Energy can be transferred between objects or transformed between stores and forms, but the total is conserved. For a device operating without a net change in stored energy, energy input equals the sum of its outputs. A lamp transfers supplied electrical energy into light and thermal energy. The thermal output may be unwanted, but it still counts in the energy account.
A Sankey diagram uses arrow width proportional to energy transfer. The input width equals the combined output widths when the account is complete. A useful branch represents the intended output; other branches show transfers such as thermal energy to surroundings. “Wasted” means less useful for the purpose, not destroyed. If a device stores energy during operation, include that storage increase as well as outgoing transfers.
Example 1
A lamp receives 100J and transfers 25J as light. If there is no storage change, how much becomes other outputs?
Other outputs = input − useful light =100−25=75J, mainly thermal transfer in this simplified account.
Example 2
A Sankey input is 20mm wide for 200J. Find the width for a 50J branch.
Scale =20200=10J per mm. Width =1050=5mm.
Watch out: It has been transferred or dispersed in less useful ways, often as thermal energy in surroundings.