In a previous blog, we talked around the result that different waveforms have actually on RMS vs. Optimal voltage. Now we’re going come look in ~ the implications of that as soon as it concerns power consumption. For purposes of discussion, we are going to usage an old-fashioned irradiate bulb with a filament – miscellaneous that’s less complicated for most of us (especially me) to know than the new-fangled irradiate bulbs that present a little different picture (a subject for another blog). Friend may have noticed that light bulbs have not only a wattage rating but a voltage rating as well. We don’t think about the voltage rating together much since we space usually talking just of a voltage rating that 120 volts. For this reason what does a bulb rating the “100 Watts” mean? precisely this – – as soon as a specific voltage (120 volts for a typical light bulb) is applied across the filament, the resistance the the filament will attract a present of .83 amps consuming energy at the price of 100 Watts. Doing a little more math, R(Resistance)=E(Voltage)/I(Amps) we deserve to calculate the the resistance that the filament as soon as operating at 100 watts is around 145 ohms. It have to be detailed that this is the resistance at the operation temperature together the resistance does change (gets higher) together the filament heats up but, because that now, let’s speak it is addressed at 145 ohms to keep things simple. This is reasonable because the resistance that the filament relies on the temperature and the temperature go not rise or diminish appreciably over a 1/60th of a 2nd cycle.

You are watching: How many amps does a 100 watt light bulb use

Note – A light bulb rated at 60 watt at a voltage the 120 volts has actually a various filament resistance come consume 60 watts instead of 100. The filament in a 60 watt bulb has actually a resistance that 240 ohms as contrasted to 145 ohms because that a 100 watt bulb.

The above assumes that the voltage source is consistent at 120 volts. But, when the source is a sine wave, the voltage is not consistent at 120 volts. One RMS voltage sine wave resource varies between minus 170 volts and also plus 170 volts. A resistive pack produces the same present flow despite polarity. A confident voltage produce the exact same amount strength (amount the light) together the same an adverse voltage in the case of a light bulb. Thinking a tiny further though, since the voltage is varying and also the resistance of the filament in the bulb stays the same, the power intake varies with time with the average power usage being 100 watts.


Current flow (amperage) varies v the voltage applied across the filament. As soon as there is no voltage (at the beginning of the cycle), there is no strength consumption. Usually the bulb is off. As the voltage increases to 120 volts, the strength increases appropriately to 100 watts. Together the voltage walk even higher (the optimal of the sine wave) the reaches a optimal of about 170 volts. Due to the fact that the resistance that the filament doesn’t adjust (145 ohms) the current at 170 volts is 1.17 amps. The resultant power intake at that immediate is just short of 200 watts. For this reason the irradiate bulb, rated at 100 watt at 120 volts is actually consuming energy at the price of 198.5 watt at the peak of the voltage cycle!


In researching because that this blog (I carry out research these things) I uncovered a video clip that illustrates the above. The filament never goes totally dark in the video because the its heat inertia but the pulsations in brilliance room evident.

See more: What Do People Celebrate At The Equestrians Of Managua Festival

The “take away” right here is the a 100 watt light bulb is rated at 100 watts only once operated at its plan voltage (direct current or RMS). Reduced voltage outcomes in much less power while greater voltage results in higher power unless the resistance the the filament is changed. A 100 watt light bulb intended for operation at 120 volts is no longer rated at 100 watt if the voltage changes!

Note – Yes, this is precisely why “long life” irradiate bulbs that the previous were rated 100 watts at 130 volts instead of 120. Once operated in ~ 120 volts, they to be dimmer – no consuming a complete 100 watts. This resulted in a much longer operating life since the top operating power was significantly reduced.