Understanding your electric bill | kW/h explained.

Originally published on 12/11/2021
Electricity flowing to a bulb

Stay informed and save money on your energy bill. By understanding what and how your electric company charges you at the end of the month, you can use it smarter and therefore save money. Here were gonna clarify and learn how to measure kW/h.

How the electric bill price is calculated?

Your electricity provider measure the electricity your house consumes in kW/h puts a price on that and after knowing how much kW/h you spend they know how much they have to charge you. In top of that many providers have a fixed rate by day, so even in your summer house if you don't spend energy you will pay something for the equipment they provided to you at the beginning of the contract (the device you have on your door), think of it like a rent. And lastly your country may charge taxes that will be charged by them too.

So what is a kW/h?

A kW/h (KiloWatt / Hour) is a unit of energy. One kW/h, breaking it down it is 1000 watts spent over one hour. Simple as that. Lets see examples so you can understand it better: Imagine this two situations:

  • An Electric heater that consumes 1000 watts if you have it turned on for one hour you will spend 1 kW/h;
  • A TV powered with 100 Watts will have to be on for 10 hours for it to consume 1kW/h of energy;
  • An electric grill rated at 2000 Watts left grilling your meat by one full hour will be 2kW/h that you will be charged;

Converting to money on bill

So knowing that you have to do some math to end with the cost of your bill. Let's say i have spent 200 kW/h this month and i pay €0,20 for KW/h, they will get the energy cost by doing this calculation:

200 * 0,20 = €40

Just for sake of reference i live in a house with 2 Adults and 2 kids and i consume about 300 kw/h month. Note that i have a induction cooker and my water heater is not electric powered so don't add nothing to that bill.

Easy isn't it? But you can make it even easier by installing an App that you just insert the Wattage of the device and get energy consumed and price. For Android i recommend eCostsCalc - Eletricity Calculator and if you are more of a Apple user give Electricity Bill iOs App a try (I never tried this one just seems simple enough for our needs). Both apps will give you insights on how much running an appliance will cost you at the end of the day / week / month.

lets see a table with some more examples so you have more insight.

Table calculating the energy cost of multiple examples.

I made a Google Sheet with the formula for making the calculations above. You can tinker with the spreadsheet here changing the wattage will update the results, if you want to to use it just make a copy and save it on your google drive. Or you can use my Electricity Costs Calculator online tool to get the calculation in real time.

How to know how much is the power of a specific device?

So now you know how calculate the cost now the question that floats in the air is how to know how much electricity an appliance uses? Lets dive into that. You basically have two ways one free but more naive so to say and another more scientific. For the first way you can simple check the box, make an web search for the rated power of the device, or simple check the power supply and check if it has some information for that matter. Its handy that the power supply can show the output power as Volts and Amperes just multiply the two to get the watts but keep in mind that those power supply don't run at max power so if it says maximum 100 W probably it never reaches that maximum. For example the laptops are a good example. Mine has a power supply with 100W maximum but hardly reaches that only for gaming or intensive computation normally it runs between 10w / 20w. At the end of the day is this method is not a thing that you can trust. If you want to be more precise you'll have to buy a dedicated device and with that you will be in position to make better decisions and ultimately save more money.

What are the devices that can help me with energy consumption?


Don't forget to be careful when handling electric devices.

Non Smart - For measuring precisely the energy used by a device you can use a device that plugs into your wall socket and next you plug the appliance and by the display you can check how much it is using, these are very simple and easy to setup but considering that the device consumption may vary at many times like a refrigerator that will turn on only when it gets warm you have to check periodically to get a good picture.

The Smart Home Approach - Get a smart energy meter to measure the power use by the device. That way you can calculate costs see device consumption history and that all even when you are not home. I think all those smart devices have the ability to allow you to turn on or off appliances remotely.

In my home i've tried a few and i can recommend some that i liked the most. By now i will let you with the shelly plug. its a simple electric plug that you can put in between the wall socket and the device plug that you can get for around 20 USD, has wifi, its very simple to use and handles up to 2500 Watts. The shelly app is very user friendly yet powerfull. I'll leave you with some images of that:

Image of a Shelly Plug SShelly App on Android visualizing my ShellyPlug

I have some Sonoffs too and i have everything synced with my home assistant but i recommend shelly because is more easy to use and integrate with another apps.


You are now ready to check your devices and see if some device is spending you more that you want or need. Being informed is the first step to make good decisions. Even with the existing devices you can use it wisely or even get your home smarter and use the more powerful devices to work when the electricity is cheaper. All the smart plugs / energy meter (at least should) support timers and schedules. Hope you enjoyed this demystification of the electricity bill and the KiloWatt / Hour if you have some question or thoughts please share it the comment section bellow. Thank you.

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