I think changing twice a year would be unneccessary, once a year makes sense, although for a regular HPS changing every 18 months to 2 years would be sufficient. It all depends on your cycles and whether the lamp is being used for veging or flowering or both.
If only used for flowering then 12 hours a day for a year is 4380 hours so a standard HPS would be good for almost 3 years before being down 10%.
Harrad - have you got depreciation figures for compact fluoros please? And, yet again thanks for the great info
