That was a great, great question. It is very easy to get lost in conversations about watts and amps and volts. I can’t count how many times I have had to review in my head what the basic units are, what they mean, how they convert, and what “typical” usage might be. It is hard to get any intuition about electrification, or a basic idea of what your home might need, if you cannot reason about the basic units.
So this is the first in a three-part series about “Electric Speak”. In this first part I’ll go over what the basic units are and how to think about them. In the second part I’ll talk about the electric panel, both what it does and how to think about it. The third part will cover how to estimate operating costs of new appliances.
Here are just some of the many things that I have found confusing about “Electric Speak”.
- My hair dryer is measured in watts (1500 watts). I think that’s a lot -- I’ve flipped circuit breakers with it on occasion. How does it compare to, say, a mixer, or a fan?
- My refrigerator label doesn’t say how many watts it uses. It says 6.5 amps. My clothes dryer says 21 amps. My EV charger is 40 amps. How can I compare these to my hair dryer?
- My EV battery is measured in kWh instead of watts (kW) or amps. Help?
- A home battery is measured in both kWh and kW. Which do I care about and why?
- Gas seems to have only one unit, a therm. Why can’t electricity work the same way? Well, there’s also a BTU. How do these compare, and how do I convert them to the corresponding electric units? Which one even is the corresponding electric unit?
We shouldn’t need to become electrical engineers in order to understand the basics of fuel switching. But it definitely seems that way. Fortunately, we all have an intuitive sense of how water works, and in a lot of ways electricity works the same way. I hope you can answer all of the above questions by the end of this blog post.
Consider a kitchen faucet. You know a couple of things:
- More water comes out if your house has good water pressure.
- More water comes out if the faucet is all the way open.
- A glass will fill up pretty fast even if you have weak water pressure, as long as the faucet is open. If you have good water pressure, you don’t even need to open up the faucet the whole way. But to fill a big pot for cooking pasta, it’s best if you have good water pressure and the faucet is all the way open.
Now let’s translate that to electricity.
Electricity is analogous to water in some ways. Background image source: freepik
Voltage (measured in volts) is like water pressure. You can think of voltage as being like the water pressure at a house. Just as a utility delivers water to the house at a certain water pressure, a utility also delivers electricity to the house at a certain voltage, namely 240 volts. (1)
In this analogy, voltage measures how hard the electricity is pushing into a home, an outlet, an electrical circuit. Every circuit and outlet in the house could use a full 240 volts, but most step down to 120 volts because they don’t need much electricity. It’s cheaper and safer that way. For big appliances like a clothes dryer or oven, though, we use the full 240 volts.
Current (measured in amps) is determined by the size of the faucet. Bigger faucets can accommodate more water. We can also adjust the faucet lever to allow more or less water. In the electrical world, we similarly design how much current a cord or an appliance can accommodate. For example, bigger cords or thicker metal can often carry more current.
Some appliances will have something like a faucet lever inside so they can dial up or down the amount of current they are using. That way they can save on electricity by using only as much as they need.
Electric current is measured in amps, or electrons per second.
Power (measured in watts) is like the water flow rate. If you multiply the “pressure” coming to an outlet or circuit (volts) by the current that an appliance is drawing (amps), you get the amount of power that the device is using. This is a well-known equation: Power = Current * Voltage (P = IV), or watts = amps * volts. (Power is measured in watts.)
Measuring electric power is like measuring how fast water is coming out of a faucet and into a glass. How fast are we pouring electricity into an appliance? How quickly is an appliance consuming electricity?
Since voltage doesn’t change much in our houses (it’s typically either 120 or 240), amps and watts are often proportional to each other. That is, if a refrigerator uses twice the current of a large television, then it also uses twice the power, as long as they are both plugged into the same kind of outlet. (2)
Energy (measured in watt-hours) is like water volume (gallons). When you pay your water bill, you pay for the gallons that you used that month. It doesn’t matter whether you used them quickly or slowly, all that matters is the total amount of water you used. The same is true for electricity. What matters is how much energy you use, not how many watts at any given time.
To find out how much water came out of a faucet, you take the flow rate and multiply by how long it was flowing at that speed. Same with electricity. To find out how much energy was used, you take the amount of power (in watts) and multiply by how long (in hours) that much power was being used. A high-power device that doesn’t run often could use the same amount of energy as a low-power device that runs all the time in the background.
The unit for electric energy is the uninspiringly named watt-hour. (3)
In sum: You pay your electric bill in terms of watt-hours, or energy used. Appliances use more energy the more hours they run and the more power (watts) they use. Power is a function of current (amps) and voltage (volts). Voltage doesn’t vary much in the context of home energy (it’s either 120 or 240), so power (watts) and current (amps) are closely related.
Let’s go back to our initial list of questions and see if we can answer them.
1. How do you know how much power (watts) an appliance uses?
This is a trick question, because you pretty much just have to look it up. For example, this toaster says it uses 850 watts (in the “specifications” section). This hand mixer uses 275 watts. This fan uses 55 watts. Usually things that generate heat use more power.
2. My refrigerator label says 6.5 amps. Is that more or less than my 1500-watt hair dryer?
Your refrigerator is plugged into a 120-volt outlet, so it uses 6.5 amps * 120 volts = 780 watts. So it uses about half as much power as your hair dryer. It probably runs more often, though, so uses more energy over the course of a day.
3. What about my clothes dryer? It says 21 amps.
Your dryer is plugged into a 240-volt outlet. So it uses 21 amps * 240 volts = 5040 watts. You can think of that as being about 5 kilowatts (5 kW). That dryer has the power of more than three of your hair dryers.
4. And my 30-amp EV charger?
That would be 30 amps * 240 volts = 7.2 kW. EVs with very big batteries will often work with more powerful home chargers so they can charge faster. Some Teslas are designed to use 48-amp home chargers. So. Many. Hair dryers.
5. How come an EV battery is measured in kWh? What is that?
The watt-hour (or kilowatt-hour kWh) is a measure of energy. It’s the amount of power that is used over time. For an EV battery, it is related to how long a car can drive between charges, which is what most people care about. (It’s like the size of a gas tank.) But an EV battery also has a watt (or kilowatt kW) rating, which is the maximum amount of power that the battery can discharge at any given time. (4) For a Chevrolet Bolt, that is about 150 kW. For a Tesla Model 3, that is a little over 200 kW. So a 0-60 mph test would be faster for the Tesla than for the Bolt, because the battery can generate more power.
If an EV has a 60 kWh battery, and it is designed to work with a 30-amp home charger, then how long does it take to charge? We know the charger is using 7.2 kW of power (see 4 above). That means it will be "pouring" in 7.2 kWh of electricity every hour. So that EV will take 60 kWh / 7.2 kW = 8.3 hours to fully charge.
6. How come home batteries have both kWh and kW specifications?
It’s for the same reason. The kWh measure indicates how much energy the battery can store. That is related to how long it will last (e.g., through a power outage). The kW measure indicates how much power the battery can generate at any given time. That is related to how many appliances can be running at once when powered by this battery. Both of the metrics matter when you are looking for a home battery. How much are you going to plug into it, and how long do you want it to last?
A solar roof has a kW measure -- how much power it can generate (e.g., based on how many panels with how much power). But it also has a kWh measure, which is how much energy it might generate over the course of a day, month, or year. That will depend on how many hours it is generating power (e.g., sunny hours per panel), and also how much power each panel can generate (e.g., angle of the sun relative to the panel). Again, both of these matter, though I’d say that matching your energy use (kWh) matters more.
7. How does the gas unit, therm, relate to these other measurements?
A therm is a unit of energy, just like a kWh. In fact, one therm is equal to 29.3 kWh. So if you use 4 therms of gas on a typical winter day, that is the equivalent of 117 kWh. But if you converted to appliances that are 3.5x as efficient as their gas counterparts, then they will use only 33 kWh.
I hope this helps more than it confuses. It’s not easy. Lots of people confuse measures of energy and power, for example when discussing the size of renewable energy farms. I try to get it right in this blog. If you have a different way of thinking about this, please share in the comments.
Next week we’ll use these basics to think about electric panels and how big they should be.
Notes and References
1. It’s important that voltage on the grid doesn’t deviate much; utilities work hard to keep the voltage stable. A “brownout” is when the voltage drops too far.
2. Since a device can use different amounts of current over time, it can also use different amounts of power over time. You may be familiar with a car using a lot of power to start moving, and then less as it drives steadily. A variable speed motor draws different amounts of power depending on how much work it needs to do. Many heat pumps (e.g., mini splits) work this way as well, so they use only as much energy as needed.
3. Energy is often measured in terms of joules. A single watt-hour is equal to 3600 joules.
4. Another unit for power is horsepower (hp). 1 kW =~ 1.34 hp. Is this based on some standard horse? I have no idea.
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