I have an appliance that consumes 5mA when in standby on 220VAC mains. I'm assuming on a 120VAC 60Hz system it would also consume 5mA in standby (can't measure it). Since I haven't measured it, I guess my first question should be: Is the appliance likely to also require the same current on a 120VAC mains?

Assuming the answer to the question above is "yes"; since power is measured in watts, and wattage is V * A, are 220VAC mains inherently less energy efficient than 120VAC mains?

  • \$\begingroup\$ What is the device and please explain why you have assumed it would draw the same current at half the voltage. In general 220 VAC mains is more energy efficient because losses in cable resistance are less. \$\endgroup\$
    – Transistor
    Dec 26, 2015 at 23:16
  • 1
    \$\begingroup\$ If its standby current is from a resistive or capacitive dropper, it may actually be less at 120V AC. If it's from a SMPS then it could be double, to maintain the same output power. No way to tell without either schematics or accurate measurement. 220V AC may be slightly more efficient overall, but then, 60Hz can be slightly more efficient (lower magnetization currents in a given inductor) than 50Hz. However, either way tells you almost nothing about standby power. \$\endgroup\$
    – user16324
    Dec 26, 2015 at 23:19
  • \$\begingroup\$ power is volts x amps if it needs the same power at a lower voltage then current rises to compensate. \$\endgroup\$
    – Andy aka
    Dec 26, 2015 at 23:21
  • \$\begingroup\$ The appliance is a microwave oven, but this question is about comparing the power efficiency of mains within a residence, not about the design of kitchen appliances, or transferring power over long distances. \$\endgroup\$
    – Verax
    Dec 26, 2015 at 23:25
  • 1
    \$\begingroup\$ @verax you are missing the point and you are also asking about a home appliance that, as a design, nobody is likely to specifically know about. \$\endgroup\$
    – Andy aka
    Dec 26, 2015 at 23:39

2 Answers 2


If we examine the power draw at the power inlet of the device (and thus rule out one advantage of 220Vac over 110Vac) then if an appliance draws 5mA when powered from 220Vac its power draw via P = I*V would equate to 220 * 0.005 = 1.1Watts. [1]

If we consider two types of loads.

  1. PASSIVE (resistive)
  2. ACTIVE (SMPS etc.)

A passive-type load would follow Ohm's law and thus a reduction in voltage would result in a reduction of current & equally a reduction in power. This is true for heaters, lightbulbs etc...

If you were to take a 5kW electric heater from the EU and plug it into a US 110Vac outlet you would find its output is 1.25kW (give or take) as the resistive load is drawing 11A instead of 22A.

Does this make 220V more or less efficient when viewing a passive load? neither because if you wanted a 5kW electric heater you would need to correctly acquire one for the operating voltage.

Active loads however are more constant power loads and thus a reduction in input voltage will be met with an increase in current draw as it attempts to maintain its operating point.

For you example of 220Vac @ 5mA == 1.1Watts. An active load would draw whatever current is required to satisfy its load's need. So if the voltage was reduced to 110Vac the current that would be drawn would be 10mA to meet the power needs of the active circuit ( [1] is still applicable here)

Does this make 220Vac or 110Vac more efficient? Well this is where copper losses come into play & thus 110Vac would be less efficient than 220Vac systems.

[1] This is assuming that 5mA was drawn at unity displacement power factor & pure sinewave... it isn't in practice but for quick calculations to prove the concept.


No, 220 V AC mains is not 'inherently less energy efficient than 120VAC mains'.

  1. If the load is resistive then Ohm's law (I = V / R) will apply and the current (and power) will be proportional to the voltage.
  2. If the load is a switched mode power supply the power drawn will remain constant (ignoring losses) at different voltages. Since I = P / V, the current will be proportional to the inverse of the voltage.

If Case 1 applies to your appliance AND that appliance is rated for 110 / 220 V then, yes, that appliance will consume more power on 220 V. This would be poor design of the appliance and not the mains system.


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