Power and energy basics
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Contents |
[edit] Energy = Power x Time
Energy equals power times time, i.e. E=P*t. Where:
- Energy is the ability to do work.
- Power is the rate at which work is performed.
| Symbol | Description | In Water | In Electricity | Base Units | |
|---|---|---|---|---|---|
| Power | P | Rate at which work is performed | Power=Current*Pressure (P=Q*H) | Power=Current*Voltage (P=I*V) | kg•m²/s³ |
| Energy | E | The ability to do work | Energy=Power*Time (E=P*t) | Energy=Power*Time (E=P*t) | kg•m²/s² |
[edit] Common units of Energy and Power
| Common Units of Energy and Power | |
|---|---|
| Energy | Power |
| joule | joule/sec |
| calorie | calorie/min |
| Btu | Btu/hour |
| orange | orange/day |
| watt-hour | watt |
[edit] Oranges
Okay so oranges are a made up unit of energy. But it could be a unit of energy. According to the USDA, one medium sized, 2-5/8" diameter orange, weighing 121g, has 59 Calories. And, because people in the USA do not want to think they are eating thousands of calories per serving, a Calorie is actually a kilocalorie, so one medium sized orange has 59,000 calories. So 1 orange = 59,000 calories and 1 orange/day = 40.97222 calories/minute (take a moment to calculate this).
[edit] So why do people have a problem with watts
Watts are a rate. In fact, one watt equals one joule per second, i.e. 1 W = 1 J/s, but there is no unit of time in the name. Likewise one kilowatt-hour is a quanity with no explicit time. In fact one kilowatt-hour equals 3600000 joules (take a moment to calculate this). Horsepower has this same issue. One kWh could be an appliance drawing:
- 1 kW for 1 hour
- 2 kW for .5 hours
- 1 W for 1000 hours (remember that the little k signifies kilo and means 1000x)
So, if you turn on 4 light bulbs, each rated at 25 W, how long can they be on before you reach 1 kWh?
4 bulbs x 25W/bulb = 100 W E=P*t => t=E/P => 1 kWh/100W = 1kWh/.1W = 10 hours
[edit] Kilowatt hour aka kWh
A kilowatt is just 1000 watts (kilo means 1000x). A kWh is 1000 watt hours. So for one 60W incandescent bulb burning for 100 hours:
60 W * 100 hours = 6,000 watt hours and 6,000 watt hours * 1/1000 kilowatt/watt = 6 kilowatt hours aka 6 kWh
Electric energy consumption is usually metered and billed in kWh.
[edit] Analogies
Energy is a measurable quantity like distance. Power is a rate like speed.
| Units | Analog | Units | |
|---|---|---|---|
| Energy | kWh | Distance | miles |
| Power | W | Speed | MPH |
[edit] Conversions
[edit] There is no such thing as kilowatts per hour
[edit] Hall of Shame
Please see the Hall of Shame for a short list of violators.
