The confusing part about these units is that they don't work like other units so when you try to apply what you already know, it just doesn't make any sense. The electricity bill tells us we have spent an amount of electricity quantified as kWh, but normally when we hear about "hour" in units we think of things like "kilometers per hour." It would be like saying that your electric car consumed "30 kilometers per hour" at the end of the month. What does that even mean? On the other hand, it's also impossible to understand the Watt. The Watt is supposed to be rating how much energy an appliance consumes, but it doesn't tell us in what frequency it consumes that amount of energy. It would be like saying that a car is "1 gallon." Is that gallons per hour, gallons per minute, or gallons per second? Of course, since we have some idea of how big a gallon is, it's easier to figure out the answer, but with Watt, we simply have no idea.
Note: because the term "Watt" comes from the name of the engineer James Watt, we capitalize it. In abbreviations, it's always an upper case W, too.
Disclaimer: electricity and physics aren't my field of expertise, I'm simply writing down my understanding from what I read and learned from others.
How the Watt works
Let's start with the simplest unit: the Watt.
A Watt is defined as 1 "Joule" per second [https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf] (page 15, accessed 2024-02-12).
Most articles would explain what a Joule is by this point, but that's not really important. What is important is that Joule is our measurement of force, of power. When we have 1 whole Joule of force being applied on average through 1 second, we have 1 Watt.
When you see a light bulb rated at 10 Watts, what that means is it consumes 10 Joules per second of power, which makes a lot more sense than just saying 10 Watts. Maybe it consumes 5 Joules for half a second, then 15 Joules for the other half. It doesn't matter. If it averages 10 Joules per second, it's 10 Watts.
How the Kilowatt works
The kilowatt is just 1000 Watts.
To convert from kW to W, we just multiply by 1000:
1 kW = 1000 WConversely, to convert from W to kW, we just divide by 1000:
1 W = 0.001 kWHow the Kilowatt-Hour works
A kilowatt-hour measures, confusingly, that a number of kilowatts have been spent DURING a whole hour.
Observe that the name of the kWh unit is "kilowatt-hour." It's not "kilowatt per hour." Since 1 Watt is 1 Joule per second, if that were the case, we would be talking about "1000 Joules per second per hour" which doesn't make any sense. We're not measuring the average consumption through a period of time, but simply the accumulation of that consumption through a period of time.
1 Watt is 1 Joule per second. There is 60 seconds in 1 minute, and 60 minutes in 1 hour. We calculate the number of seconds in an hour as 60 * 60 = 3600 seconds[how to read math]. Essentially, this means that:
1 kWh = 3600 kWWith weight units, we would simply say a "ton" when we have 1000 kilograms.
1 ton = 1000 kgYou can think of it as 1 kWh meaning a "ton" of kilowatts, except the factor is 3600, not 1000.
Another way to put it is:
[...] a kilowatt-hour is 3.6 million Joules [...]
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/houseenergy.html (accessed 2025-02-12)
Since 1 killowatt means 1000 Joules were applied over one second, 1 kilowatt-hour means 3.6 million Joules were applied over one hour.
Examples for Calculating Electricity Bills
Let's see a couple of examples to understand the concept better.
Light Bulbs
Let's say we have a light bulb that is rated at 10 Watts. We leave it on all the time because we're too lazy to flip a switch to save the planet. Just as we have the kilowatt-hour, we can also have a Watt-hour (Wh) for our light bulb that consumes so little electricity.
If it's rated at 10 W, it consumes 10 Joules per second. If we leave it on for a whole hour, we say it has consumed 10 Wh. In other words, it consumes 10 Joules per second for a whole hour.
How many hours do we have in a month? Let's say a month is 30 days, because it often is and that makes math easier. There are 24 hours in a day, and 30 days in our month, so 24 * 30 = 720 hours per month.
Our 10 W lightbulb consumes 10 Wh if we leave it on for an hour. If we leave it on for a whole month, it consumes 10 * 720 = 7200 Wh. We can convert this into kWh by dividing it by 1000: 7.2 kWh.
This means that if we have a 10 W light bulb, which is the energy efficient LED light bulb, not the 60 W incandescent ones, and we leave it on all the time, it's going to show up as 7.2 kWh on our electricity bill.
Appliances
Appliances consume a lot more electricity than light bulbs, but they are also powered for shorter periods of time.
For example, a dishwasher may be rated at 1.8 kW, which means it consumes 1800 Joules every second. Let's say it takes 1 hour for the dishwasher to wash the dishes. That means every time we use the dishwasher, we consume 1.8 kWh or 1800 Joules per second on average for a whole hour.
Note: dishwashers don't consume a constant amount of energy. They spend different amounts of energy pumping water, heating it, and spraying it, and they don't do all three things all the time, so it's often not possible to have a precise figure for how much energy they consume.
We don't wash dishes every day. To make the math easier, let's say we have enough space for 10 dishes, and we just wait until the dishwasher is filled with dishes to use it. We use 2 dishes per day, one for each meal, so it takes 5 days to use the dishwasher once. With our 30 day month, that means the dishwasher is used 30 / 5 = 6 times per month.
If we multiply 1.8 kWh by 6 times, we get 10.8 kWh, which is how much this dishwasher would consume through a whole month.
Variable Wattage
Some electrical and electronic appliances have knobs and buttons that let you change how much power it uses to do their tasks. I assume these are rated for their maximum power.
A simple example is an air fryer. The air fryer consumes electricity to generate heat. You can have a knob to choose how much heat you want in degrees. Naturally, the less heat, the less electrical energy will be used, so the less the wattage for that specific use.
In this case it only really matters for different dishes, because if we try to cook the same dish with less heat, you'll probably just leave it in the air fryer for longer, and that won't save any electricity.
Another important thing to consider is that some electronics like TVs have a LED that is lit when the device is plugged in an outlet even when the device is "off." This LED consumes very little electricity, so it's just not worth considering at all. That little light on the TV isn't wasting a lot of electricity compared to the amount of electricity that is consumed when the TV is actually displaying things on its screen. It's just not worth worrying about it.
Graphics Cards & Computers
Since this is a website about computers, we can't forget to talk about graphics cards and other internal computer parts somehow.
Generally speaking, processors like CPUs and GPUs do not consume energy to do their work. That's because all processors do is process data, and to process data you don't need to convert energy into heat, light, or force. You will need to do that in order to display the data on a monitor or on speakers, but you don't need to do that in order to just process the data.
The only thing processors care about is the voltage, the flow of electrons, that they use to represent the state of their algorithms. If the processor was made out of superconducting materials, theoretically it could process data without using any energy at all. Electrons would come in and come out with the same amount of energy, not having spent it to do anything, because the processor doesn't need their energy for anything.
However, processors are made out of normal conductors, and normal conductors have a resistance. When electrons encounter resistance, they "slow" down, i.e. part of their energy is lost. But energy can't just vanish from the universe like that. The energy must have been converted into something else. In this case, to heat.
So the processor doesn't consume energy, but having electricity pass through it wastes energy, which becomes heat.
Semiconductors allow electronics to conditionally open and close electric circuits inside of them. The more circuits that are open, the larger the area electricity will pass through inside the circuit, which means the more resistance it will encounter. In other words, the more processing that is done, the more energy that will be wasted, because electricity is just going more places inside the CPU.
This heat can raise to over 100 degrees Celsius, and at that point it will damage the processor and other electronic components inside the computer, meaning that we need a way to cool the processor to prevent it from just burning all the wires and plastic connectors around it. To do that, we need a cooling system, such as a cooling fan or a water cooler.
These systems consume electrical energy in order to work.
Basically, this means that we need to spend energy to fix the problems caused by the wasted energy when the actual thing doesn't even use any energy.
As you can imagine, this means it's very difficult to figure out how much electricity a computer consumes. Not only this depends on what the computer is processing at a given moment, but it also depends on the current room temperature.
If you want to measure how much energy your computer consumes precisely, you might need a device that can measure how much power a device is pulling from an outlet.
Although not practical, let's consider how we can make a computer spend less electricity to have a better idea.
First, we can generate less heat by processing less things. However, if we are processing things, we're doing it for a reason, and we need to process them eventually, so "just don't process it" isn't really an option. The only thing we can do then is change whether we process more things simultaneously or not.
If we process more things at once, we generate more heat at once, so we might reach critical temperatures faster, which will require more energy to cool it down. On the other hand, if we process less things at once, we're still wasting some electricity per cycle (each time electricity runs through the processor circuitry and loops around), but heat will dissipate in the air even if we don't spend energy to cool it down. It's not clear which is the best option. Maybe someone has measured it, but I haven't.
Second, and perhaps more practically, we can spend less on cooling by processing more data when the room temperature is lower. For example, the temperature difference between day and night can be around 10 degrees Celsius where I live, and between Summer and Winter that difference would be much greater. Although it's not very sensible to wait until Winter to process things, it's possible to save energy by simply not doing intensive tasks with the computer at the hottest hours of the day.
This is something that you might want to consider if you have the choice because the heat will also lower the lifespan of your electronic components.
Again, this isn't very practical advice, since it's hard to think of any very intensive tasks for consumer-grade hardware that can just be postponed like that. Watching videos doesn't consume that much electricity these days, so the only thing I can imagine is if you are rendering videos from a video editor, generating fractal art, or using something like Stable Diffusion to generate images with AI, or to train an AI model. You could include playing graphics-intensive games as well, but if you have a job, you probably can't play during daytime anyway, and if you can and want to, you probably won't want to postpone that to night time just to save some energy.
Solar Panels
If you have a solar panel installed on your house, you can generate power instead of consuming it, which is pretty nice. Solar panels are rated based on how much electricity they can generate, not consume.
A solar panel can generate around 2 kWh of electricity per day, which is pretty good actually since it can completely pay for the energy that some devices use.
If we take our 10 W light bulb from before, it consumes 10 Wh per hour, 240 Wh per day, or 0.24 kWh per day. This means we can have 18 light bulbs on all the time for free with just one solar panel.
Likewise, we get to use our 1.8 kWh per use dishwasher for free once if we have a solar panel. Since we don't use it every day, the other days we will have extra energy, and that's problematic since if we don't put in a battery or do something with it, it's going to be wasted because we haven't used it for anything.
The way solar panels work is very awkward for computers. The energy generated by a normal power plant is Alternating Current (AC), which means we will have a turbine with a rotating magnet that moves electrons back and forward dozens of times per second, and that makes the voltage oscillate from positive to negative.
A computer needs those electrons to go only one direction so its algorithms carved in the electronics actually work, and this means it needs to make the voltage always one direction. We do this using an electronic circuit called a rectifier that converts the Alternating Current (AC) to Direct Current (DC) which is always a positive voltage instead of an oscillating one.
The funny thing is that solar panels do not generate an AC current, because there is no turbine. The sunlight reaches the panel without oscillating anything, so the electrical current is a DC one. I don't know a lot about electricity, but I assume you could just feed this directly into an electronic device without a rectifier. However, we have lots of devices in our homes that use AC current instead, so the solar panel's energy needs to be converted from DC into AC using a device called an inverter.
This means that for my PC to use electricity from a solar panel, the DC current is inverted into AC then rectified into DC back again. I don't know but this sounds a bit wasteful for me. But I'm no electrical engineer, though.
The solar panel is setup so that the energy generated by the power of the Sun flows naturally into the house's appliances, and if that energy isn't enough, energy from the power plant will be pulled in as well. There is no automated switching happening. This is a natural process like having a plastic cup with water in it and oil on top and making a hole at the bottom. When one runs out, the other starts coming out.
As one would expect, solar panels only generate energy when the Sun is out. The ones in my home generate from 6 AM to 8 PM, gradually raising in efficiency through the day. The extra energy that goes unused I mentioned earlier is bought from me by the electricity company in the form of credits, which I can spend to pay my electricity bill when the solar panels doesn't cover everything.
I was told the credits expire after 5 years, so in my case if I were generating far more energy than I was consuming, I wouldn't be able to use those credits, and the electricity company would get my electricity for free.
For this reason, when purchasing solar panels it's important to know how much electricity you spend. A single solar panel is going to save you money in the long run, but having too many solar panels than might not be a good idea depending on how things work where you live. Always a good idea to consult some expert in your region about it.
Calculating how much solar panels will save you money is a bit difficult since the price per kWh varies a lot, and you can get hit with extra fees for consuming too much electricity, or too much electricity during critical periods of the day, or of the year. Well, those details actually favor solar panels since you don't have to worry about these random extra fees anymore.
I'm going to use my electricity bill as an example.
It says I pay around R$1.25 per kWh. I'm not sure, but I think the panels cost around R$10000. The inverter connects to the Internet for some reason, and there is an app, for smartphone, that lets me view how much they generate. By the way, I was told there is no way to access this information on the desktop, you know, through a website, in a web browser, although the company that installed the panels can see the reports through the website. They just don't let consumers view it through the website, it must be through the smartphone app. I can't help but feel like this didn't even need Internet and my inverter is going to get hacked like IoT things usually are and become part of a solar powered botnet these days. But I digress. Anyway, it says the panels generate around 20 kWh per day.
Since I pay R$1.25 per kWh, and they generate 20 kWh per day, they're saving me R$25 per day (optimically). I paid R$10000 for them. R$10000 divided by R$25 is 400. Assuming it keeps generating this exact amount of energy, these panels will pay for themselves in just over 1 year (365 days) and 1 month.
Unfortunately, things aren't so simple. It's Summer right now, so more electricity is being generated from the ball of fire in the sky. I've asked the company about this, and they've shown me that through the year the production of electricity almost halves when Winter comes around compared to Summer.
Even at this slower rate, in a few years it will have paid for itself, so there is no question that this is a good investment, the only question is when the returns will start happening, and if there is a risk that they just break somehow before they pay for themselves.
A simpler way to look at this is that since they are making R$25 per day, they will make R$750 per month. This is more than my electricity bill, so during the warm months the solar panels will just pay for the whole bill, and perhaps give me some credits that can be spent during the cooler months, or when it's cloudy.
Observation: I'm not sure, but I think the credits are worth less than just using the energy directly from the solar panel, in which case you save more money by using the electricity while the panels are generating energy. In that case, we have a dilemma here, because the panels generate more energy during the hottest hours, but computers consume more energy during the hottest hours. Personally, I think it's a good idea to avoid the heat even if you have solar panels since the heat reduces the lifespan of the computer parts, and that is a hidden cost that can sometimes be very costly compared to just the electricity bill.