If you have a newer house, this may not be an issue. But as we found out, older homes with more antiquated systems may not deliver enough juice. One of our test houses, which was built in 1946, was sending only about 10 volts of electricity to the existing doorbell, which wasn't enough to power the two doorbells in the roundup that lack built in batteries. After we upgraded the doorbell's circuit to a 20 volt transformer, everything worked as advertised. Most people shouldn't have to upgrade their transformers, especially with newer houses, and the two doorbells that have built in batteries don't require power from the doorbell at all. Some doorbells, like the Ring Video Doorbell 2 and the RemoBell, can run on battery power.