Energy flows, matter cycles
Energy enters as sunlight and leaves as heat, losing about 90% per trophic level, while carbon, nitrogen and water are reused.
A worked example
Producers holding 50 000 kJ pass roughly 5 000 kJ to the herbivores above them and about 500 kJ to the carnivores above those.
The mistake people actually make
Do not say energy is destroyed going up the pyramid; it is spent on respiration and lost as heat. Matter cycles, energy does not.