The loss factor is the ratio between the average power losses (energy losses) and the losses during peak load in a period of time. In other words, the loss factor is simply the load factor of the losses. There are two methods to calculate the loss factor:

IEC 60853 (CIGRE)

For transient calculations acc. IEC 60853, the loss factor is determined by decomposing the load cycle over 24 hours into hourly rectangular pulses.
The loss factor may also be calculated trough summation of weighted measurement values over the past 24 hours.

Symbol
$\mu_{\mathrm{IEC}}$
Unit
p.u.
Formulae
$\frac{\sum_{n=0}^{23} I_{n}^{2}}{24 I_{max}^{2}}$IEC 60853, hourly rectangular pulses
$\frac{1}{I_{max}^{2}} \sum_{n=0}^{86399} \frac{I_{n}^{2} t_{n}}{86400}$IEC 60853, individual measurements with duration $t_n$ seconds over 24 hours
Related
$\mu_{\mathrm{w}}$
$\mu_{\mathrm{y}}$