ok so a 400w hps uses 0.4 kwh per hour 0.4 x 18 = 7.2kw for 18 hours of light per day my eletric company charge 13.4 pence per kw so 7.2 x 13.4 =96.48 pence (uk) per 18 hours of light. this look about right?
im runnin a total of 300 - 350w and only using 4p an hour. it makes no sense, i dont see an increase at all with my bill lol youll be fineeeee i pay 30 quid a month, i run my lights, xbox all day, pc all day, other random shit and nothings changed.
13.4p divide by 1000 x 400 = 5.36p per hour for your 400w light 18hrs use will be 96.48 so your right weekly use £6.75 hope it was helpfull