The 400w heater is going to be kicking on and off during the day which makes measure average power consumption over a period of time a little harder to calculate vs a MH light being turned on for "x" hours a day.
If you're just trying to figure out loading (will the circuit support this device) I would imagine that they draw roughly the same current based in the same wattage rating. However, I've never actually hook up a meter to verify my 250w heaters actually consume 250watts.
They sell cheep meters at hardware stores called "kill-a-watt" meters that will tell you what the current and wattage draw are for plugged in appliances. That might be worth investing in if you need hard numbers.
You are right, I am figuring load. I am disconnecting a heater an need to replace it with a light. Running close to max cap with heater going. Just wanted to make sure. Thanks guys
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