The CEB's website said on Monday 29.6GWh hours of energy was generated of which 25.6 percent of 7.6GWh were hydro. A strong shower causes most small ponds in the Laxapana hydro power complex to spill.
A strong shower can usually bring inflows in to the reservoirs for two or three days as water held in the upper catchment seeps down. Sri Lanka's hydro catchment areas are under threat from human encroachment.
Peak power demand was 1,822MW on Monday down from 1900MW plus level seen in recent weeks amid somewhat cooler weather.
CEB's published data totals centrally dispatched plants and the contribution of mini-hydro power is effectively shown as a negative number from the total.
A strong shower tends to increase generation at run of the river mini hydros, which pushes peak power demand down. Sri Lanka has about 200MW of mini-hydros installed.
The 29.5 GWh of energy generated was also much lower than a highest ever peak generation of 35.09 GWh reached on May 23.
The CEB is estimated to be losing 200 million rupees a day due to under pricing power generated by liquid petroleum.
