This is a very helpful website letting you calculate how much money $ (usd) it cost to run certain electrical device like grow light. Just in input the wattage (watts) of the device use, in my case a 18watts LED light, then the time duration how long you run it for, in my case 8 hours, then how much your electrical company charge per watts, in my case PECO about 12 cent, comes out to be 2 pennies a day. So visit this site https://www.rapidtables.com/calc/electric/electricity-calculator.html
Or just google search https://www.google.com/search?q=electricity%20cost%20calculator