When it comes to trying to define "to be energy efficient" or "energy efficiency", there does not seem to be a single commonly-accepted definition of energy efficiency. it is generally thought that an increase in energy efficiency is when either energy inputs are reduced for a given level of service, or there are increased or enhanced services for a given amount of energy inputs.
There are many ways of saving energy.
I can turn down the thermostat and put on a sweater. or use a programmable and ignore the setting by wearing more clothes. My bill drops a number of BTUs.
I can take a military shower. Water on, wet down, Water off. Soap up, scrub. Water on Rinse off, water off. In 90 degree F water of course. My bill drops a number of BTUs. and my water bill drops.
I can add some insulation to the house. Leave the thermostat setting the same. My bill drops a number of BTUs.
I can install a tankless hot water heater. The bill drops a few BTUs and my water bill stays the same.
You can change the way people work.
You can change the way things work.
I believe 'Conservation' is the way people work.
I believe 'Efficiency' is the way things work.
As an Energy Auditor, I can explain to people how the conserve. They have to make the choice. They have to make it a habit. I'm not going to follow them around and watch to see they are doing it.
Very much like my physician telling me to eat less, get extra exercise and not to use as much salt. All on me.
As an Energy Auditor, I can recommend how people can improve their homes to increase efficiency. They have to buy the stuff (and pay for or provide the labor) to improve their home.
The Dr. wants me to change my habits, and energy conservation is changing my habits.
Efficiency is something that changes energy consumption without requiring more then a one time purchase or install to change how things work.
Both are important. One is less expensive in $$. One is less stressful to remember.
Design wise I would say that energy efficiency is defined by the Btu / square foot. This is usually expressed in kilo Btu / square foot annually as well as summed kilo Btu annually for the entire structure.
In design terms, you might want to consider how that a home for instance, could be powered by renewables, where the energy efficiency can make it possible to power a home completely on renewables. Of course there are days when we do not have enough wind and solar and so we have to use grid power. The thing then would be that summed together over a year, a home could produce more energy than it uses once renewables are added. So we have to consider then something like the Btu / square foot and we can compute a design using the old school method of construction using R 11 in the exterior walls and R 19 in the attic as a standard and then do the model with lets say R 19 in the exterior walls and something over R 30 in the attic. Attic wise I would say R 60 is a good place to start with a design. Anyways we compare the two insulation values for the same model. We can then express this comparison in percentage terms.
If we do not increase the energy efficiency of our future designs, then we make it hard for the building owner to invest in renewable energy since they will have to upgrade their buildings energy efficiency to reduce losses. If the building is already designed to be efficient then a more reasonable investment in renewables is possible.
In software we use "heat degree days" for our area to determine the annual energy cost. For design purposes we use the lowest average temperature of the winter, and the lowest temperature on record to design a building that we can heat with solar heating or solar electrical power. Where I live the lowest temperature we reach in the winter is 9 degrees F.
If we are not using renewables and use grid power as well as natural gas, our bills are going to be lower which is something that interest people in itself without them having any interest in renewables.