Project Euler Problem #755
A low hanging fruit first. You don't need to work through the entire powerset. Given that \$i_0 &lt; i_1 &lt; ... &lt; i_k\$, and \$N = F_{i_0} + F_{i_1} + ... + F_{i_k}\$, how small \$F_{i_k}\$ could be?
Recall that \$F_0 + F_1 + ... + F_n = F_{n+2} - 2\$. As long as \$F_{n+2} - 2 &lt; N\$, a representation is impossible, so the term \$F_n\$ cannot be the largest. Therefore, the largest term in the representation must satisfy both \$F_n \le N\$ and \$F_{n+2} - 2 \ge N\$. Since the Fibonacci numbers grow exponentially, this observation drastically reduces the choice of the largest term (one or two possibilities, depending on how \$N\$ compares to \$F_{n+1}\$).
That alone shall give you a boost.

Yet it is still bruteforcing. As you correctly say, there must be an underlying math.
I played with your program, printing the total number of representations for numbers between \$F_n\$ and \$F_{n+1}\$. The results are surprising:
Range     Total
3..4      3
5..7      5
8..12     11
13..20    21
21..33    43
34..54    85

Hmm... \$T_{n+1} = 2T_n - 1\$. Something to prove about, isn't it? Try. I don't want to spoil the fun.

For a proper review:

I don't see the reason for sumOfList, fibList, repWay, and rangerepWays being generators. yield only make sense if it breaks the natural control flow, while preserving some execution context. Those above have no context to preserve; they can safely return. Making them generators only adds overhead.
powerset is a different story. It yields in the midst of a (nested) loop.

Naming deserves some attention. It took me a while to understand what, say, repWays is supposed to accomplish. Along the same line, mostList is a list of the Fibonacci numbers, isn't it? - so name it accordingly.