There is a technique for converting a value of one physical quantity (an input physical quantity) into a value of another physical quantity (output physical quantity).  For example, a technique has been proposed in which a value of an input physical quantity such as electric power, energy, or the like into an output physical quantity, for example, a physical quantity desired by a user, based on a pre-defined rule for conversion between physical quantities. 
This allows a user to intuitively determine a value of a physical quantity which is difficult for the user to intuitively determine. 
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication No.  2012-191825 and Japanese Laid-open Patent Publication No.  2005-25654. 
When an input physical quantity is converted into an output physical quantity on the basis of a pre-defined conversion rule, as in the related art noted above, there is a problem in that the physical quantity is not converted when a corresponding conversion rule does not exist. 
In particular, physical quantities into which users desire to convert are diverse, and thus when an output physical quantity is a physical quantity desired by a user, it is highly likely that there is no conversion rule for directly converting an input physical quantity into the output physical quantity.  Even when available conversion rules are simply combined together, it may be difficult to convert the input physical quantity into the output physical quantity.