Why is understanding volts, amps, and watts important?
Understanding volts, amps, watts, and how they relate to each other is the foundation of every perfectly-sized solar electric system. Every decision made will be based on this understanding, so it’s important that you’re crystal clear about what these terms mean.
In order to size your solar electric system, you will need to know the following three things about each device that you plan to power with your system:
- how many volts the device requires to run
- how many amps the device draws
- how many watts the device consumes
How will this information be used? Well, first, you’ll need to make a list of all the devices that will be powered by your system. Then, you’ll use this list to figure out what size your battery bank should be, and how many watts of solar you need. Finally, you’ll use the list to figure out which kind of inverter to buy.
These might sound like simple decisions, but all of these decisions involve spending heaps of money. You’ll be purchasing hundreds of dollars worth of gear—maybe even thousands, and it’s important to buy the right gear the first time around because solar gear is often not returnable.
Volts: Technically speaking, voltage is a unit of electrical potential. In laymen’s terms, volts is a measure of “push.” 12 volts is much “weaker” than 120 volts.
Amps: Amps is short for amperes. Amps measure the rate of flow of electric charge; one amp is the equivalent of one coulomb per second. In laymen’s terms, amps refers to the amount of charge a device draws. For our purposes, knowing amps by itself doesn’t mean much unless we also know voltage, and you’ll see why in a minute.
Watts: Watts is a measure of power. Power is the measure of the rate at which
energy is consumed. One watt is equivalent to one joule per second (a joule is a unit of energy).
These definitions will make more sense after you start reading through examples of how to apply these terms.
The relationship between these three terms is:
Volts x Amps = Watts.
Understanding the Volts x Amps = Watts equation
There are 2 key points that will help you understand this equation.
- The amount of power a device draws (watts) never changes (unless is has multiple power settings).
- It is possible to change the voltage of the electrical system that is providing power to a device. Changing the voltage will, in turn, change the number of amps that the device draws.
For example, I have a refrigerator that will run on either 12 or 24 volts and uses 60 watts. No matter which system I plug the refrigerator in to (12 or 24 volts), it will always use 60 watts. However, the number of amps that it draws varies with the voltage.
The formula that explains the relationship between volts, amps, and watts is as follows:
Volts x Amps = Watts
So, if the refrigerator draws 60 watts, we can plug that into the equation:
Volts x Amps = 60 Watts
Now, let’s say we’re powering this refrigerator with a 12 volt system. We’ll plug in 12 for volts.
12 Volts x Amps = 60 Watts
Amps = 5
By doing a little simple algebra, we know that this refrigerator will draw 5 Amps on a 12 Volt system.
Are you with me so far? Good.
Now, let’s say we plug the same refrigerator into a 24 volt system (remember, the fridge uses 60 watts). Now the equation looks like this:
24 Volts x Amps = 60 Watts
Amps = 2.5
Again, we simply do a little algebra to find out that now the refrigerator will draw 2.5 Amps.
If you need a little help visualizing these numbers, here’s a chart listing the volts, amps, and watts for this example.
Note that the amps changed because the voltage changed, but the wattage stayed the same. Since the refrigerator does not have power settings, the wattage will never change. (However, for an item that does have power settings, such as an electric blanket that has a switch for low, medium, and high, then yes, the wattage will vary with those settings. The wattage listed on the package is the maximum wattage.)
Why did I take you through this example?
There are a number of misconceptions about voltage, and how it relates to power. I’ve actually had a solar professional tell me that a 48 volt system will provide more power to my appliances. Not true. Power is measured in watts. The number of watts any appliance draws will be the same no matter the voltage (given that the appliance doesn’t have multiple power settings).
I’ve heard solar “professionals” say that 48 volt systems are better because 12 volt systems aren’t powerful enough to run 120 volt appliances. Also not true. Whether your system is a 48 volt system or a 12 volt system, you can certainly run 120 volt appliances, and you’ll need an inverter to do so.
The last big lie that I’ve heard is that 24 volt batteries are twice as powerful as 12 volt batteries. This can be true, but as a blanket statement, it’s very much false. In order to make any sort of comparison, you also need to know how many amp hours a battery contains. Remember, power is measured in watts.
If you understand the true relationship between volts, amps, and watts, you’ll learn to recognize misconceptions like this on your own.
A few words about voltage
Every electrical device needs a specific voltage to run properly, and it’s important that you know what voltage each of your electrical devices needs. You can usually find this information either printed on the device itself, or in the user manual that came with the device. If you supply a device with voltage that is too low, it may not run. If you supply voltage that is too high, you might fry your device.
What determines the voltage of an off-grid system?
In short, the batteries. Batteries for solar electric systems come in a variety of voltages, most commonly 2, 6, 12, and 24. Most solar DIYers use either 6 or 12 volt batteries. You can wire batteries in series to achieve whatever voltage you need, as wiring batteries in series adds voltages. For example, if you want to run a 48 volt system, you can do that by wiring (8) 6 volt batteries in series (because 8 x 6 = 48), or (4) 12 volt batteries in series (because 4 x 12 = 48).
Decoding the lingo
Manufacturers and retailers don’t always advertise all of the specifications of the item that they are selling. Sometimes they’ll mention how many amps an item draws, and what voltage the item runs on, but not the wattage. Sometimes they’ll mention the wattage, and the voltage, but not the amperage. So that’s when knowing the Volts x Amps = Watts equation is handy. You can use it to figure out missing information.
Not to confuse you, but some manufacturers use variations of these terms, indicating orders of magnitude, and it’s important to point this out now so that you’re aware. For example, a manufacturer might use milliamps instead of amps. 1 amp (A) = 1000 milliamps (mA). Another common one is using kilowatts (kW) instead of watts (w). 1 kilowatt (kW) = 1000 watts (W). Here are a couple tables to help you remember:
|milliamps (mA)||1000 mA = 1 A|
|kiloamps (kA)||1 kA = 1000 A|
|millwatts (mW)||1000 mW = 1 W|
|kilowatts (kW)||1 kW = 1000 W|
So, I hope that explains volts, amps, watts, and their relationships adequately. The main thing to remember is that Volts x Amps = Watts.