Hi folks…

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 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 is my machine using?

This is not very difficult to calculate (approximately).

First we have to know how much electric power does 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. 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 a maximum power that the power pack can provide when 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 to kilowatts. How? Divide it 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 You will get the result in kilowatt hours. Let’s say that my laptop runs 24/7. All the time. 365 days per year… Ok… 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 during a one 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 with the current price of the kWh unit I will know approximately how much money will I 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 fully bright screen, with WiFi, Bluetooth etc. enabled, 24/7 then it would cost me approximately €16 / 2 months to power it up. This is a very pessimistic estimate. If You use power saving features of the laptop ie. if You disable screen when it’s not used, if You are scaling CPU frequency down and if You disable devices like WiFi or Bluetooth when they are not needed – You can bring that estimate down to 1/3 or even less.

You can use info from this post 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 leaks power.

Cheers.

Andrzej