  We analyze long-term memory properties of hourly prices of electricity in the
Czech Republic between 2009 and 2012. As the dynamics of the electricity prices
is dominated by cycles -- mainly intraday and daily -- we opt for the detrended
fluctuation analysis, which is well suited for such specific series. We find
that the electricity prices are non-stationary but strongly mean-reverting
which distinguishes them from other financial assets which are usually
characterized as unit root series. Such description is attributed to specific
features of electricity prices, mainly to non-storability. Additionally, we
argue that the rapid mean-reversion is due to the principles of electricity
spot prices. These properties are shown to be stable across all studied years.
