The Visayas and Mindanao power grids were placed under yellow alert Monday as electricity demand rose and available supply remained tight, according to the National Grid Corp. of the Philippines (NGCP).

In the Visayas, a yellow alert will be in effect from 4 p.m. to 5 p.m. and again from 10 p.m. to 11 p.m. The grid is also expected to be under red alert from 5 p.m. to 10 p.m.

NGCP said the Visayas has 2,119 megawatts of available power capacity, while demand is expected to reach 2,406 MW during peak hours.

The grid's supply was affected by nine power plants that were forced to shut down and 17 others operating at reduced capacity. Together, these plants account for 956.2 MW of power unavailable to the grid.

NGCP said the additional yellow alert was partly due to a higher forecast for electricity demand in the Visayas, which increased by 112 MW for 5 p.m.

Limited power being imported from Mindanao also contributed to the alert, as Mindanao's projected demand increased by 157 MW for 5 p.m.

Meanwhile, the Mindanao grid will be under yellow alert from 5 p.m. to 9 p.m.

Mindanao has 2,566 MW of available capacity against an expected peak demand of 2,389 MW.

NGCP said 19 power plants in Mindanao were on forced outage, while five were operating below their full capacity. This left a combined 755.1 MW unavailable to the grid.

The additional yellow alert in Mindanao was also prompted by a 106-MW increase in projected electricity demand for 6 p.m.

NGCP explained that a yellow alert means the power supply is still enough to meet expected demand, but the available reserve is not sufficient to meet the grid's required backup capacity in case of an unexpected problem.