MadSci Network: Engineering
Query:

Re: What does it mean when an electrical appliance is marked '12 Volts,240 Watt

Date: Sun Nov 29 13:02:08 1998
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Engineering
ID: 911051000.Eg
Message:

Hi Leon.

You pose a very good question about basic electricity. It is interesting to note that even though electricity has become central to our everyday lives, even critical to suvival in many cases, most do not understand what it really is and how it is measured.

Electricity is a form of energy, meaning it can be used to perform useful work. Electricity is one of the most practical forms of energy because it can be easily transmitted with little loss and converted to other forms of energy like heat, light, and mechanical energy. A typical power plant will convert chemical energy from coal and oxygen into heat which is converted to mechanical energy, which is finally converted to electrical energy and sent to your house.

Now to your question. Energy is measured in Joules (metric or MKS units). The rate of energy production or consumption is called power and is measured in Watts. Power is also equal to voltage (in Volts) times current (in Amperes). When an appliance is marked 240 Watts at 12 volts, the appliance will require, on average, 240 Watts to operate. This means that if 240 Watts from a 12 volt power source is available, the appliance will consume 240 Watts. The average current draw will be 240/12 or 20 Amps. If the power source cannot deliver 20 Amps, the power source may be damaged or it may just operate at reduced voltage. The voltage of the power source does influence how much power the appliance may consume. In most cases, a reduction of voltage of the power source voltage will result in a reduction of power draw by the appliance. In some cases, e.g. computer switching power supplies, the power draw will remain the same, or increase slightly. In this case, the current draw will increase to compensate for the loss of voltage.

One way to picture what is going on is to make an analogy to a car. Automobile engines are similar to electric power sources, but produce mechanical energy in the form of a spinning shaft. If we want to make a car that can climb a hill at a certain speed, we need an engine capable of delivering a specific amount of power. For example, a 1000 kilogram car climbing 30 degree hill at 90 km/hr will require about 14.4 kilowatts or 19.3 horsepower (HP). If the engine can deliver 19.3 horsepower at 90 km/hr, there's no problem. If the engine cannot deliver that much power at that speed, you won't get up the hill at 90 km/hr. Your engine may be capable of delivering over 100 HP, but the amount of power that is acutally used depends on how fast you want to climb a hill.

I hope this answers your question. If not, drop me a line at madhu@madhu.com


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