In any large installation, lamp failure rates increase with age in a predictable way.  The optimal point for group re-lamping is typically when 8% to 12% of lamps have failed, usually around two-thirds of the lamp’s rated life. At this point, both lumen output and lamp color may have deteriorated, yet the cost of electricity remains at “full price.”  The per-lamp labor cost of installing new lamps is usually significantly less when an entire site is re-lamped, compared to the per-lamp labor cost of replacing lamps one at a time.  As a result, the building receives a lighting “face-lift” and the maintenance costs over the next two or three years are significantly reduced.