If 2000 kcal of energy would be available in grass, how much energy would be availabele to the cow that eats the grass? to the human that eats the cow?

Generally, consumers only receive 10% of the energy from what they eat. So, if the grass had 2000kcal of energy, then the cow would receive 10% of that -  200kcal. And then when a human eats the cow, it will only receive 10% of the cow's energy. This is because within a food chain, energy is lost as organisms use their energy to survive - like moving and producing heat (if endothermic). So consumers do not get 100% of their food's energy. Hope this helps a little.