How to build an energy flow Sankey diagram

The Sankey diagram was invented for energy — Matthew Sankey drew one in 1898 to show the efficiency of a steam engine — and energy is still where the format is most rigorous, because the physics forces you to account for everything.

Three columns are usually enough

Primary sources on the left (wind, solar, gas, nuclear), carriers in the middle (electricity, heat, fuel), end uses on the right (industry, transport, residential). Losses branch off wherever conversion happens.

Losses are a destination, not a gap

Every conversion step loses energy, and in a correct diagram those losses are their own ribbon flowing to a Losses node. A chart where 100 TWh of gas produces 100 TWh of electricity is not optimistic, it is wrong.

Pick one unit and stay in it

TWh, petajoules or ktoe all work; mixing them does not. Convert everything up front and put the unit in the node labels so the reader never has to guess.

Colour by source, not by rainbow

A common, readable convention is renewables in green, fossil in grey or red, and losses in a muted tone so they recede. That way the balance of the chart is legible from across a room.

• Sources → carriers → end uses, left to right.
• Every conversion gets a loss ribbon.
• One unit, named in the labels.
• Round to whole units; nobody audits a national energy chart to three decimals.

The same structure scales down perfectly: a single building's energy audit is the identical chart with smaller numbers and a shorter list of end uses.