Tapping costlier diesel-fired power plants during the series of red alerts imposed in Luzon and the Visayas last week could trigger a slight increase in electricity rates.

Although no final estimate has been made so far, Energy Undersecretary Rowena Cristina L. Guevara told reporters at a recent media briefing that more expensive diesel plants were activated to help stabilize the grid during the red alerts declared from Wednesday to Friday.

“Potentially (rates could inch up), because we were on red alert, we had to run the more expensive power plants like the diesel power plants because all hands were on deck last Wednesday, Thursday, and Friday,” Guevara said.

She clarified, however, that the possible increase in rates is expected to be minimal since only a small amount of diesel-generated capacity was added to the country’s power supply mix.

“Potentially, there will be small increases, if any, but these are very small plants. Only a few megawatts (MW) from such plants were added to our generation mix,” she added.

Separately, Energy Secretary Sharon S. Garin assured consumers that the red alerts were not expected to cause major increases in electricity rates, saying the priority was to maintain stable power supply to households.

“Technically, there is no direct correlation to that because it is not about the rates; it is more about the delivery of power to households. There is no (major) impact on rates — it is more about ensuring that households have power,” Garin said.

At the height of last week’s outages, nearly two million Manila Electric Co. (Meralco) customers in parts of Bulacan, Cavite, Laguna, Metro Manila, Quezon, and Rizal were hit by rotating brownouts after the National Grid Corp. of the Philippines (NGCP) ordered manual load dropping to stabilize the grid.

The affected users represented around 23 percent of Meralco’s 8.3 million customer base as of end-March, equivalent to nearly one in every four customers served by the country’s largest power distributor.

The Department of Energy earlier attributed the strained grid conditions to a combination of forced outages, derated power plants, and elevated electricity demand driven by extreme heat conditions.

At one point, the Luzon grid’s available capacity fell to 12,479 MW against the projected peak demand of 12,595 MW.

This week, while the Luzon and Mindanao grids operated normally, the Visayas grid remains under Yellow Alert for the third time since Monday.

NGCP on Wednesday extended the Yellow Alert from 4:00 p.m. to 8:00 p.m., longer than the earlier 5:00 p.m. to 7:00 p.m. advisory issued in the morning.

Available capacity stood at 2,692 MW against a projected peak demand of 2,503 MW.

NGCP said the extended alert was caused by the continued outage of major coal plants TVI 1, TVI 2, and PEDC 3, along with an expected increase in electricity demand during peak hours.

The grid operator also said 847 MW remained unavailable to the Visayas grid due to plant outages and derated operations. Nine plants had been on forced outage since May, while 15 others were running below normal capacity.