Irrespective of usage, the receiver must be active all the time waiting
for
a call - like leaving a radio switched on - with the volume turned
down -
it
will eat the batteries. Plug in ones are better.


I would suspect the receiver battery lasts at least as long
as the transmitter battery. Supposing one with a separate
transformer plug in supply costs $10 more, it may come close
to erasing any cost difference too over the life of the
unit, though the plug in type would reduce # of batteries
going into a landfill.

The transmitter is only consuming power for a few seconds at a time, when
somebody presses the button. The receiver is on 24/7, and it has to power
the bell/buzzer.



While that is true, that doesn't mean it necessarily
consumes a lot of current. Take an LCD watch for example,
it runs years from a tiny battery. How long do you think it
would run from 2 x C cells? It is an irrelevant question
because the cells will have self discharged faster than the
watch would have drained them.

As a % of time the buzzer isn't running very often and the
rest of the receiver may be using on a few uA. Since I have
one that uses 2 x C cells and they have lasted a little over
a year already, it is already clear it uses significantly
less than 1mA on average.

I stand by my point - the bell unit is acting as a radio receiver all the
time - ready to ring if it receives the correct input. It is bound to use a
significant amount of power. I would only consider having a mains powered
one - as many have a 13 amp plug through facility you are not even losing a
socket.
Battery quality is also an obvious factor.