The seaside Encina Power Station, a Carlsbad landmark since the 1950s with its iconic boxy concrete building and 400-foot smokestack, has officially been retired to prepare for demolition. Owner NRG Energy Inc. switched off the old plant at 11:59 p.m. Dec. 11 and turned on its replacement. The new, smaller, more efficient “peaker” facility can be fired up in 10 minutes to meet the demands of a summer heat wave, a fallen transmission line or some other sudden development on the power grid.