How does varying voltage level affect power consumption?

Answer 1

Theoretically, Ohm's law says V=I*R where V is the Voltage, I is the current and R is the resistance. Consider a purely resistive load. As the resistance 'R' is fixed, above equation implies I=V/R.

Next, consider the way power is calculated: Power = V*I. Substituting I from above, Power = V*(V/R) = (V*V)/R. Assuming that R is constant, this implies that as you increase the voltage, the power increases by square of it. So, if one increases the voltage by 10% (say, from 100 to 110), The power increases from (100*100)/R to (110*110)/R, in other words, from 10000/R to 12100/R - an increase of 21%.

In general:

Increase by 5% in Voltage implies increased power consumption by 10%

Increase by 10% in Voltage implies increased power consumption by 21%

Increase by 20% in Voltage implies increased power consumption by 44%

Increase by 30% in Voltage implies increased power consumption by 69%

HOWEVER, please note that Resistance is a function of Heat as well. As a resistive load (like Incandescent Bulb) heats up on passing current, R actually decreases so actual savings may be slightly lower than above theoretical values. MOREOVER, not all loads are purely resistive - e.g., fans, refrigerators etc are inductive. So, some experiments suggest that net voltage drop of 10% lead to a power saving of about 14-20%, drop by 20% lead to 18-28% saving...depensing on the load composition and also the usage.

Many electric distribution companies across the globe try to increase the Voltage by up to 25% in low loss areas while decreasing it by similar amounts from permissible levels in high loss areas! This way, at the cost of consumer, they cover their losses.

Answer 2

Lowering voltage will lower power consumption, but there will come a point where the desired operation of the device will not be met.

For example if you had a variable voltage like a dimmer on a light bulb, you can reduce the voltage to a point where the bulb no longer emits a useful amount of light.