In the penalty announced Thursday, $10 million will go to the United States Treasury, $10 million will go to the reliability council and $5 million is to be spent by Florida Power & Light on reliability improvements. Those improvements will be subject to prior approval by the reliability council and the Federal Energy Regulatory Commission. The settlement also requires the company to make additional “substantial, wide-ranging and specific reliability enhancement measures.” The agreement specifies that the company cannot collect the penalty from ratepayers.

The blackout was caused by an engineer who disabled protective circuits while he tried to fix an equipment problem at a substation west of Miami. The company said Thursday that its engineer had done so “without authorization and contrary to F.P.L.’s policies and procedures.”

The failure led to the sudden shutdown of several generating stations, including the two nuclear plants at Turkey Point. About a third of the homes and businesses that lost power were customers of other utilities, illustrating the interconnected nature of the power grid.

As part of the settlement, Florida Power & Light did not admit any violations or acknowledge any liability. It said the standards it was accused of violating were “ambiguous and subjective.”

In a statement, the company’s president and chief executive, Armando J. Olivera, noted that the blackout took place more than 18 months ago and that litigation to resolve the case would be expensive and “would require the time and attention of the same people who are responsible for the reliability of the grid.” “As a result,” Mr. Olivera said, “we believe a settlement is an appropriate course of action at this time.”