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The block heater is 1000 watts, at least the one in 2002 is. It is imprinted right on the side of it, I saw it when I pulled it while rebuilding the oil cooler a few weeks ago. Also for those who haven't had one apart the block heater screws into the adapter that houses the oil filter mount and the rear mount for the oil cooler which really is a heat ex-changer as oil and coolant travel through it. So when you plug it in it is heating the coolant in the cooler which also heats the oil in the cooler which is what makes it start so much easier. Several on average -15 days last week and it started without being plugged in but plugging it in makes life better.
Currently I have been plugging mine into a photo-eye timer rated for 1000 watts that will operate from dusk to dawn or dusk to 2,4,6,8,or 12 hours. I have it set to 12 hours and it is still warm a few hours later when I go to start it. I usually use a timer but it wont fit in the current outlet due to an outdoor cover without modification, but make sure your timer is rated for at least 1000 watts.
I plug mine in when the forecast says I'll likely need my defroster. I have a timer set to start 3 hours before the earliest I would go to work (flex hours).
I plug in whenever it's below 30 or going to snow. I've started close to 0 not plugged in, but it's not real happy about it. Just because it can start unassisted at that temp don't me it should. My truck is just much happier plugged it. It also keeps the hood clear of snow and keeps the windshield warm enough the snow just wipes off. Then there is the near instant heat.
My heater don't draw near what is rated at. My 1000 watt Honda generator runs it just off idle, while a 900 watt hot plate will have it wide open. When I was using my truck everyday for work and it was plugged in 12-14 hours a day my electric bill was $15-20 dollars more a month. Well worth it for the benefits, IMHO. Hell, I probably saved more then that in fuel by plugging in.
I'm with Tugly too... 3 hours before is plenty. Even though it'll start in extreme cold, the oil is thick and takes a bit longer to reach to the metal... I guess if you don't mind metal to metal until the thick oil gets to it... well...Can't be good for the motor/HPOP over time... The 50 cents it costs to run it for 3 hours is worth it to me...