Imagine the busiest moment at you feeder you seen, a moment where there was just crazy activity and such a large number at once. Now Take that moment and replace the feeder with a bath, in the winter drinking stations are more popular than feeders and with all kinds of life (not just seed eating birds).
I always believed Ground mounted bathes are better and more active being it is natural for the birds to be on the ground drinking. I have this one..
http://www.amazon.com/Farm-Innovators-FS-1-Seasons-Birdbath/dp/B000HHSMTW/ref=pd_sim_lg_10
I had a cheap plastic pedestal bath before so i recently bought one of these for it.
http://www.drsfostersmith.com/product/prod_display.cfm?c=9089+8776+26441&pcatid=26441
Both these heaters are small but will do the job if placed correctly (like out of direct wind). I think most heaters on the market are over kill for the average size bath (who needs 250 watts of power for a bowl of water??). I believe the most heaters are on a thermostat that just switches on and off the power at a certain temperature. So if you have a 250watt heater when the thermostat turns on it turns on the full 250 till the temp goes up. So the only difference between the larger output heaters vs the smaller is the full blown power. That brings the question how much water needs how much wattage to keep the water liquid? You have a bunch of factors that can affect this question also like winds, sun, warmth or insulation around the bath, etc. How cold does it get by you? what is the aver cold you get in the winter? Can you place something infront of the bath to break the wind up? is the bath cheap thin plastic or thick cement? these questions really matter when it comes to what wattage you really need to heat the water.
most heated bird baths are shallow and don't hold lots of water and to make matters worse as the heater does its job you will actually lose water from that alone. This may sound like a bad idea but whenever you are warming up water you to a lesser degree are cooking what is in the water like poo and chemicals used to treat the water. Cleaning will be needed every other day as with a refill would be..these two kind of work out together.
the more heated baths you have the less traffic on each one which means the water will last a bit longer. Going with two 80 watt heaters will use less than one 250 watt heater, so again go smaller and if there is lots of traffic add another one later.
In the summer months everyone has a pool, pond, fountains, watering their grass, etc. In the winter where is there water? the closest river? a heated bird bath in your back yard may be the only place for water in your area...