I will try to calculate the durability required to survive inside the sun. For that I will try to determine the heat radiated from the star and the heat that is transmitted through conduction. I will also do one calculation for surviving just under the surface and one for surviving in the core of the sun.
Surface
1. Radiation: For radiation we need to know the emissivity, surface area and temperature.
The temperature of the sun is about 5500°C per Wikipedia.
For the surface area we take the surface of the average human body, since we assume that the person is submerged in the sun. The average body surface area is about 1.73 m^2 per this article.
The emissivity is about 1.2 at this temperature per this article.
Now we input this values into this calculator and get 130756044.60407 J/s.
2. Conductiont: For conduction we need to know surface area, thickness of the material that the heat is transmitted through, the thermal conductivity of the material and the heat of the sun and the object.
Surface area and temperature of the sun can be taken from the radiation part.
Now for the material were the heat is transmitted through I will take human skin.
Human Skin is around 3mm thick. (https://en.wikipedia.org/wiki/Human_skin)
It has a thermal conductivity of about 0.209. (http://users.ece.utexas.edu/~valvano/research/Thermal.pdf)
Normal skin temperature is about 33°C. (http://hypertextbook.com/facts/2001/AbantyFarzana.shtml)
With that we have everything we need. We use this calculator to get a result.
The result is: 658901.0633333334 watts = 658901.0633333334 J/s.
Now we add both results together to get a final value: 658901.0633333334 J/s + 130756044.60407 J/s = 1.3141494566740333*10^8 J/s.
Core
Now a similar procedure for the core. The core of the sun is about 15.7 million Kelvin hot. The emissivity of the sun at temperatures such as this isn´t known, but the article that I linked to emissivity states that the minimum lies at 6900°C. So we will use the minimum emissivity of 0.92 for this. Now we just need to input all values in the calculators again.
1. Radiation: 5.4829665830548E+21 J/s
2. Conductiont: 1892212356.0633333 J/s
5.4829665830548E+21 J/s + 1892212356.0633333 J/s = 5.4829665830566922E+21 J/s
Note: This is for a human in the sun. If the character is a lot bigger or smaller than an average human, or if the character is made from another material, like for example metal, this numbers change.
Maximum internal energy intake
If an object is heated it usually doesn´t get hotter than the source of the heat. If the object is as hot as the heat source the energy itself emits to its surroundings should be equal to the energy it is infused with.
That means there is a maximum amount of thermal energy an object can take in through a certain source of heat.
In order to calculate this energy I will just measure how much energy will be necessary to heat the object to this temperature, from the point that it has no internal energy, which should be 0K.
The specific heat capacity of a human body is 3470 J/kg.oC
Average weight of a grown human is around 62 kg.
Surface: The surface of the sun has a temperature of 5.773.2K.
3470*62*5773.2 = 1.242046248E+9J
That is Building Level.
Core: The core of the sun has an temperature of 15 700 000K.
3470*62*15 700 000 = 3.377698E+12J
That is Multi-City Block Level+.