I was once confused about this as a rookie and I've recently had some friends that were confused about it. I just wanted to help clarify it for others. I live in the US, so this is how I calculate my electricity costs.
The simplest method to find your actual cost per kilowatt hour, (kWh,) is to divide the total monthly electric bill, (amount due,) by the number of kilowatt hours charged in the billing cycle.
Example:
My total household electric bill for last month was $143.15 (US).
You can subtract the meter readings from one another to find the number of kWh's used. Sometimes bills are estimated, but the number of hours billed should be clearly stated. Mine was 1142.
$143.15 ÷ 1142 = $0.12535
NOTE: Don't make the mistake of using the "Price to Compare" as printed on the bill. This may not include distribution charges, taxes, and fees.
In order to break out how much your mining rig is truly costing you, (or your mom,) you first need to know the wattage draw of your rig. For example, I have 3 GPU's, (1 GTX1070 and 2 GTX1080's). I have a wattage/amp meter plugged into my mining computer that displays Watts. It seems to always read between 600 and 700 Watts, depending on which coins I am mining. I'll use 650 watts as an average per hour. You can use published numbers from hardware components as long as you include all hardware in the rig.
Take the constant number of Watts being drawn from the wall outlet and divide it by 1000. This converts Watt Hours into Kilowatt Hours (kWh). Then multiply that by 24 hours per day, then by 30 average days per month, then by your cost per kWh.
In my Example:
650 ÷ 1000 = 0.65 (kWh)
0.65 X 24 = 15.6 (kWh per day)
15.6 X 30 = 468 (kWh per month)
468 X 0.12535 = $58 (actual cost per mo.)
We can conclude that if you are not mining your electricity cost, (or more,) in coin each month, you are mining at a loss. Mining at a loss, such as in my case, can still be justified if your plans are to hold all coins until the market recovers. That profit may come at a later date.
Hope this helps.