"Paul" wrote in message
...
Can someone refresh my memory as to why 240V electric baseboard heaters
are "more efficient" than 120V?
Paul:
Electric heaters no matter what voltage are for all intents and purposes
100% efficient.
In other words they turn all the electricity fed into them into heat.

So whether 240 or 120 volts is not material from point of view of amount of
heat and nothing to do with efficiency!

The discussion about 240 versus 120 sounds more about the practicality of
wiring with no availability of fuse or breaker positions in an already full
panel.

As someone said 750 watts, at 120 volts is only around 6 amps. On 240 it
would be around 3 amps!
See addendum:

But whether electrical codes allow you to mix an electric heater and outlets
on the same 120 volt circuit is a question!

FWIW: Our bathroom is small and without a window. The bathroom heater
located under the towel rail is 240 volt 500 watt, it is one of several each
with it's own room thermostat on a 20 amp electric heaters only circuit. It
complies with our codes; the heater being completely enclosed except for
upper and lower small air vents through which you could put a pencil but not
a finger (and a child's finger would not be long enough to reach the heater
element). Also it is GROUNDED and that grounding is bonded to the copper
plumbing and fixtures. The bathroom shaver outlet is on a separate circuit
and is a GFCI type. We chose that type (GE type) heater 35 years for safety
after other home's, more open types, had resulted in scorched bedclothes,
but fortunately no fires.

Oh btw; we also have six 40 watt bulbs in a fixture above the mirror. If
left on these contribute noticeably to the heating of the bathroom.

As a completely separate issue that full panel should worry. Also if you DID
decide to install a subpanel IT would itself need a (probably a double pole)
breaker in the main panel to supply it! Your panel, due to additions and
changes, may be 'at capacity'?

Addendum:

1000 watts @ 240v = 4.2 amps
750 watts @ 240v = 3.1 amps
500 watts @ 240v = 2.1 amps

1000 watts @ 120v = 8.4 amps
750 watts @ 120v = 6.2 amps
500 watts @ 120v= 4.2 amps

At electricity cost of 10 cents per kilowatt hour; if the heater was 'on'
continuously, both 1000 watt heaters would cost 10 cents per hour.

For improbability and to further demonstrate volts/watts and amps!
Imagine a 12 volt 250 watt heater? 250/12 = 21 amps.
Enough to drain a car battery in average condition in less than two to three
hours!
Oops; car won't start!

Note: For another comparison a large microwave oven plugged into a 120 volt
outlet can require 1200 watts (1200/120 = 10 amps). Usually, for typical
m.wave oven use, for a fairly short period of time.