It's not an easy question depends on the room temp itself and how much it will vary as well as the distance from bulb to floor area.
To give you an example to warm up a hot spot of 90 degrees in a cage that was 12 inches high in a room that was never no less than 75 degrees I used to use a 50 watts bulb.
I am not a huge fan of bulbs in general it's a lot of heat loss.








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