In this video, we focus on the many ways that energy is measured. While there are different measures of energy, they're all measuring the quantity of energy just as meters and yards both measure length. Formally, energy is defined as a quantity, the amount of work to be done. We'll explain more about that later. The Joule is the SI unit of energy. A joule is a very small amount of energy. One joule is equivalent to the energy required to heat one gram of dry cool air by one degree Celsius. The energy required to lift a small apple, one meter against the Earth's gravity. The energy as heat that a quiet person produces every 100th of a second, or 100th of the energy a person can get by drinking a single five millimeter diameter droplet of beer. Imagine that. There are many metrics used for measuring energy, all of which can be converted from one to another. The standard SI unit of energy is the Joule, which we've just seen, often expressed as megajoules, which is a million joules or exajoules, which is 10^18th joules. That's a lot of joules. When measuring electrical energy, the standard measure is the Watt-hour often expressed as 1000 Watt-hours or Kilowatt-hours. We'll have much more to say about Watt-hours in upcoming lectures. The heating and air conditioning industry measures heat energy using British thermal units. This is often expressed in Therms, which is 100,000 BTUs. The petroleum industry uses barrels of oil equivalent energy or tons of oil equivalent. The coal industry uses tons of coal equivalent energy and the natural gas industry uses energy in cubic meters of natural gas at standard pressure and temperature. The food industry uses the calorie or precisely the kilocalorie or Calorie with a capital C to measure the energy content of food. We see this measure on food packaging all the time. The most commonly used measure of electrical energy is the Watt-hour. One watt-hour is equal to 3,600 joules. One kilowatt hour is equal to 1000 watt-hours, a megawatt hour is equal to 1000 kilowatt hours, and a gigawatt hour is equal to 1,000 megawatt hours. Is examples of the scale of electrical energy. 0.00028 Watt-hours, that's one joule is the energy required to lift a small apple one meter into the air, 100 Watt-hours is the energy expended by arresting human for one hour, 150 watt-hours or 225 Calories is the energy in a typical candy bar, 100 kilowatt hours is the energy in a typical battery for an electric vehicle, 4,000 kilowatt hours is the annual household electricity consumption in Scotland, 10,000 kilowatt hours is the annual household electricity consumption in the United States. Then finally, 131 petawatt hours. That's 131 times 10^15 is annual human energy use currently. The terms power and energy are often used interchangeably, but they have very different meanings. Power is the rate that energy is transformed or converted per unit of time. The watt is the SI unit of power, defined as the conversion of one joule of energy per second. That's a very small rate of energy conversion. Comparing energy and power, energy is a quantity, power is the rate at which electrical energy is produced or consumed. Here we see a picture of a coffee cup and a jug. The coffee cup contains little water or little energy, but it's pouring it out at a rapid rate. If this were energy, you'd be at a lot of power. On the right is a jug that contains a lot of water or if in the case of energy, a lot of energy, but it's pouring it out at a slow rate, so it exhibits lower power. Another example, our marathon runners, they have to conserve their energy for a long period of time. They're not pouring it out or using it very quickly. In contrast, there are sprinters who need to use all the energy they can in a very short period of time so they need a lot of power to win the race. When measuring electricity, Watt-hours are a quantity. Watts are a rate, and it's easy to get the two confused. Another important fact to know is that electrical equipment is usually sized according to its maximum power generation or consumption. For example, wind turbines are measured by the maximum rate that the wind energy is converted to electrical energy. For example, 1.5 megawatt wind turbine will produce 1.5 megawatt hours of energy each hour under ideal conditions. Solar PV systems are measured by the maximum rate at which sunlight is converted into electrical energy under ideal conditions. For example, 25, 125 Watts solar panels on a residential roof can produce 3,000 Watt-hours or a three kilowatt hours of electrical energy per hour on bright sunny days. Hydroelectric dams are measured by the maximum rate that the kinetic energy of flowing water through the dam is converted to electrical energy. For example, a small 10 megawatt hydroelectric dam can produce 10 megawatt hours of electrical energy per hour under ideal conditions, equivalent to the energy use of five or 10 households in a year. That's is lot of energy. Summarizing, energy is a quantity. The Joule is the metric unit of measure for energy. The Watt-hour is the unit of measure for electrical energy. Power is the rate of conversion from one form of energy to another. The Watt is the unit of measure for electrical power. Different units for measuring energy and power are easily converted from one to another. They all measure the same two things, energy or power. In the next video, we conclude with an overview of the four laws of thermodynamics. As a teaser, the second law of thermodynamics is particularly important for renewable energy. Will see you there.