Efficiency is normally calculated as a percentage - something 90% efficient is considered good at its job. Devices that transfer only 5% of the energy they use into something useful are inefficient (very wasteful). We can calculate efficiency if we know the total energy used, and how much is transferred into useful forms. Most cars are inefficient - petrol engines convert about 30% of the energy they release from fuel into kinetic energy! Electric cars are far better as electric motors run more smoothly and quietly, wasting much less energy.
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