When Do We Say Something is Energy Efficient?

Something is energy efficient if it uses energy efficiently. It can be used to refer to energy conversion efficiency which refers to the proportion between the amount of input of energy used in the system compared to the useful output. Useful output can mean either heat, mechanical power, or electrical power. Energy efficiency was brought to the attention of the average person on the street mainly because of the 1973 oil crisis. Amory Lovins, a physicist, popularized the concept of boosting energy efficiency rather than promoting heightened energy production at around the same time.
Lovins' idea has been expanded to apply to whole economies which are attempting to grow yet are hesitant to spur energy production as well. One US state which has managed to integrate energy efficiency policies into the daily activities of the common man, industries and even the government is the state of California. In the over 30 years since the mid-1970s when California started adhering to strict energy usage policies and programs, the state has been able to keep a flat rate of energy consumption while the rest of the country has seen their energy consumption double over that same time period. California also prioritizes energy efficiency above all, with use of renewable electricity supplies as its secondary priority.
Despite the energy savings that can be achieved when systems become energy efficient, there are still enterprises that are reluctant to set energy efficiency measures in place. It is believed that as much as 75% in savings on the current electrical consumption in the US alone can be reaped if efficient systems and measures were to be set in place and religiously observed.
Another side effect of using energy efficient systems is the corresponding decrease in real emissions without jacking up costs in the end, as stated by the Vienna Climate Change Talks 2007 Report.
We can categorize energy conversion efficiency into the subdivisions of electrical efficiency, mechanical efficiency, and thermal or fuel efficiency. The first sub-category of electrical efficiency can be computed by measuring how much useful power output is created when electrical power is used to power the system. Mechanical efficiency, on the other hand, is defined as the use of potential mechanical energy (such as water flowing from a river into a dam) to create mechanical energy. The last category refers to the use of fuel to produce heat.
In modern society, energy efficiency is used to refer to the volume of power supply (such as electricity) used by a certain system (let's say your radio) to carry out its function (which is to produce music over a certain period of time.) An energy efficient radio would be one that can produce good audio while using as little electricity from your electrical outlet as possible. A more thorough computation of the energy efficiency of your radio would mean factoring in running expenses and its expected lifespan as well.
When you, the average consumer, takes steps to choose items that are more energy efficient over items which are less energy efficient (such as switching to more fuel-efficient cars and selling your gas-guzzler vehicle to the scrap yard), that is called energy conservation. Energy conservation can be said to be successfully applied if you are able to reduce your usage of energy yet are still able to achieve the same level of functionality. In simple terms, if you use a more fuel-efficient car and can still get to work, but are able to use less gas in the process, then you can say that your attempt at energy conservation has been successful.