The diagram below shows the energy flow in the ecosystem.
Plants being primary producers, absorb solar energy to prepare food. This food is then used up by primary consumers, so they obtain only a small part of energy from the producers and rest is wasted in decay of producers.
Afterwards, a small part of energy stored as food in primary consumers is obtained by secondary consumers (carnivores). They again use a part of energy in respiration and rest is stored in them as food.
Now, the tertiary consumers (carnivores) obtain energy as food from the secondary consumers and utilise a small part in respiration and rest is wasted in their decay and decomposition.
The energy flow in ecosystems is thus linear i.e., it moves in a fixed direction. At the end, the energy reaches to the degraded (or unuseful) state. It does not return to the sun to make the process cyclic.