In article , fred writes
If the theory scales up it suggests the following:

300W at full brightness costs 300W (100% _relative_ efficiency)
Dimmed to half brightness costs 230W (65% rel eff)
Dimmed to quarter brightness costs 170W (65% rel eff)

correction, 45% rel eff for 1/4 drive
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fred
Plusnet - I hope you like vanilla