Ethereum storage - how does this cost happen?

Question

contract A { 
      
    uint x = 10;
    
    function test2() public {
        x = 0;
    }
          
}

I can see that:

• Transaction cost is 13204
• Execution cost is 5136

How can this be ? I calculate differently.

Those 3 are taken from white paper.

• Paid for every transaction - 21000
• Paid for an SSTORE operation when the storage value’s zeroness remains unchanged or is set to zero. - 5000
• Refund given (added into refund counter) when the storage value is set to zero from non-zero - 15000

So, 21000 + 5000 - 15000 = 11000. How did we get 13204 ?

Answer 1

There's a cap to the maximum refundable gas. Only half of the total used gas can be refunded. (*)

In the example at most you can refund (21000 + 5000) / 2 = 13000 gas.

So it will be at 21000 + 5000 - min(13000, 15000) = 26000 - 13000 = 13000. Very close to the measured amount.

(*) The exact details are in Ethereum's Yellow Paper Section 6.2 Execution.