## So How Much Electricity Does My Computer Consume?

by AndrzejL

I was looking for an answer to this question for a while now and thanks to some websites, and thanks to my colleagues on the #pclinuxos-pl channel (a Polish language IRC channel for the discussion of PCLinuxOS), I finally figured it out. It’s not difficult.

When you are paying for the electricity, you are paying for the amount of kilowatt hours (also called “units”) that you have used in the billing period. This is all great, BUT how do I know how many of those kilowatt hours my machine is using?

This is not very difficult to calculate (approximately).

First we have to know how much electric power your machine needs. Sometimes you know exactly (or you can read on the label on the back of your computer) that your machine has X Watts adapter. This is what you need. I was not so lucky with my laptop. The label on the adapter states:

Output: 16V, 4.5A

I had to calculate the power (watts) myself. To do so, I had to use this formula:

P(t) = V(t) * I(t)

where:

P(t) is the instantaneous power, measured in watts.

V(t) is the potential difference (or voltage drop) across the component, measured in volts

I(t) is the electric current, measured in amperes

Ok, so the output values of 16 volts and 4.5 amperes multiplied by each other will give me the power (watts) of my laptop’s ac/dc adapter:

P = 16V * 4.5A = 72W

My laptop’s power pack uses 72 watts. This is a very simplified/approximate value. Why? Because it’s the maximum power that the power pack can provide when the laptop is using 100% of it. This means screen is on and on full brightness, WiFi, Bluetooth and all other devices are on.

What can I do with those watts then? I can convert them to kilowatts. How? Divide by 1000. This means that you take the power of the device in watts and you divide it by 1000:

72W / 1000 = 0.072kW

Now, knowing the amount power in kilowatts and multiplying it by the amount of hours the machine runs, you will get the result in kilowatt hours. Let’s say that my laptop runs 24/7. All the time. 365 days per year. First, I am gonna find out how many kilowatt hours it uses in one day. To do that, I am gonna multiply the amount of kilowatts and the number of hours.

0.072kW * 24 = 1.728kWh

So my laptop is using 1.728 kilowatt hours (maximum) during a single day. My bills are sent to me approximately every 60 days. This means that if I multiply the daily usage times 60 I will get the rough estimate of how many kilowatt hours this machine will eat in one billing period.

1.728kWh * 60 = 103.68kWh

So my machine will consume roughly 103.68kWh in 60 days right? Right. Now if I will multiply that by the current price of the kWh unit, I will know approximately how much money I will have to pay for the electricity devoured by this little devil.

103.68kWh * €0.15 = €15.55

This means that if this machine was running full speed, with a fully bright screen, with WiFi, Bluetooth etc. enabled, 24/7 then it would cost me approximately €16 every 2 months to power it up. This is a very pessimistic estimate. If you use power saving features of the laptop (for example, disabling the screen when it’s not used, scaling CPU frequency down and disabling devices like WiFi or Bluetooth when they are not needed), you can bring that estimate down to one-third of that amount.

You can use the information from this article to calculate the price of electricity used by any other electric device over any chosen period of time. It will work, provided that the device is not faulty and that it does not leak power.

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