How to compute the power comsumed by the balance resistor

I used Wilkinson 2-way splitter/combiner to combine PA power. The frequency is 900MHz. The port 2 power is 20W, and port 3 is also 20W, so port 1 is 40W. Normaly, the balance resistor (100Ω) is lossless and can provide good isolation between port 2 and port 3. But how to choose the power rating for the balance resistor? And how to compute the power consumed by resistor for mismatch? Please help me. Thanks.

We need more info before we can answer. At the two 20W amplifier outputs. Are the two outputs the same signal? ie are they the same frequency and output phase (coherent signals), or are they two independent signals with different frequency/modulation?

If they are two of the same (coherent) signals, how well do they track in amplitude and phase?

What is the worst case load VSWR (like if the antenna ever ices up, etc)?

In Wilkinson splitter, the signal splits into equal amplitude and phase signals appearing at ports 2 and 3. Since each end of the resistor is at the same potential, no current flows through it. So, the resistor used in the Wilkinson splitter generally has a power rating of just 100mW.
This in case that all ports are well terminated.
When one of the ports is open or shorted, is enough to damage this resistor.

Here you can find how to calculate the power of the resistor due to mismatch.
http://www.rflabs.com/IMG/pdf/Power%...20dividers.pdf