Energy Flow and Voltage Rating
Country: United States
Date: September 2007
What is the relationship (in kilowatt-hours) of energy usage
between a 1000w appliance running on 220/240volts vs. 110/120 volts?
It depends on what is meant by "1000w appliance". Some appliances are
intended to run from a range of supply voltages, and those will consume
roughly the same energy on either. A 1000w appliance intended to run only
on 110 will do something unpredictable (maybe spectacular) on 220, and
will in general consume a different amount of energy, but nobody can guess
how much without more information about the device, except to say that it
will probably be either more or zero. The opposite case is similar, but
less likely to be spectacular.
If it is 1000W, that is its power consumption. In general, an appliance
designed to run on 220 V will not work the same at 110V and vice versa.
For a given resistance in an electrical component, the power consumption
is proportional to the square of the applied voltage. So a 110-V
appliance would typically burn out at 220 V, and a 220-V appliance would
be underpowered at 110 V.
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
A 1000 W (1 kW) appliance running for one hour will use 1 kWh (one killowatt
hour) of energy. This is true whether it is running on 240 V or 120 V (V =
The difference is that the current going through the appliance is different
in the two cases. Since P = IV (power in watts (P) is equal to current in
amperes (I) times voltage in volts (V). So at 240 V a 1 kW appliance is
using 4.17 A and at 120 volts it would use 8.33 A.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
An appliance with a POWER consumption rating of 1000 W (that is, 1 kW), when
run for an hour, consumes 1 kWh of ENERGY.
The same energy is used whether we use 110 or 220 V voltage. If it were
different, we could save energy by switching. We do not. It is that with
110 V, the appliance would draw twice the current that it would from a 220 V
supply line. The product of V (voltage) x I (current) is P (power), which is
the energy an appliance needs to run on per second, remains the same.
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
Both appliances use 1000 Watts, so both appliances clearly
use exactly the same power. Operating voltage is irrelevant
to energy or power usage. Since Power = Voltage x Current,
and since both use 1000 Watts of power, by simple arithmetic,
a 1000 Watt appliance that operates from 220V uses 4.55
Amperes. The 1000 Watt appliance that operates from 110V
(half the voltage) uses twice as much current 9.10 Amperes.
As for your question on energy usage, this depends on how
long the appliance is switched on for, since Watts x Hours
equals Watt-Hours of Energy. A more universal measure of
energy is the Joule; 1 Joule = 1 Watt-Second (that is, 1 Watt
being consumed for one second). Therefore 1 Watt-hour = 3600
Joules (one hour has 3600 seconds).
So, if either of your 1000 Watt appliances (the 220V one or
the 110V one, makes no difference) operates for 1 hour, they
will both use 1000 Watt-Hours of energy, or 360,000 Joules.
Hope this clears things up!
Both are 1000 watts. Watts are the unit of measure of energy usage.
So by definition, before your question was ever asked,
these appliances were measured and found to have the same power demand.
Both put 24 kilowatt-hours on your electric bill for every day of
There is a second-order effect you might be fishing for, but it is pretty
especially if your house wiring is as thick as it is supposed to be.
1000w at 120v requires 1000/120 = 8 amps.
1000w at 240v requires 1000/240 = 4 amps.
All the wiring from pole-transformer to heater element might add up to 0.2
so small additional energy losses are incurred:
P= I^2 x R: (power = (current squared) times resistance)
120v: (8 amps)^2 * 0.2 ohm = 12.8 watts
(The wire is using up 8 amps * 0.2 ohm = 1.6 volts, out of 120v.)
240v (4 amps)^2 * 0.2 ohm = 3.2 watts
(The wire is using up 4 amps * 0.2 ohm = 0.8 volts, out of 240v.)
The difference is about 10 watts in 1000w, advantage to 240vac.
That is a 1% savings with a heavy 1000w load.
Not very motivating.
For smaller loads it would matter even fewer percent.
The more important disadvantage of using 120v here, in my opinion,
is fire hazard.
Wires crack, corrode, or get thermally smothered to uncontrolled amounts,
later in a house's history.
That 13 watts is 4 times more wire-heating power than 3.2w,
and far more likely to be able to heat up a long wire to a temperature
that makes something go bad and start a fire.
The usual something bad is oxidation at some metal-to-metal connection,
which greatly increases the resistance and heat generated.
After that a fire can start.
Old dark-colored copper wiring is a warning for you,
if heavy loads are to be used.
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Update: June 2012