First, remember a well designed coop usually stays warmer (sometimes a good bit warmer) than the outdoor low temperature. Put a max-min thermometer in there at roost level and see whatcha actually got.
Second, most chickens are really quite cold tolerant as long as they're kept in dryish air with no drafts pointed at them and have a good wide roost to snuggle down on to let their tummy feathers cover their toes. Exceptions (to some degree) would be very small breeds, silkies, and things with huge giant single combs.
If you just have normal ol' chickens, chances are pretty good that in Downingtown (I grew up not too awful far away from there) you will not need a lamp at all. Certainly not (with normal, basically cold-adapted chickens) til the COOP temperature, which remember will probably be warmer than the outdoor temperature, gets down towards the single digits (although you won't necessarily need heat then either).
If you do use a heat lamp, it needn't be a HEAT (i.e. high wattage) lamp as such -- a simple 100w bulb hung (safely) over the roost is probably all you need. Kinder to the chickens, kinder to the electric bill, much less of a fire hazard. Make sure it has a guard and is safely hung, i.e. from two independant points on the lamp to two independant and secure points on the coop ceiling.
Good luck, have fun,
Pat
