If you're new to indoor growing, you may want to read up. Don't be stupid. Don't get in over your head with utility bills. Never assume you know how much your grow room is going to cost. Use this formula to estimate the cost of your lights (can be applied to anything that draws electricity): (watts x hours per day) / 1000 = daily kilowatt-hour (kWh) 1 kilowattkW = 1000 watts (W) Once you've determined the daily cost to run the lights, multiply it by the number of days you run them during the year. Example: My 2400W grow tent (4 x 600W): VEGETATIVE PHASE (2400W x 18 hours per day) / 1000 = 43.2 kWh per day If I veg for 4 weeks, that's 28 days. 1 month out of every 3 translates to 4 months per year. 28 x 4 = 112 43.2 kWh per day x 112 days per year = 4838.4 kWh per year 4838.4 kWh per year / 4 months per year = 1209.6 kWh per month BLOOM PHASE (2400W x 12 hours per day) / 1000 = 28.8 kWh per day If I flower for 8 weeks, that's 56 days. 2 out of every 3 months translates to 8 months per year. 56 x 4 = 224 28.8 kWh per day x 224 days per year = 6451.2 kWh per year 6451.2 kWh per year / 8 months per year = 806.4 kWh per month Now that you've determined the monthly cost of both the veg and bloom phases, you need to find out what your utility provider's monthly fee is. You also need to find out their rate per kWh. Mine: $20.08 / month + 10.05 ¢/kWh VEG PHASE 1209.6 kWh x 10.05 ¢/kWh = 12156.48 ¢/kWh Divide by 100. 12156.48 / 100 = 121.56 Now add their monthly fee. 121. 56 + 20.08 = 141.64 This is the dollar amount it will cost me per month during my veg phase. BLOOM PHASE 806.4 kWh x 10.05 ¢/kWh = 8104.32 8104.32 / 100 = 81.04 81.04 + 20.80 = 101.84 Now that you've determined the cost of your lights, which will be the bulk of your utility bill, you can do the same thing to calculate the cost of the inline fans and everything else. Cheers. - Dredd