Bigger is better is you can control the heat. 1000w light is a 1000w heater and a 600w light is a 600w heater. Uses a bit more than a large fridge with kids leaving the door open to stare whats inside because they want something but have no ideal so they waste my electric and stand in front of the fridge with the door open. About 33 bucks a month in flower12/12 period for just the light plus whatever you need to cool the area.
as in a home appliance i have no idea, it's power usage has to on it somewhere. check your owners manual.
as said before a 600w light will use the same as a 600w heater/fridge/dryer/playstaion........
if you want to know how much it will cost to run, use something like this. Costs Calculator
look at your power bill and find how much you pay per kWh.
A 1000w @ 18hr's for me would cost.
Cost per day 3.03
per quarter 276.10
per year 1104.42
Is it better to use 600w's or 1000w's in a grow room.
That all depends on your circumstances, which you haven't stated.
more plants grown = more light needed.
If penalty's are high, less heat and less power usage to tip off the man are the go.
If your ok with what would happen if TSHTF, or are doing it to make money, bigger = better