The efficiency of a transfer shows what percentage of the energy at one trophic level is passed on to the next. Use: efficiency (%) = energy passed on to the next level ÷ energy available at the earlier level × 100.
Worked example: the producers in a field hold 8000 kJ and the primary consumers hold 960 kJ. 960 ÷ 8000 = 0.12, and 0.12 × 100 gives an efficiency of 12%. The energy not passed on is 8000 − 960 = 7040 kJ.
The same method works for biomass. If 500 kg of grass supports 40 kg of herbivores, the percentage of biomass transferred is 40 ÷ 500 × 100 = 8%. Another example: if the primary consumers hold 2400 kJ and 240 kJ passes to the secondary consumers, then 240 ÷ 2400 = 0.1, so the efficiency is 10%.
You can also work backwards: the energy passed on is the efficiency as a decimal × the energy available, and the energy available is the energy passed on ÷ the efficiency as a decimal.
Always divide by the amount at the earlier trophic level, the one the energy comes from, never the later one. The answer is a percentage below 100, and transfers in food chains are usually somewhere around 10%.