Watt in the World Are You Talking About?
As cars become less mechanical machines and more computer-controlled electronic devices, car owners will want to get familiar with a few electronic terms. It’s easy to be confused or overwhelmed by the enormous amount of technical jargon and “geek speak” that all too often is passed off as good communication. “Twenty kilowatt electric motor, huh? Is that good?” This article is meant to serve as a very basic introduction to the electric-related terms you may encounter as you begin to delve into the world of hybrid gas-electric vehicles.
Understanding the meaning of energy, power, volt, current, resistance, and watt will help you talk the talk when shopping for a hybrid, or extolling its virtues to family and friends.
Power to the People
The first important issue to address is the fundamental difference between energy and power. If you can’t readily tell the difference between the two, don’t feel bad. Experts, officials and the media frequently confuse these terms. The difference between energy and power is actually quite simple:
- Energy is the ability to do work
- Power is the rate at which work is done
Understanding the difference is very important to the overall understanding of electricity and electrical systems, including hybrid vehicles. The amount of energy in your batteries (or in your gas tank for that matter) is indicative of the distance you can travel before having to refuel. The power rating of your electric motor (or gas engine) tells you how quickly you can turn that energy into useful work, such as vehicle acceleration.
The batteries in a hybrid vehicle are rated for a specific voltage and maximum current. For example, the battery output on a Toyota Prius is 275 volts. But what the heck is a volt?
- Voltage is the force that “pushes” and excites the electrons in a conductor and makes them flow (as current). Voltage is measured in volts and can be thought of as the potential for charge to do work.
An analogy that some people use to describe voltage is water pressure, where a large water pressure built up behind a dam is like a large voltage stored in a battery. In the case of the dam, if the water is released it will flow at some rate and can be made to do work (for example, by turning a turbine and generating electricity). Friction in the pipe can slow down the flow of water out of the dam.
For a battery, when a load is applied (such as a light bulb or electric motor), current will flow out of the battery and that current can be used to do work, like lighting up the light bulb or spinning the shaft on the electric motor. All real systems have some amount of resistance in them, and that resistance will slow down the flow of electrons and cause the electricity to turn into heat. This is why all real electrical systems get hot, and also why it is important to not pull more current through a device than whatever it is rated for, unless you really enjoy putting out electrical fires.
More About Current and Resistance
A smart physicist named Georg Ohm noticed that there is a very simple relationship between voltage, current and resistance. They named a law after him, and it says that the voltage across any load can be determined by multiplying the resistance of that load by the current flowing across it. So, how is that useful to you, the investor in a new hybrid vehicle?
If you multiply the current by the voltage, you can determine the maximum power produced by your batteries. Because of the internal resistance of your batteries, motor and other components, you won’t ever be able to produce the maximum power that your batteries are capable of, but you can at least get an idea of your “upper limit” battery performance. The more energy your batteries can store, the longer you can drive on electrical energy (instead of gasoline combustion energy). The more power your batteries can produce, the less gasoline you will have to use to meet your driving demands. That is, when you put your pedal to the floor, a high-power battery can meet most of that load and simply use the gas engine to make up a small difference in the power requested.
The most common unit of electrical power is the watt. It is a rate of energy used in a given amount of time.
A watt is a pretty small unit of measure, especially when you’re talking about the power used by a personal vehicle, so you’ll often see kilowatts referred to rather than watts (1,000 watts, abbreviated as kW). The electric motor used in General Motor’s hybrid pick-up trucks are able to generate up to 14,000 watts or 14 kW of continuous electric power.
If you turn on an appliance that requires one kilowatt of power to operate, such as a common microwave, and leave it on for one hour, the energy used would be equal to one-kilowatt hour (kW-hr). Although prices fluctuate throughout the day and are different across the U.S., it would cost you somewhere in the neighborhood of $0.08 to $0.25 for that one kW-hr of electricity used.
The Civic Hybrid’s hybrid battery is rated at 1 kwh. This means that it could generate enough power to run your microwave for one hour of non-stop use. But don’t think about running it even for three minutes to cook some popcorn as you drive down the highway. You’ll want to use every kilowatt of power to reduce the energy suppied by the gasoline engine. You understood this, right? More power to you.