I have problems understanding the difference between electrical power and consumption. Can you help me understand what electrical energy consumption is?

Let's say that I have a bulb, on which is written 100 W. How much does this bulb consume in 1 hour? Can it have a power lower than 100 W? Can it have a power greater than 100 W?

I would like to know what is the relation between electrical power and electrical consumption.

I'm very sorry for the big disambiguation from my head. But I don’t know who else to ask. I hope that you will not consider my question as stupid, even if is so.

  • \$\begingroup\$ The monitor and laptop questions should be in a separate question. But since that isn't about electronics design, I removed them rather than closing the question. \$\endgroup\$ – Brian Carlton Oct 22 '12 at 17:40
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    \$\begingroup\$ Actually it's too bad the monitor and laptop questions were removed, as devices with switching power supplies illustrate a counter example where unlike with the lightbulb the power consumption probably does not depend very much on the line voltage, but only on the needs of the load. \$\endgroup\$ – Chris Stratton Oct 22 '12 at 18:03
  • \$\begingroup\$ use this.. all ur questions will be solved.. bijlibachao.com/Electricity-Bill/… \$\endgroup\$ – user29116 Sep 16 '13 at 13:38

The issue you're confused about seems to be the difference between power and energy.

Energy is how much work you can do. Common units are joules or watt-hours.

Power is how fast you do work. It's a rate of change. Common units are watts or horsepower. Horsepower is probably an instructive unit to consider. Say you wanted to move a large pile of straw. Whether it's moved by a horse or a housecat doesn't affect the amount of work done. But the horse does it faster, because it's a more powerful animal.

For the purposes of discussing grid electricity consumption, watts (W) and kilowatt-hours (kWh) are the most common units used. To know how much energy is consumed, multiply the power by the time. 100 W x 1 hour is 100 watt-hours, or .1 kWh. In short, the relationship between power and consumption is time.

A 100W bulb consumes 100W assuming the voltage across it is what's specified on the package, which is usually 120V in my experience. If the voltage at your socket is lower, the bulb will consume less power. It's approximately a fixed resistance, so the power consumed is $$ P=\frac{V^2}{R} $$

As an aside, remember energy conservation. If something consumes 100W, that energy is being converted to some other form. Either it gets stored (potential energy), it's used (light, motion, etc.), or it's wasted as heat. For an incandescent bulb, ~90% of the power consumed is converted to heat. So a 100W incandescent bulb consumes 100W, but only outputs 10W of light. It gets hot because the other 90W is being wasted. Which is why CFL's run so much cooler and consume less power for the same light output.


The others already told you that energy is power \$\times\$ time. And that 1 kW during 1 hour is 1 kWh. A year has 8765 hours, then a device which is in standby most of the time will consume 8.76 kWh in a year if it consumes 1 W in standby.

The kWh is a practical unit for electrical power, but the SI unit is the joule ("J"), which is much smaller. A joule is the energy consumed by a 1 W device in 1 second.

\$ 1 J = 1 W \cdot s \$

So a kWh is 1000 W \$\times\$ 3600 s = 3.6 MJ (megajoules), so that's not so practical, especially when we talk about industrial machines consuming tens of kWh; you would soon be talking about GJs (gigajoules). That's why the kWh is accepted as alternative unit.

(An older unit for energy is the calorie, where 1 cal \$\approx\$ 4.2 J. An odd number, but it's the energy needed to heat up 1 g of water by 1 °C (from 14.5 °C to 15.5 °C). You can do that by adding 4.2 W during 1 second, or 42 W during 100 ms.)


The power rating on the light bulb is the rate at which it consumes energy. How much energy is consumed is a function of how long it is turned on. Energy = Power × time. If it is on for one hour, it will consume 100 Watt Hours. This is the unit that the electric company bills.


When first starting to learn physics (or most any topic,) here's a metal trick: always cross out the complicated terms, and replace them with their 'unpacked' definitions. Simple, right?

This greatly helps in understanding flow rates. Don't use single units, instead give the quantity-per-second names. For example, the word "Current" means charge-flow. So (at least in your early years) make it your habit to never say current. Always change it to "charge flow." And never say Amperes, because the ampere is really just the charge-flow rate in coulombs-per-second.

The answer to your question may become obvious if you stop saying the word "Power." If the word has a fuzzy definition in your mind (or, a wrong definition,) then your question may answer itself once you habitually insert the correct meaning into all sentences. "Power" means energy-flow. It's the rate of energy-transfer, or energy propagation across a distribution network (including electrical, and drive belts, and drive shafts.) Don't say Power, say energy-flow. And similarly, stop saying Watts. Watts are really just the rate of energy-flow in joules-per-second.

Let's say that I have a bulb, on which is written 100 joules/second. How much does this bulb consume in 1 hour?

See what I did there?

One hour is 60min or 3600 seconds. So the bulb consumes 3600 X 100 = 360,000 joules of energy per hour.

  • \$\begingroup\$ +1 This is a really good answer - I like how you almost converted the question to a tautology :) \$\endgroup\$ – Morten Jensen Jul 3 '18 at 15:39
  1. 100 Wh (0.1 kWh). Energy is power x time, so W x h = Wh.
  2. The 100 W is assuming the usual household voltage. In the US that is fixed 120 V. This only varies slightly, so the power may be a bit above or below 100 W

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