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Well, if your brooder is indoors (in your heated house that is) you won't need that high of wattage, ever. Unless maybe you like to keep your house below 40*F, or if you brood in an unheated garage or shed. The heat from the lamp is concentrated directly below it, so IMO it doesn't really matter how big your brooder is, as long as you can get it to at least 95*F under the lamp. So long as they are not piling on each other, I use the lowest wattage that works.
I use a 75 or 85 watt red floodlight for indoor brooding. Usually that is sufficient with ambient household temps being 65-70ish. My brooder box is a 4ft long rectangular rubbermaid tote. If I have wet chicks fresh from the incubator sometimes I'll set them up with two 75 watt lamps for about a day since they chill easier being wet, but generally you don't need that much for indoor brooding.
I don't move the lamp or adjust the temp much at all. I just put it at one end of the box and they start sleeping further away from it as they get older. The 85 watt red floodlights used to be really easy to find, even at
WalMart, but this fall they didn't have any when mine burned out, so I bought a bug light in the same wattage. I've used white 60watt bulbs temporarily, it's warm enough but the bright white light keeps them up.
I have some month-olds that were out in a medium size (25# size) doghouse with a 85 watt "anti bug color" flood light this fall, down to at least 22*F and they are fine. I put their water in the house with them (instead of in their run) so I didn't even have to worry about it freezing either.
I HAVE used the 250 watt "heat lamp" bulb for week-old chicks in a 3-sided shelter when it's still cold on spring nights. As you probably noticed, I move my chicks out to chicken tractors at a young age. They do fine with it being cold out in the "run" portion of their brooder pen, so long as they can come back to their "mama-lamp" to warm up. The whole brooder doesn't have to be 95*F just the sleeping area. IMO with the increased heat there's more of a fire risk IMO, and plus it costs more electricity. Of course, when you need it, you NEED it so it doesn't hurt to have one on hand. I think I have 2 on hand, as they tend to go out when you need them the most - per Murphy's law.
Of course your mileage may vary
I don't baby my birds much, those who insulate their coops would be horrified to see mine.
However, I've also gone out and put a lamp on 12 week old chicks who seemed to be shivering when it dropped below freezing - they didn't have enough down yet. I guess you could say I tend to go by the reactions of my animals, rather than a set temperature or wattage or "fully feathered age" standard.