Designing voltage summer without op-amp
The output voltage is 

$$\begin{split}
V_{\text{OUT}} &amp;= R_1||R_2||..||R_n||R_L \cdot \sum{(V_i/R_i)} \\ &amp;=  \biggl(R_1||R_2||..||R_n\biggr)||R_L \cdot \sum{(V_i/R_i)} \\ &amp;= \frac {R_K R_L}{R_K+R_L}\cdot \sum{(V_i/R_i)}
\end{split}$$

where \$R_K = R_1||R_2||..||R_n\$

If you consider the ideal output voltage is when \$R_L = \infty \$, then 

$$ \frac {V_{\text{OUT}}}{V_{\text{IDEAL}}} = \frac {R_L}{R_K + R_L}$$

To look at it intuitively, \$R_K\$ is simply the Thévenin equivalent source resistance of the divider without the load resistor.