The efficiency is always less than one because some of the energy is always useless for what you want. Most wasted energy eventually turns into heat. therefore the efficiency is like a percentage with 1 being 100% a device can't be 100% efficient as some energy is wasted.
To work out the efficiency of a device you need to do the total useful energy produced divided by the total energy that is inputted into the device
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Well, honey, the efficiency of a machine is less than 1 because it's basically a way of saying the machine is not 100% perfect at converting input energy into useful output energy. It's like trying to bake a cake but ending up with a burnt mess - you're not getting the full bang for your buck. So, in simple terms, efficiency less than 1 just means the machine ain't working at its best.
An efficiency factor of 1 (or 100% efficiency) is not possible due to losses that cannot be reduced to zero. These losses take the form of friction, heat loss etc.
The Law of Conservation of Energy states that energy can't be created or destroyed. Thus, efficiency is 1 (i.e., 100%) if there is a perfect energy conversion. Since this is usually not the case, and some energy is wasted, the efficiency of most energy conversions is less than 100% (or less than a factor of 1).
less than
1 is 8.7 less than 9.7
0.9 is less than 1