How much solar energy required to produce enough food for a herbivore?
BMR is the amount of energy an animal uses just by being alive. BMR is proportional to the mammal's mass. More precisely, BMR is proportional to the mammal's mass to the power of 3/4. A 0.1 kg mammal has a BRM of 10 kcal/day. Using the aforementioned proportionality, a 600 kg mammal (average mass of a cow) will have a BMR of 6800 kcal/day.

This means that a cow would need about 6800 kcal of energy per 24 hours (1440 minutes) to survive.

The amount of solar energy reaching the earth’s atmosphere is 0.00194 kcal per square centimeter per minute. Since, according to you, there is 10 times the amount of sun in Mercury, there should be 0.0194 kcal per square centimeter per minute hitting Mercury. 

0.0194 kcal per minute for 720 minutes (counting only day - since there is no sun during the night) equals 13.968 kcal per square centimeter per day. At this rate, the sun has to hit an area of 486 cm squared to harness enough energy for the cow.

In summary:


The average cow needs about 6800 kcal per day to survive (BMR)
On Mercury, the sun generates 13.968 kcal per square cm
Which means, the sun has to hit an area of 486 square cm


In one sentence:


In order to power a cow on Mercury, the sun has to hit an area of 486 cm for 720 minutes.


To learn more:


https://en.wikipedia.org/wiki/Kleiber%27s_law
http://www.ase.org/resources/measuring-number-calories-sunlight