That is a formula I will need...thanks. I was able to add a device from Amazon called “kill-a-watt†and once you add the cost of electricity, I plugged one into my lights for the hydro (3-300 real watt leds, 1000-1200 watt each) . After 1 month, the 12 hour on cycle was around $9.75 for the 3 lights alone. Still need to add in the water pump, nutes etc.etc. Basically, it’s a start my next goal is ....what did the HPS AND 315LEC with the exhaust fan cost to operate. Then , I’ll eventually get some real numbers, or close too it