It won't be running for 12 hours.. Or what ever phase of growth you use the heater in. You set the thermostat at around 73-78,when lights are off. It will only come on when the temps get below setpoint. I can't even tell you how much it would cost,considering when you calculate electric usage it's running for that amount of time. I can't really see that much of an increase in my bill.

Ga.