The transference of energy is an extremely important concept that affects every building on earth. The designs of all heating and cooling systems depend on calculations that determine how much energy will enter or leave the building via various methods. Energy transfer is also important for computer fans, engines, and various types of machinery. You may find it complicated to calculate exactly how much energy is transferred in a particular application, but the general process is always the same.
Create a schematic boundary around the system you are analysing. When you are analysing heat or energy transfer, it is easy to get overwhelmed with all of the factors affecting a situation. Define exactly where, in space, the boundary is located, and over what period of time energy transfer is of interest.
List all of the factors affecting energy transfer. If you were analysing a room, you would note that energy enters the room via lighting, electrical appliances and heaters, and leaves via conduction through the walls and air leakage through any unsealed crevices.
Convert all sources and drains of energy into common units. You cannot add BTUs (British Thermal Units) and kilowatt-hours any more than you can add pounds and kilograms. You need to pick one common unit and apply an appropriate conversion factor to all other units.
Convert any sources of power into quantities of energy. Power is defined as energy expended per unit of time. Therefore, if a 60-kilowatt light bulb is running within your system for one hour, you need to multiply the power rating of 60 kilowatts by the time of one hour to get the quantity of energy, 60 kilowatt-hours.
Set up a net energy-balance equation. An energy-balance equation simply adds and subtracts all energy that enters or leaves a system over the relevant time frame. If you are working with consistent units of energy, you will find that calculating the net energy gained or lost is a matter of simple arithmetic.
This process assumes that the system analysed is under "steady-state" conditions, meaning that all power gains or losses behave consistently. For example, when calculating energy transfer for a room, steady-state conditions entail that the outdoor temperature is unchanging. "Dynamic" conditions would indicate that the outdoor temperature is either rising or falling over the time frame of interest. Dynamic conditions often require the use of sophisticated software or copious number crunching for an accurate calculation.