Save energy, save money!
Do you have a light in your house that is always on?
Or are you using an electric heater to stay warm in winter?
Have you ever asked yourself:
How much am I actually paying to keep that thing on?
When trying to save energy it is important to know where energy is being used! By knowing this, you can make informed decisions about how you want to change your energy consumption.
Energy vs Power
There is a common mix-up between energy and power. When we talk about energy in the context of energy consumption, we refer to the amount of energy being used over a period of time. When you receive your electricity bill you pay for the energy that you have used since your last bill. Power, on the other hand, is the rate of change in energy.
I always find it helpful to think of it with an analogy to pouring a glass of water.
If you buy a glass of water, you pay for the amount of water in the glass (energy). Not for how fast it was poured (power).
The volume of water is measured in liters and the speed of pouring is liters/second.
In the same way, the amount of energy used is measured in kWh (kilowatt hours) and the speed at which the energy is used (the power) is measured in kW (kilowatts).
Kilo-whatnow?
Okay, I know these units can sound a bit confusing.
Just remember:
- l, liters → kWh, kilowatt hours
- l/s, liters per second → kW, kilowatt
One kilowatt (kW) is equal to 1000 watts (W) which is equal to 1000 Joules per second (J/s). If you are interested in the definition of a Joule you can check here. Otherwise just think of it as a small amount of energy.
A toaster typically uses around 1kW of power. If we leave the toaster on for 1 hour, we will have used up 1 kWh. Pretty convenient!
If we leave it on for 2 hours, we will have used up 2 kWh, and so on.
We could say that:
Time (h) x Power (kW) = Energy (kWh)
So if we take a bigger toaster that uses 2 kW but only leave it on for half an hour, we will end up using...you might have guessed it... 1 kWh!
Now that we know how to calculate the energy used by our toaster we can find out how much it is going to cost us.
Most electricity companies charge their customers in cents/kWh. So if the price was 50c/kWh and we used a 1kW toaster for one hour, it would add 50c to our upcoming electricity bill.
This might sound cheap, but using multiple devices over time can really add up! Especially heating and cooling can significantly contribute to energy consumption depending on the season.
Let's put our knowledge to some good use!
Let's look at some examples of how we can estimate the electricity cost of various appliances.
For these examples I will assume an electricity price of 50c/kWh.
Sleeping with an electric heater
I am sleeping with an electric heater in my room. The heater uses 800W and it is on for 8 hours per night. I want to know how much this costs me every month.
To find the cost we must first find the energy we are using.
Remember the formula from before:
Time (h) x Power (kW) = Energy (kWh)
The power was given as 800 W which we can divide by 1000 to get the kilowatts as 0.8 kW.
If we have 30 days in a month and use the heater for 8 hours per day. Then we end up with:
8 (hours/day) x 30 (days) = 240 hours/month
The monthly energy can now be calculated:
0.8 (kW) x 240 (hours/month) = 192 kWh/month
Great! Now to find the cost of using this much energy:
192 (kWh/month) x 50 (cents/kWh) = 9600 cents/month = 96$/month
If you dont use cents and dollars replace with your own currency. In Sweden, for example, this would be öre and kronor.
A light that is always on
I have a light in my house that is always on. I have checked the lightbulb which says it uses 10W. How much am I paying every year for this light to be constantly on?
Same as before we can convert the power from W to kW by dividing by 1000.
10/1000 = 0.01 kW
Now to find the time. How many hours are there in a year?
24 (hours/day) x 365 (days/year) = 8760 (hours/year)
Now we can take the power times the hours to get the energy per year.
0.01 (kW) x 8760 (hours/year) = 87.60 kWh
Finally, we get the cost by multiplying the energy with the price per kWh.
87.60 (kWh/year) x 50 (cents/kWh) = 4380 (cents/year) = 43.80 $/year
Great!
Now you have a good understanding of power and energy allowing you to make smart decisions about how you can potentially save some energy and money!
By knowing where the majority of your energy is being used, you have a better chance at making a dent in your electricity costs! Is it enough to run your heater at 500W instead of 1000W? Compare the costs and decide what is best for you.
If you want help with the calculations check out this Electricity Cost Calculator I have created to help!