What Are Amps, Volts and Watts—And Why Is My Phone Charging So Slowly?

July 3, 2023

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Whether you’re looking for answers on charging your phone, tablet, laptop, or other high-end devices, our technology expert staff reviewers have put together this primer to help you figure out what all those little numbers on charging device labels really mean, to go along with our comprehensive round-up of the best power modules for standing desks.

Fear not, this will not be an engineering course. It’s pretty much the same stuff you learned in high school science class. But we understand not everyone has continued to use amps, volts and watts in their daily life, so hopefully this brief refresher course will help you separate the wheat from the chaff when it comes to all the different edge-clamped, grommet-mounted, under-desk mounted and tabletop power modules on the market. 

Getting Charged Up, Safely

While mobile device manufacturers are continuously evolving battery technology to charge faster, hold more power, and perform better for longer periods of time, most of the chargers on the market today are cheaply-made for planned obsolescence. After all, how long does any one new cable or connector standard last before there’s a new one taking its place? So a consumer might be tempted to go for the cheapest unit they can find to fit the bill, thinking perhaps if it lasts only a couple of years there’ll be a newer USB standard or wireless charging platform that they’ll soon need to invest in anyway. 

The problem with cheaply-made power modules boils down to two things: a) they either don’t provide the optimal charge that your device is designed for—resulting in infuriatingly slow charging times or b) they can pose considerable safety risks. These risks range from potential damage to your devices to the module itself potentially emitting noxious fumes, making loud buzzing sounds, or even catching fire and possibly putting life and property at significant risk. We’re not exaggerating; the Electrical Safety Foundation International (ESFI) reports thousands of homes per year burning down every year to power strip overload alone.

Between the high-voltage AC socket you plug these power modules into and your delicate electronics there are a lot of electronics involved in converting that 115V AC power into the various voltages and amperages needed by your devices. These devices should all have safety rating placards on them that detail how many Joules they’ve been certified to. As any fireman can tell you, most people don’t know what a Joule is, much less how to calculate when they’ve added too many devices to a power strip for it not to get overloaded and potentially catch fire. 

Consumers may think their shopping savvy can end at checking out the number of stars a product receives on Amazon. Such is probably the case with AmazonBasics 6-Outlet Surge Protector Power Strip, 790 Joule, which picked up more than 19,000 ratings, 84 percent of which were five stars. But as CNN pointed out, users are now flagging AmazonBasics items over safety concerns — and these complaints are being made both to government regulators and in reviews posted on the product site. Since 2016, at least 1,500 reviews covering items from power cords to phone and battery chargers, describe products sparking, catching fire, melting, smoking, or triggering some kind of electrical malfunction.

To summarize, just because something is UL or ETL certified it doesn’t mean it is perfectly safe to use. If it seems too cheap to be a quality electrical device, it probably is. Likely ways it can be produced so cheaply is that it was made with extremely cheap components that have a high likelihood of failure, it has a very low total power rating (Joules), and that while its design might be been lab-certified at one point, once receiving the certificate the manufacturer reverted to cheaper components (this is more common than you might think with Chinese-made products sold through Amazon and elsewhere).

What You Need to Know

Whether you’re dealing with a single port USB charger or a multi-device charging station with Qi (pronounced chee) wireless charging, USB ports, and AC outlets, you should be asking yourself three things:

  • Do the amps, volts and watts advertised match that of the mobile device I want to charge?
  • Will it provide optimal charging speed to those devices?
  • Is it safe to use?

In this article we’ll be discussing: 

  • The relationship between Amps, Volts, Watts and charging speed
  • How to choose a multi-device charging station
  • Battery health for iPads, iPhones, Androids, laptops, and more

And answering these common questions that apply to any mobile device:

  • Why isn’t my device charging?
  • Why is my device charging too slowly?
  • How do I optimize my battery’s health and longevity?

Amps, Volts, Watts and Their Relationship to Charging Speed

The amount of charge or power that a device is consuming is measured in watts. What’s more important to understand in regard to mobile device charging, however, is amps versus volts. “Amps,” or amperage, is a measurement of how much electrical charge is flowing past a given point per second. “Volts,” or voltage, on the other hand, is a measurement of the pressure behind those amps.

Consider the classic analogy of a hose filling up a bucket of water.

When the faucet is opened halfway the bucket will fill at say, 1 gallon per minute.

If you reduce the size of the nozzle, the water pressure (Volts) will increase but the bucket will still fill up at the same rate.

The same goes for the inverse, opening the nozzle won’t increase or decrease the fill rate but it will reduce the overall pressure (Volts).

Now let’s say you opened the faucet all the way. The increase in water volume (Amps) would, in fact, increase the fill rate whether the nozzle was narrow or wide.

In layman’s terms, think of voltage as pressure and amperage as volume. In order to fill the bucket completely and quickly, what you really need is volume or amps. But, consider a slightly different scenario where that’s the only information you had. You might think to yourself ‘‘well then, I’m just going to pour a dump truck full of water onto the bucket and we’ll be set.”

Although humorous, this exaggerated example applies to your mobile devices. You can not dramatically increase the voltage and hope to get a full charge in a second without causing damage. Your device could literally explode. You also don’t want the pressure to be so low that the existing charge in the battery resists the incoming charge. That said, there’s a balance between the size of the hose (preferably not a dump truck) and the pressure behind it.

The Amps x Volts = Watts Formula

As mentioned above, watts are simply units that are used to measure power output or power consumption. But what does that really mean?

Given that watts are a unit of power this is the number you’re most likely to see advertised on products. Although chargers and devices both have ratings for amps vs. volts, these numbers can vary depending on the pairing between charger and device as well as the amount of charge a device requires at any given time. Watts, on the other hand, summarizes that differential in overall power provided to a device.

The Takeaway

For those of you who are not electrical engineers, don’t worry about the technical jargon. Here’s what you need to remember:

  1. The difference between amps vs volts is a matter of electrical pressure and volume, respectively.
  2. The amps x volts = watts formula allows you to understand the output and input ratings on your chargers and devices.

Why Isn’t My Phone Charging?

Maybe you have a slow phone charger or multi-device charging station that just isn’t putting out the power you thought it would. This is a common frustration with cheap, Chinese-made consumer electronics but one that can be resolved with the right knowledge. Knowing the amps, volts, and watts ratings on both your charger and device is the first step.

To understand charging speed and optimize battery health, it’s important to consider how power is being distributed between the ports on your charger. 

Multi-Device Charging Station

Dealing with multiple chargers and cables is a hassle, especially with sit-stand desks where short cables can get snagged and unplugged during height-changing operations. Multi-device charging stations provide simple cable management solutions and can make the difference between a comfortable ergonomic workday and one where you’re constantly crawling under your desk to plug and unplug various devices. The caveat is that they distribute power in various ways.

Modern multi-device charging stations are typically built to accommodate an AC power outlet with USB ports so that you can charge your larger and smaller devices at the same time. This is great, in theory, but even after purchasing what seemed to be a quality multi-device charging station you saw an ad for on Facebook and impulsively purchased you might still be asking yourself  “why isn’t my phone charging?” or “why is my phone charging too slow?” or even “is my phone charging at all?” 

Charging Speed Depends on Power Distribution

Power distribution is actually as easy as it sounds. The total available power a multi-device charging station can provide is either shared between all of the outlets and ports on a single circuit or split between multiple circuits, just like the wiring in your home.

Imagine if every time the hairdryer tripped the circuit your entire house blacked out. This is how most of the cheap chargers on the market today work; they share a circuit between multiple ports.

Shared Circuits

Shared circuits divide the total amperage and voltage between all ports. Take the following example which demonstrates common power distribution on cheap, Asian-made chargers:

Shared Circuit Charging Station (Cheap Off-Brand):

3 USB Ports

  • 2.4 Amps Total = 0.8 Amps / Port
  • 5 Volts Total = 1.66 Volts / Port

If you only have your smartphone plugged in, you’re fine. But what if you wanted to charge your iPad and a third device too? The larger devices would require more Watts than the smaller devices and leave them charging at a dismal rate. It starts to become clear then that the difference between amps vs. volts isn’t just a matter of total output or Watts, but of how those numbers are split between ports.

Split Circuits

To consistently deliver more power, multi-circuit charging stations provide separate output ratings for amps vs. volts that are either split between fewer ports or, even better, isolated to a single port. Take the domestically-produced iMovR Mojo Power Block with USB-C Laptop Charger for example:

Split-Circuit Charging Station (iMovR Mojo):

2 USB-A Ports

  • 2 Amps Each
  • 5 Volts Each

1 USB-C Port

  • 60 Watts*

1 AC Outlet

  • 15 Amps
  • 120 Volts

With split-circuit multi-device charging stations like the Mojo you can be confident knowing that every port will deliver the full amps + volts available to that circuit, providing a fast and reliable charge for all of your mobile devices.

See our review of the iMovR Mojo with this exact split circuit configuration here.

*Note that USB-C, which is quickly becoming a universal power source, is rated in Watts because of its unique variable power delivery that can provide a different voltage based on the demand of a given device. T

The Takeaway

Slow phone chargers are typically a result of one of two things:

  1. Incorrectly matched charger output ratings to device input ratings,
  2. Power being distributed across too many devices.

Battery Health 

It doesn’t matter what type of device you’re dealing with, battery health for iPads, iPhones, Androids, off-brand laptops, you name it… is easier to understand than it once was. Consider that

  1. Most modern mobile devices have built-in protection against overcharging your battery,
  2. Knowing the Amps Volts Watts Formula is all you need to select the optimal charger

It may be obvious by now why a slow phone charger, or any mobile device charger for that matter, is underperforming or even leaving you wondering about overall battery health and performance. Yet, we’ve been asked questions like this time and again and aim to provide you with as much information as possible so that you can make an educated decision when it comes time to purchase.

Of course, your phone is likely charging to some extent, but a slow phone charger is next to garbage in today’s fast-paced world. Understanding Amps vs Volts vs Watts is necessary to match the output rating of your charger with the input rating of your device. But what if an exact match isn’t an option? Here’s a breakdown of what happens when either Voltage or Amperage is either too high or too low;

Voltage Too Low

If the charger has the same Amps as your device but lower Volts, then your device will effectively have “low blood pressure”. Recall that Amps is a measure of electrical pressure. This typically won’t cause any damage but if you’re using the device at the same time that you’re charging it you won’t receive an effective charge.

Voltage Too High

If the charger has the same Amps as your device but higher Volts, think of your device as having high blood pressure, and this can result in overheating your device, overcharging the battery, reduction in overall battery health, and long-term damage.

Amperage Too Low

If your charger has the same Volts as your device but lower Amps then your device will demand more power/Watts than your charger is rated to provide, leading to your device to charge slowly, or not at all, and your charger to overheat or fail entirely.

Amperage Too High

If your charger has the same Volts as your device but higher Amps, not to worry. Your device can set the pace. The charger doesn’t have to provide the amperage it’s rated for, simply what your device demands of it.

Optimizing Battery Health

The best thing you can do to optimize battery health is to limit charging to 80% of the battery capacity for your device. While the modern lithium ion battery is designed to hold a full charge, doing so will deteriorate its health over time. The same goes for letting it get too low, typically below 20%. That said if you plan to store your device, such as a laptop or tablet, for weeks at a time without use, try to stow it with a charge between 20% – 80%.

The Takeaway

  1. Today, most devices protect against overcharging your battery but unnecessary wear can still occur above an 80% charge and below a 20% charge
  2. To maintain optimal battery health, only charge your devices up to 80%, don’t let them get lower than 20%, and if being stored for long periods of time, make sure they’re stowed with a charge in this range.

Learn More

At WorkWhileStanding & WorkWhileWalking we are committed to serving our readers with objective and educational articles and product reviews. Learn more about power management, electronics, and household electronics safety below:

USB 2.0 vs USB 3.0


Wireless Charging

UL Listed vs ETL Listed


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Leave a response >
  • J.Coffee July 3, 2023

    Nicely worded. I asked myself if I completely understood the best V/a ratio so I Googled it and this article came up on top.
    I found a straight to the point answer and satisfied my curiosity without having to decipher random formulae or skip through videos.
    I appreciate the time the writer put in to this. Thank you, J.Coffee

  • Scott December 29, 2021

    Really helpful information. I won’t look at Amazon chargers the same way ever again. Thank you so much.