Calculating Percentage Efficiency

The efficiency of energy transfer between two trophic levels is calculated as:percentage efficiency = (energy transferred to the higher level / energy available at the lower level) × 100The value on the bottom is always the energy at the level the energy came from. Both values must be in the same units.Read the question carefully to see which values to use. For producers the lower level is often their NPP, and for consumers the higher level is often their net production (N). A question may instead ask for the efficiency of converting light energy into GPP.Worked example 1: Producers have an NPP of 20 000 kJ m⁻² yr⁻¹. The primary consumers' net production is 1800 kJ m⁻² yr⁻¹.
Efficiency = 1800 / 20 000 × 100 = 9%.Worked example 2 (rearranging): Energy is transferred from primary to secondary consumers with 15% efficiency. The primary consumers contain 1800 kJ m⁻² yr⁻¹.
Energy in secondary consumers = 1800 × 15 / 100 = 270 kJ m⁻² yr⁻¹.Worked example 3 (working back): Secondary consumers receive 50 kJ m⁻² yr⁻¹, which is 10% of the energy at the level below. The level below contained 50 / 0.10 = 500 kJ m⁻² yr⁻¹.Give answers to a sensible number of significant figures. An efficiency can never be more than 100%: if yours is, the fraction is upside down.