Hi Han,
When entering system load information as a percentage of the peak load, then peak load P, in MW, is given by:
P = A / [Sum_i s_i * h_i]
where
A = annual energy demand in MWh,
Sum_i indicates summation over all i time slices in the model,
s_i = the percentage of peak load in slice i, given by your load curve,
h_i = the width, in hours, of time slice i.
Hope this helps,
Taylor