Efficiency is lower in a colder filament because more of the radiation is in the infrared for a given wattage. So you get more heat for an equal amount of visib

Efficiency is lower in a colder filament because more of the radiation is in the infrared for a given wattage. So you get more heat for an equal amount of visible light. Here is an example with plausible (but not real) numbers. Higher current (hot filament): 1000 lumen, 100 W, 10 lumen/watt, 1000 hour lifetime Lower current (colder filament): 333 lumen, 50 W, 6.66 lumen/watt, 3000 hour lifetime. To emit 1000 lumen for 3000 hours you'll use 3 of the former lightbulbs (one after another) and consume 300 kWh, 100 per lightbulb. And you'll also use 3 of the latter lightbulbs (in parallel) but you'll spend 450 kWh, 150 per lightbulb.