T&T: Appliances 110v versus 12v

[Ken Bloomfield]

Your memory is good Leonard, but you forgot to include one factor.  As 
you say, watts equals amps times volts, so it is true that the current 
at 12 volts for a 120 watt bulb is 10 amps and only 1 amp at 120 volts.  
However, on board your boat, you don't have any 120 volt batteries.  So, 
instead you use an inverter, and the inverter consumes power at 12 volts 
and outputs power at 120 volts.  To achieve this boost in voltage, it 
has to consume (in a perfect case) 10 times the amperage at the 12 volt 
feed that it provides at the 120 volt output.  Also, there are no 
perfect cases.

As such, the inverter 120 volt output driving the 120 watt bulb is 
providing a mere1 ampere, but the input to the inverter (in a perfect 
case) will be consuming the 12 volts at a 10 x 1 = 10 amps.  Now, having 
said this, there is some loss in the inverter process, and this will 
likely be in the 5% to 10% range.  So, the 120 watt bulb on the 120 volt 
side of the inverter is causing the inverter to consume 10.53 amps or in 
other words 10.53 amps x 12 volts = 126.36 watts.  Thus, an appliance 
that can run directly on 12 volts is more efficient.  The rub, of 
course, is that as soon as you try to get some serious wattage for a 
coffeemaker, or hair dryer, or hot plate, etc.  -- the wire size you 
would need would be ridiculously big.  Not too many folks would like 
having a 1/0 or 2/0 extension cord laying around!  Thus, there are not 
many realistic 12 volt appliances.

All the best,
Ken Bloomfield
m/v Tellico Lady

PRINTMORE1 at aol.com wrote:
> If I remember my HS physics; P=VA where P is power, V is voltage and A is  
> amperes. For the same power a 12 volt appliance uses more amperes whereas a 
> 110  volt appliance uses less amperes. Therefore from a boat point of view a 
> 12 volt  appliance is less efficient because the batteries will get 
> discharged at a  greater rate. Of course I may be wrong and I am sure others more 
> verse will  correct my statements. Leonard Stern, INDIFFERENCE.