The "grade" you refer to is simply temperature. To efficiently extract energy from a heat flow you want a really hot hot side and a really cold cold side. On earth we can't get a cold side much colder than outside air temperature (not without wasting more energy than you gain) but luckily that is cold enough when the hot side is thousands of degrees. But you can't get much energy from roomtemp and roomtemp+10. A warm cup of coffee will run a Stirling engine for a party trick, but it's not going to charge your phone, much less your car.