If you have a device that has an energy efficiency of 0.7 and you put in 150 units of energy how many units of useful energy will you get out of it?

To calculate the useful energy output, multiply the input energy by the energy efficiency. In this case, with an energy efficiency of 0.7 and an input of 150 units, the useful energy output would be 0.7 × 150 = 105 units. Therefore, you would get 105 units of useful energy from the device.