliances and devices in our homes.
Generators and UPS Systems
Generators rate electrical power output in kilovolt amperes (kVA). At the same time, UPS systems rate their electrical output in volt-amperes (VA). These ratings allow you to know the total electrical power your backup system supplies. You’ll then multiply the apparent power by appliance or device power factor to determine the Wattage consumed.
Effects of (VA) and (W) on Performance and Efficiency
While volt-amperes signify the total electrical power applied to a circuit, Watt is the actual power consumed to do real work. Wattage is the electrical power remaining after reactive power. A device that consumes fewer watts and does more work is energy-efficient.
Real-World Scenarios Where VA and Watt Play Out
Home appliances and commercial equipment have their electrical power needs rated in volt-amperes and Watts.
Home Appliances
Some home appliances have both (VA) and (W) ratings to help you understand their electrical power needs. The volt-amperes (VA) rating on a refrigerator, for example, shows how much electrical power it draws from the wall outlet.
The VA power is essential to run the compressor and maintain the selected temperature setting. On the other hand, the wattage (W) shows the actual power it uses for cooling, usually 100 - 800W. Other home appliances like microwaves and air conditioners have a VA rating for proper circuit sizing.
Commercial Applications
Industrial motors require additional power to start and run. VA rating shows their power requirement to start working. On the other hand, Watts shows the power commercial equipment consumes to do useful work.
In A Nutshell
Volt-Amperes and Watts are two units of measuring electrical power. However, they measure different aspects of electrical power. Volt-amperes measure the total electrical power supplied to a circuit, while Watts measure electrical power consumed to do useful work.
