{"id":2242,"key":"Power_and_energy_basics","title":"Power and energy basics","latest":{"id":1224115,"timestamp":"2026-01-09T00:07:12Z"},"content_model":"wikitext","license":{"url":"https://www.appropedia.org/Appropedia:Copyrights","title":"CC-BY-SA-4.0"},"source":"Energy is a property of physics that can be a little hard to describe completely. The most common definition is one that will work well for us. Energy is the ability to do work.\n\nPower is just a rate of that work being done. Specifically, power is rate of energy per time (e.g. power generation, power consumption, etc.).\n\n== Energy = Power x Time ==\n\nEnergy equals power times time, i.e.\n\n E = P * t\n\nwhere:\n\n* Energy is the ability to do work.\n* Power is the rate at which work is performed.\n\n{| class=\"wikitable\"\n!\n! Symbol\n! Description\n! In Water\n! In Electricity\n! Base Units\n|-\n! Power\n| P\n| Rate at which work is performed\n| [[:Category:Microhydro|Power=Current*Pressure]] (P=Q*H)\n| [[Electricity basics|Power=Current*Voltage]] (P=I*V)\n| kg•m²/s³\n|-\n! Energy\n| E\n| The ability to do work\n| Energy=Power*Time (E=P*t)\n| Energy=Power*Time (E=P*t)\n| kg•m²/s²\n|}\n\n== Common units of Energy and Power ==\n\n{| class=\"wikitable\"\n|+ Common Units of Energy and Power\n! Energy\n! Power\n|-\n| joule\n| joule/sec\n|-\n| calorie\n| calorie/min\n|-\n| Btu\n| Btu/hour\n|-\n| [[#*Oranges|orange]]*\n| orange/day\n|-\n| watt-hour\n| watt\n|-\n| kilowatt-hour\n| kilowatt\n|}\n\n=== *Oranges ===\n\nOkay so oranges are a made up unit of energy. But it could be a unit of energy. According to the [http://web.archive.org/web/20120729071321/http://www.nal.usda.gov:80/fnic/foodcomp/cgi-bin/list_nut_edit.pl 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 <tt>'''1 orange = 59,000 calories'''</tt> and <tt>'''1 orange/day = 40.97 calories/minute'''</tt> (take a moment to calculate this).\n\n=== So why do people have a problem with watts ===\n\nWatts are a rate. One watt is equal to one joule per second, i.e. <tt>'''1 W = 1 J/s'''</tt>, but there is no unit of time in the name. Likewise one kilowatt-hour is a quantity with no explicit time. In fact, one kilowatt-hour equals 3,600,000 joules ''(take a moment to calculate this)''. One kWh could be an appliance drawing:\n\n* 1 kW for 1 hour\n* 2 kW for .5 hours\n* 1 W for 1000 hours (remember that the little k signifies kilo and means 1000x)\n\nExample:\n\nIf you turn on 4 light bulbs, each rated at 25 W, how long can they be on before you reach 1 kWh?\n\n<math>4 bulbs \\times \\frac{25W}{bulb} = 100 W </math>\n\n<math>E = P\\times t \\Rightarrow t = \\frac{E}{P} \\Rightarrow \\frac{1 kWh}{100W} = \\frac{1kWh}{.1kW} = 10 hours </math>\n\n=== Kilowatt hour aka kWh ===\n\nA kilowatt is just 1000 watts (kilo means 1000x). A kWh is 1000 watt hours.\n\nExample:\n\nHow many kilowatt hours would be needed to run one 60W incandescent bulb for 100 hours?\n\n<math> 60 W \\times 100 hours = 6,000 watt hours </math>\n\nand\n\n<math> 6,000 watthours \\times \\frac{1 kilowatt}{1000 watts} = 6 kilowatthours = 6 kWh </math>\n\nElectric energy consumption is usually metered and billed in kWh.\n\n=== Analogies ===\n\nEnergy is a measurable quantity like distance. Power is a rate like speed.\n\n{| class=\"wikitable\"\n!\n! Units\n! Analog\n! Units\n|-\n! Energy\n| kWh\n| Distance\n| miles\n|-\n! Power\n| W\n| Speed\n| MPH\n|}\n\n=== Conversions ===\n\n{| class=\"wikitable\"\n!\n! Joule\n! Calorie\n! Btu\n! Watt-hour\n|-\n! 1 joule\n| 1\n| ~0.239\n| ~9.478 x 10^-4\n| ~2.778 x 10^-4\n|-\n! 1 calorie\n| ~4.184\n| 1\n| ~3.966 x 10^-3\n| ~1.163 x 10^-3\n|-\n! 1 Btu\n| ~1,055.056\n| ~252.164\n| 1\n| ~0.293\n|-\n! 1 watt-hour\n| 3,600\n| ~860.42\n| ~3.412\n| 1\n|}\n\n'''There is no such thing as kilowatts per hour'''\n\n== Hall of Shame ==\n\nPlease see the [[Power and energy hall of shame|Hall of Shame]] for a short list of violators.\n\n== See also ==\n\n* [[Low voltage connection basics]]\n* [[Electrical wiring and switching]]\n\n{{Page data\n| license = CC-BY-SA-3.0\n| description = Power and energy concepts are explained clearly on Appropedia, with units, examples, and links to real-world devices and systems.\n}}\n\n[[Category:Electricity]]\n[[Category:Energy]]"}