Most household electrical appliances have a label or metal plate that shows the number of watts. This label is usually found on the base or back of the appliance and indicates the maximum power consumption. To estimate the amount of total energy your device uses, you need to convert this into kilowatt hours (kWh).
Steps
Method 1 of 3: Estimate Kilowatt Hours Considering Appliance Label Data
Step 1. Find the electrical power of the device
Appliances that consume a lot of energy typically have a specific label that is placed behind or on the base of the item. Find the electrical power shown on this label; generally the data is indicated with the letter "W". Usually the number corresponds to the maximum power absorbed by the device in operation and could be well above the real average value. The steps below will help you estimate kilowatt hours from this number, but remember that the actual consumption of the device is typically lower.
Some electrical appliances report a power range, such as "200-300W". To get accurate results you should consider the average value between the two extremes; for this example, the number to consider is 250 W
Step 2. Multiply the watts by the number of hours per day the device is in operation
Watts measure power, i.e. the electrical energy absorbed. If you multiply this value by the unit of time, you will know the amount of energy consumed, that is the important data for the bill.
- Example: A large window fan absorbs 250W and runs an average of 5 hours a day. The daily consumption of the fan is equal to: (250 watts) x (5 hours a day) = 1250 watt hours per day.
- If you have to calculate the consumption of the air conditioning and heating system, consider one season at a time.
- Refrigerators actually only draw energy 1/3 of the time, which is about 8 hours a day, when they are never unplugged.
Step 3. Divide the result by 1000
One kilowatt equals 1000 watts, so this step is essential for converting watt hours to kilowatt hours.
Example: you have calculated that the fan absorbs 1250 watt hours per day; (1250 watt hours / day) ÷ (1000 watts / 1 kilowatt) = 1, 25 kilowatt hours per day.
Step 4. Now multiply the result by the number of days you want to consider
At this point you know the kilowatt hours (kWh) absorbed each day by the appliance. To find out how many it consumes in a month or a year, just multiply the figure by the number of days taken into consideration.
- Example: in the course of a month consisting of 30 days the fan should consume (1, 25 kWh / day) x (30 days / month) = 37.5 kWh per month.
- Example: if the fan worked every day of the year, then it would consume (1, 25 kWh / day) x (365 days / year) = 456, 25 kWh per year.
Step 5. Multiply the cost of electricity by the kWh
You should find this on your electricity bill. At this point you just have to multiply the cost of a kilowatt hour by the number of kWh absorbed and you can estimate how much you will have to pay.
- Example: if electricity is priced at 17 cents / kWh, running the fan will cost you (0.17 euros / kWh) x (456.25 kWh / year) = 77, 56 € per year (value rounded to the nearest hundredth).
- Remember that this estimate is based on the data reported on the appliance label and that these represent the maximum absorption. In reality, the bill should be lower.
- To obtain accurate data, always check the cost of the kilowatt hour shown on your bill or call the customer service of the operator in your area.
Method 2 of 3: Calculate the Kilowatt hours Starting from the Current Intensity and the Difference in Potential
Step 1. Find the current drawn by the appliance
Some labels do not indicate watts; in this case you have to look for the value of the amperes, indicated with the symbol "A".
The battery chargers of laptops and mobile phones can report two amperage values: it uses the data that refers to the intensity of the incoming current
Step 2. Find the potential difference used in your country
In the United States and a few other states, civil electrical systems have a voltage of 120 V; in Europe and in the rest of the world the domestic voltage is between 220 and 240 V.
If you live in the United States, remember that some large appliances, such as washing machines, need to be connected to particular 240V circuits. Always check your device's specifications for exact voltage ratings. The label usually shows the recommended potential difference, but the appliance installation by a professional technician is assumed to meet all criteria
Step 3. Multiply the number of amps by the number of volts
In this way you get the watts, that is the electrical power.
Example: A microwave draws 3.5A of electricity when plugged into a 220V outlet. The appliance then consumes 3.5A x 220V ≈ 780 W.
Step 4. At this point multiply the data obtained by the hours of the day in which you use the appliance
The power figure only indicates how much energy is consumed by the active device, so you must multiply it by the daily hours of use.
Example: if the microwave is active for half an hour a day, then multiply 780 W x 0, 5 hours / day = 390 watt hours per day.
Step 5. Divide the data by 1000
This will convert the number to kilowatt hours.
Example: 390 watt hours / day ÷ 1000 W / kW = 0, 39 kilowatt hours per day.
Step 6. Now you can multiply the number of kilowatt hours consumed over a longer period of time
If, for example, you want to know how many kilowatt hours will be billed in a bill that considers 31 days of supply, you will have to multiply the result by 31 days.
Example: 0, 39 kWh / day x 31 days = 12, 09 kWh.
Method 3 of 3: Using a Counter
Step 1. Buy a counter online or in the most well-stocked hardware stores
It is an instrument that measures the real amount of energy absorbed by an appliance. This is generally the most accurate method of knowing the electrical absorption of the device compared to the information on the label.
If you know how to use electrician tools, you can also use a multimeter. In this case you must have access to the appliance wiring while it is connected to the system. It goes without saying that you don't have to take anything apart if you don't know what you are doing
Step 2. Insert the counter between the socket and the plug of the appliance
First plug the instrument into the wall socket, then insert the plug of the electrical device into the meter.
Step 3. Measure the kilowatt hours
Set the counter to calculate this value; as long as the device remains connected to both the system and the appliance, it will continue to count the kilowatts consumed.
- If the meter only measures watts, you will need to use the method explained above to convert the data into kilowatt hours.
- Read the meter instruction booklet if you don't know how to change the settings.
Step 4. Use the appliance as usual
The more you leave the counter in action, the more accurate your calculations will be.
Step 5. Find monthly or yearly consumption
The kilowatt hours indicated by the meter are cumulative, i.e. the data refers to all the energy absorbed since you put the device into operation. You can multiply the figure to estimate the kilowatt hours consumed over a longer period of time.
For example, if the meter has been running for 5 days and you want to find the estimated consumption for 30 days, divide 30 days by 5 and you get 6. Now multiply the kilowatt hours indicated by the meter by 6
Advice
- If the label doesn't indicate watts consumed, check the instruction booklet. The labels that are currently affixed to electrical appliances, such as the white and blue ones of the European Community and the yellow Energy Guides widespread in the United States, provide all the information. Generally, the annual consumption of kilowatts is indicated with the words "kWh / year", "kWh / annum"; these data refer to standard domestic use and are generally more accurate than the calculations you can make yourself.
- Some devices may be set with different power. In this case, the label may show the consumption data for each setting, or only the maximum one.
