Divide the watts of a given electrical item by the total number of volts available from the electric outlet to calculate amperage draw. The amount of current flowing through the wire is measured in amperes, or amps. The equivalent of available electricity at the power source is voltage, or volts. Finally, the power produced by the electricity is measured in watts. All of these measurements are interrelated when calculating electricity usage.
Calculating from Watts and Volts
Find the wattage load of an device that requires electricity. Any device that draws energy is called a load. Examples of loads include a light bulb and a microwave. The wattage is often printed on the device itself, but if you can’t locate the number, you might need to check the owner’s manual.
Determine the voltage of your power source. In the United States, most household outlets run at 120 volts, although some, such as those for electric stoves or dryers, often run at 220 volts. If your power source is a battery, you will need to look up the voltage. Larger batteries are often 9 or 12 volts, while smaller closed cell batteries, such as C, AA or AAA, run between 1 and 3 volts, depending on size and composition.
Divide the wattage rating by the voltage from your power source. For example, if you have a 100-watt light bulb in a lamp that is plugged into a 120-volt outlet, it will draw 0.83 amps.
Calculating from Ohms and Volts
The electricity flowing through the wires in your house is often compared to water running through a hose. You can observe the size of the hose, the amount of water flowing through it, the water pressure and the result of the water spraying out. For electricity, the flow of the current is limited by the resistance to flow, measured in Ohms.
Use Ohm’s law to calculate amps using resistance. Many appliances have a listed resistance. The wire connecting the circuit also has a variable resistance. In the same sense, you can fit less water through a garden hose than a fire hose. You don’t need to include this resistance unless you have a lot of wire or need to be very accurate.
Find the voltage of your power source as you would when calculating from watts and volts.
Ohm’s law states that the voltage equals the amperage times the resistance, so if you divide the voltage of your power source by the resistance of the load, you will find the amps. For example, if you plug a 40-Ohm dryer into a 220-volt outlet, the appliance will draw 5.5 amps.
Things You’ll Need
- Object or owner’s manual
- Specifications for the electrical system
The calculations described are for a single load. When calculating amperage over multiple loads you can simply add wattage ratings together, but resistance can change depending on how the circuit is configured.
Use caution when working with electrical energy, and have your calculations double checked by a trained professional if you are calculating amps for a home electrical system.
Enter the power and voltage to convert watts to amps for DC, single-phase AC, and three-phase AC circuits.
On this page:
- How to Convert Watts to Amps
- How Many Watts Are in an Amp
- Single-Phase AC Circuit Watts to Amps Conversion
- Three-Phase AC Circuit Watts to Amps Conversion
- How to Convert Watts and Ohms to Amps
- Equivalent Watts and Amps at 120V AC
- Equivalent Watts and Amps at 12V DC
How to Convert Watts to Amps
Converting watts to amps can be done using the power formula, which states that I = P ÷ E, where P is power measured in watts, I is current measured in amps, and E is voltage measured in volts.
Given this, to find amps given power and voltage use the following formula:
Thus, the current I in amps is equal to the power P in watts divided by the voltage V in volts.
For example, find the amperage of 1200 watts at 120 volts
current = power ÷ voltage
current = 1200W ÷ 120V
current = 10A
How Many Watts Are in an Amp
Using the formula above you can figure out how many watts there are per amp. At 120 volts, 120 watts are equal to 1 amp, but this will change depending on the voltage of the circuit.
For instance, at 240 volts, 240 watts are equal to 1 amp.
Single-Phase AC Circuit Watts to Amps Conversion
Converting watts to amps for a single-phase AC circuit with power factor uses a slightly different formula.
In other words, the current I in amps is equal to the power P in watts divided by the voltage V in volts multiplied by the power factor PF. If you’re unsure what the power factor is then a power factor calculator can help.
Three-Phase AC Circuit Watts to Amps Conversion
Using Line to Line Voltage
For three-phase AC circuits where the line to line voltage is known, the formula to convert watts to amps is:
The current I in amps is equal to the power P in watts divided by the line to line voltage V in volts multiplied by the power factor PF multiplied by the square root of 3.
Using Line to Neutral Voltage
For three-phase AC circuits where the line to neutral voltage is known, the formula to convert watts to amps is:
The current I in amps is equal to the power P in watts divided by the voltage V in volts multiplied by the power factor PF multiplied by 3.
How to Convert Watts and Ohms to Amps
It is also possible to convert watts to amps if the resistance of the circuit is known using the formula:
The current I in amps is equal to the square root of the power P in watts multiplied by the resistance R in ohms.
It is not possible to convert watts directly to amps without also knowing voltage or resistance.
Because 1 kilowatt is equal to 1,000 watts, it is possible to use the formulas above to also convert kW to amps, but watts need to be converted to kW first. Use our kW to amps calculator to solve for kilowatts.
All Deep Cycle batteries are rated in Amp Hours (AH). An ampere hour (abbreviated Ah, or sometimes amp hour) is the amount of energy charge in a battery that will allow one ampere of current to flow for one hour. An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor.
For example, if you have an appliance that draws 20A and you use it for 20 minutes, then the amp hours used would be:
A x H = AH
20 x (20 mins / 60 Mins (1hr) = AH
20 x 0.333= 6.67AH
As you’ll note from the equation above, The faster a battery is drained (discharged), the less overall amperage is available. The battery’s AH rating decreases the faster you use it. This is This is called the Peukert Effect. The Peukert Effect is directly related to the internal resistance of the battery. If you discharge a battery over the course of 100 hours, the AH rating looks higher than if you discharge that same battery over the course of an hour. Because of the variances that could occur in AH ratings, an industry standard was implemented.
For deep cycle batteries, the 20 Hour Rate is the accepted AH rating time period for the majority of Deep Cycle Batteries. The 20 Hour Rate means that the battery is discharged down to 10.5V over a 20 Hour Period while the total actual AH that it supplies are measured. Sometimes ratings at the 6 Hour Rate and 100 Hour Rate are also given for comparison and for different applications. The 6 hour Rate is often used for industrial batteries, as that is a typical daily duty cycle. Sometimes the 100 Hour Rate is given just to make the battery look better than it really is, but it is also useful for figuring battery capacity for long-term backup.
For example, let’s take the below scenario into account.
If you do not have the Amp rating, you can use the Watt rating to determine the Amperage using Ohm’s law below.
Your Required Load is 500W
2.4 Hours is the theoretical run time before the battery is flat. The term theoretical is used because in practice, as the battery voltage decreases, the Amps drawn by the load increase proportionally. Due to this, total runtime will be slightly lower.
The type of load described above is higher than the rated 5A for the 100AH Battery and will shorten its life considerably. You should not use more than 5A load on a 100AH Battery as Deep Cycle Batteries don’t like being overloaded for long periods of time. To work out the total battery capacity required to run this equipment for twenty hours we need to establish how many AH we require.
Watts / Volts = Amps
500W / 12V = 41.67A
We’d like the appliances to run for 20 Hours.
A x H = AH
41.67A x 20 Hours = 833.40AH
8.33 AH = 8 x 100AH Batteries
By using the suggested 8, 100 AH Batteries you will be able to run your load of 500W for 20 Hours without damaging the batteries or considerably shortening the battery’s life. This is because each battery in your battery bank of 8 will supply only 5.21A, which is closer to the suggested load rating for a 100AH Battery.
- Always try to recharge your batteries as soon as possible after each use.
- Try to buy a ‘fresh’ battery initially. Buy batteries from outlets that frequently sell batteries and check the battery production dates.
- Do not let the battery sit unused/uncharged for long periods of time. Batteries discharge during storage time. This shortens their life. If possible disconnect any load from the battery while in storage and use intelligent charger for the top-up.
- Use a good quality intelligent charger with at least 2 charging stages. For example; Soft Start (Slow Charge), Bulk (Fast Charge), then trickle charge (maintain). Some of today’s 7 stage quality chargers offer proper battery maintenance and will largely increase your chances of prolonging your battery’s life cycle.
The above-mentioned chargers offer Battery Test & Desulphation processes which can bring a badly damaged battery back to life. Using such a charger takes complete care of your battery maintenance, ensuring long life, top performance and availability since you can also leave them connected to the battery forever without fear of overcharging it.
For more information about calculating any of the information in the above blog post, don’t hesitate to contact a member of our friendly sales team on 1300 559 953. Alternatively, you can contact us via the form below.
Isn’t it scary and overwhelming to power up your home without knowing the amperage consumed? It’s unlikely that you continuously use your electricity to the max every day. However, when you don’t know how to calculate total amps in a breaker panel, chances are you may encounter electrical, or worse, fire hazards.
To find the amp capacity of your breakers inside the panel box itself, you can use the Power formula (I=P÷V). Our goal here is to get the amps in your breaker panel before adding more circuit branches.
Table of Contents
What You Will Need
Electrical panel load calculation is a minor electrical project that doesn’t necessarily require a professional contractor, so don’t worry much about complicated tools because you’ll only need three things:
Since you’re going to solve the total amps of your panel, a total amperage calculator is a big help. Although you can crunch the numbers yourself, a digital or scientific calculator can make computations easier.
If you have an electrical code calculator, you may use it. However, since we’re doing simple equations, which only involve adding, multiplying, and dividing, a simple but accurate calculator will fit the job.
Pen and paper
You are required to write down the rated amperages of your circuit breakers, so you should have a pen and paper. Using a whiteboard will work as well. Jutting down the important numbers when you calculate amps in a circuit is beneficial when we purchase materials for the new circuit system.
Also, these written notes will serve as a basis for future load calculations. Sometimes, when there’s a breaker panel inspection, you can also show this to your contractors and refer to it if there are any anomalies.
Main breaker panels are not usually placed in well-lit areas. Contractors hide them inside your basement, attic, or any room which is not frequented by people.
Use a flashlight to check amps on a circuit breaker. Circuit breakers have their amps written on them in small fonts, and reading it in a dark area can potentially result in a misreading.
A lamp, headlight, or handheld flashlight is good. What’s important here is that you’ll have a good vision of your CB amp ratings when you write them on paper. A single mistake will result in an inaccurate calculation, so make sure to view your breaker panel with proper lighting.
Protective Equipment (Gloves and Boots)
Steps to Calculate Your Breaker Panel’s Total Amps
The listed tools you’re going to use are accessible and commonly found in most homes. Now, learn how to tell how many amps your electrical panel is supplying by following these easy steps.
What we’re trying to achieve here is to get the hypothetical amount of the total amps of the sub breakers installed in your panel.
This step is necessary to get the correct amperage rating for your panel. At the end of this tutorial, you’ll know if you’ll need a panel upgrade or if it’s spacious enough for a new circuit branch.
Step 1. Open the cover of your breaker panel
Before everything else, for you to be able to inspect all of the circuit breakers installed in your load center, open the breaker panel door or the cover itself. This step is the most dangerous part of the process. Please note that the surface of and around the breaker panel is an electric-prone area. Use insulated gloves and boots for your protection.
There’s no rocket science behind opening a breaker panel cover. What you need to remember is when you unscrew the lid, start from the bottom, going all the way to the top. And as you reach the last two screws, hold the cover in place until you’re ready to pull it out.
Step 2. Determine every circuit breaker’s amp rating
Once you’ve revealed all the circuit breakers, use your flashlight to check each installed breaker—single-pole or multi-pole. Remember that we need to exclude the Main circuit breaker in totaling the amps of the breaker panel; you can find this breaker either at the top or bottom.
Now, write each amp rating of your circuit breaker on a piece of paper. Note that a multi-pole breaker has the same ampacity for each leg. For example, a 15-amp double-pole breaker has 7.5 amps for each leg and not 15-amps.
Step 3. Add them all and get the load limit of the circuit breakers
Next, add up all of the circuit breakers’ amps, then multiply it by the load limit requirement, which is 80%. To determine electrical panel amperage easily, let me give you a sample problem as a basis:
For instance, a house has one 15-amp breaker for light fixtures, two 20-amp breakers for bedrooms, one 30-amp breaker dedicated to a water heater, and another 20-amp breaker for the garage. In total, this home has 105 amps.
However, considering the load limit, which requires a homeowner to use only 80% of the total load, the actual limit of the circuit breakers is 84-amps. (15+20+20+30+20= 105A, multiplied by 80% = 84A)
Step 4. Get the amperage for the electric panel
Now that you’ve got the total amps in your breaker panel, you can determine the best size for the main panel. The panel’s rating should be at least as high as the amps you use. Your panel should have a label that notes its amperage.
And that’s a wrap! I hope you’ve learned how to calculate total amps in a breaker panel with the example I gave you. Keep in mind that you’ll need to follow the 80% load limit for every load computations. This safety regulation is the best way to avoid electrical accidents.
Did you find this article informative? You can support us by sharing this link with your friends or posting a comment down in the section below!
Calculating watts from amps or volts is easy. It simply takes a few steps. You can’t necessarily convert amps directly to watts, since each unit represents something different, but with an extra step or two you can find the exact measurement you need. Generally, a household has three different types of appliances, and ACUPWR can offer a unit conversion technique for each of them. For simplicity’s sake, we’ll abbreviate watts to W, amps to A, volts to V, and power in watts to P.
1. DC Amps to Watts
If you want to know the power consumption in watts for an appliance with a current of 3 amps and a voltage of 110, your calculation would be:
Amps ⨯ volts = watts
3 ⨯ 110 = 330 watts
2. Single-Phase AC Amps to Watts
If you need calculations for single phase devices, then real power in watts will be given in terms of a power factor (PF) multiplied by the root-mean-square voltage multiplied by the phase current. The power factor is the ratio of “real” electric power used to do work to the “apparent” power given to the appliance. So your calculation would be:
W = PF ⨯ A ⨯ V
You need a transformer for appliance that is rated as 0.8 power factor, 3-amp phase current, 110-volt RMS voltage. Power in watts can then be calculated as:
P = 0.8 ⨯ 3 ⨯ 110 = 264 watts
3. Three-Phase AC Amps to Watts
Power in watts is calculated by multiplying the square root of three by the power factor, the current in amps, and the line-to-line RMs voltage in volts. The equation is:
P = √3 ⨯ PF ⨯ A ⨯ V
Line to Natural Voltage
Assuming the power loads are balanced, the equation for determining power in watts for line to natural voltage would be almost exactly the same, except you multiply the amps, voltage, and power factor by three instead of the square root of three. Your equation would be:
P = 3 ⨯ PF ⨯ A ⨯ V
Why Choose an ACUPWR Power Converter
ACUPWR is the leading manufacturer for voltage transformers. We provide high-quality power converters for refrigerators, freezers, coolers and more, with the ability to switch effortlessly between the power standards you need. Most of the appliances on market do not make accurate adjustments for hertz rate, and because of this, the performance of your appliance is slowed. Its internal circuitry may suffer a huge loss. Give your appliances the ability to achieve optimum performance around the world with ACUPWR products, with voltage transformers and power converters for any voltage and any major application.
When we talk about sophisticated refrigerators, we need to take additional care when it comes to converting voltage, because one low-quality product can cause serious damage to your expensive appliance. Instead of taking that risk, choose an American-made, UL-approved ACUPWR power converter. Our products come with guaranteed protection.
Generally, it is not possible to connect appliances with different supply ratings. For example, you should not use an appliance rated for 110 volts and 50 hertz with a power supply rated for 220 volts and 60 hertz. However, ACUPWR voltage transformers can adjust the frequency rate in order to match the needed speed of your appliance. If you need to use your device with variable ratings, ACUPWR power converters can serve you better, as they can effectively make conversions between one voltage level to other.
ACUPWR has a huge collection of voltage transformers with different power ratings that can be used all over the world with all kinds of devices. If you are thinking about moving to another country, but you are worried about connectivity of your appliances, ACUPWR is here to help. We design all our power converters with surge protection, so you don’t have to worry about voltage fluctuations damaging your devices, and on top of that, we offer damage recovery up to $10,000 on our products.
A solar panel will generate electricity when placed in the sun. Current will flow from a panel connected to an electrical circuit. How many amps of electricity the panel will produce depends on the power of the solar panel, the amount of sunshine falling on the panel and the characteristics of the circuit to which the panel is connected. Calculate the amps produced by the panel making measurements using a digital multimeter.
Look at the back of the solar panel or consult the installation manual and find the maximum rated power of the panel in watts. Look also for the maximum power voltage, Vmp, which is in volts.
Calculate the current produced by the solar panel when it is generating its maximum power. Calculate the current in amps by dividing power in watts by the voltage in volts. For example, if the solar panel is rated at 175 watts and the maximum power voltage, Vmp, is given as 23.6 volts, then calculate the current as 175 watts divided by 23.6 volts, which is equal to 7.42 amps. This is current produced by the solar panel at full power.
Take a digital multimeter and switch the dial to direct-current volts. With the solar panel connected to an electrical circuit, measure the voltage between the positive and negative terminals of the solar panel. Make a note of this value. Disconnect the solar panel from the circuit and switch the digital multimeter to measuring resistance. Measure the resistance of the electrical circuit in ohms and write down this value. Reconnect the solar panel to the circuit.
Calculate the current in amps flowing through the circuit by dividing the voltage by the resistance. This relationship is Ohm’s law (see References 1). For example, if you measured the voltage as 22.1 volts and the resistance of the circuit as 3.2 ohms, divide 22.1 by 3.2 ohms to get 6.91 amps. This is the actual current produced by the solar panel, given the amount of sunshine on the panel and the characteristics of the circuit.
Measuring the voltage of a single solar panel is not dangerous. If you have several panels connected together, however, consult a qualified technician to assist you in this procedure.
The wattage of an appliance (in watts) is often said to be current (in amps) multiplied by voltage (in volts).
Whilst this is true for simplified or direct current (DC) circuits, it’s not the case for the mains power we use every day.
This conventional wisdom or ‘rule of thumb’ will have you calculate Apparent Power rather than Real Power.
How NOT to Calculate Wattage – Apparent Power (VA)
Amps (A) x Volts (V) = Volt-Amps (VA)
The formula above can be used for calculating apparent power consumption in volt-amperes (VA). This equation will give you a rough idea of power use in watts but is not strictly correct. For this, you need to take into account the power factor.
How to Calculate Wattage – Real Power (Watts)
Amps (A) x Volts (V) x Power Factor = Watts (W)
This formula takes the power factor into account and shows accurate power consumption (what you are billed for).
What is Power Factor?
Power Factor is a measure of the effectiveness with which an electrical device converts volt-amperes into watts. Power factor is represented as a dimensionless number between 0 and 1.
The closer the number is to one, the ‘better’ the power factor. The higher the power factor, the more effectively electrical power is used. Resistive loads, such as most electric heaters, will have a power factor of 1 as they convert all electrical power supplied into heat. Equipment with motors, such as fridges and air conditioners, will have a lower power factor.
How Does This Relate to Watts & Wattage?
Power factor is crucial if you want to know the actual power consumption of an appliance. See below for a demonstration of how power factor is used with our power meter to calculate the real energy consumption of a small TV.
Larger businesses need to have a power factor close to ‘unity’ (1) as they may be charged a fee if they have a low power factor. This is because the utility has to supply more current (amps) to the site than required. In so doing, they incur more transmission losses. The good news is that businesses can take steps to increase their power factor.
Example – Calculate the Actual Watts Of A TV
The compliance label on this TV shows the power use as 130 Watts.
The problem is that compliance labels often show maximum power rather than actual power. The only way to know the real power is to measure it with a plug-in power meter. Over a two hours, the power meter showed a power draw of between 70 and 110 Watts – substantially less than indicated on the label.
A watt meter like this will calculate the actual wattage.
At one point the power meter showed the TV to be using 243 volts and 0.421 Amps. If we follow conventional wisdom and just multiply Volts and Amps together without power factor, we’d work out the apparent power draw as follows:-
- Amps (A) X Volts (V) = VA
- 243 V x 0.421 A = 102.3 VA
. then falsely present it as 102.3 W
When we add power factor into the calculation, we get a very different figure. Since the power meter showed a power factor of 0.65 at that time, the calculation becomes:
- Amps (A) x Volts (V) x Power Factor = Watts (W)
- 234 V x 0.421 A x 0.65 = 66.5 W
Hopefully, you can now see why it’s essential to get this calculation right.
Thankfully, our plug-in power meters will do these calculations for you. Our power meter displays real power (Watts) as well as Amps (A), Volts (V), and power factor so you can verify the calculation if you need to.
The Reduction Revolution Plug-in Power Meter is our cheapest and most popular option. The Power Mate Lite (pictured above) is a high accuracy model used by professional energy auditors.
See also: our free online appliance running cost calculator .
Subscribe to Our Free Email Newsletter
Get exclusive access to new products, energy efficiency advice, and special offers. We only send one email every few weeks, and you can unsubscribe at any time.
Reduction Revolution is an online store that specialises in energy efficiency. Our products help slash your energy usage, improve comfort, save time, and reduce maintenance costs.
Quality Products & Service
We’ve received thousands of 5-star customer reviews from customers all over Australia and NZ. Read Our Customer Reviews!
Free Email Newsletter: subscribe here to get exclusive access to new products, special offers, and more.
Have you ever wondered why power outages happen? There are a few possible reasons, but if your home or business frequently experiences power outages, you may be overloading your circuit breaker with more appliances than it can handle. To help you better understand how to measure power and know how much demand you’re placing on your circuit breaker, here is a helpful guide to amps, watts, and volts.
FIRST, LET’S DEFINE OUR TERMS
Before we dive into the details, it’s important to define a few basic terms . Here’s a look at the electrical measurements you’re likely to encounter:
Amps: Short for ampere, an amp is the base unit of electrical current in the International System of Units (SI).
Volts: The SI unit of electromotive force, or the difference of potential that would drive one ampere of current against 1 ohm resistance.
Watts: The SI unit of power, equivalent to one joule per second, corresponding to the power in an electric circuit in which the potential difference is one volt and the current one ampere.
After reading these definitions, it may still be unclear what the terms actually mean. A helpful analogy is to think of electricity like flowing water. Amps would indicate the volume of water that’s moving, and volts would indicate the water pressure. Different combinations of volts and amps would yield different types of flows. For example, high pressure with low volume would be like a dental Waterpik, while high pressure and high volume would be like a fire hose. Watts measures how much force is produced by a type of electrical flow.
CALCULATING POWER MEASUREMENTS
Your circuit breaker can only handle a certain amount of amperage, or a certain volume of electricity. It has a specific amperage rating that allows it to work and provide your home with electricity. If this limit is exceeded, your breaker will shut down in order to prevent your home’s wiring and appliances from being damaged.
HOW DO YOU FIND OUT YOUR HOME’S AMPERAGE?
This is pretty simple. All you have to do is go to your circuit breaker and check the handle. Most household circuits carry 15-20 amps, and the newer your home is the higher the amperage is more likely to be. By knowing what your amperage is, you can know how many devices you can support with it.
HOW MUCH AMPERAGE DO YOUR DEVICES USE?
First, make sure you know how many amps your circuit carries. Then, check your device’s label or user manual to see how many watts and volts the device will use. Divide the number of watts by the number of volts, and that will give you the maximum amount of amperes it will require from your circuit. It might be a good idea for you to keep track of how many amperes each device uses. This way, you can keep track of how much power you’re using. If you end up exceeding your limit, you will trip the circuit.
Voltage refers to the amount of power that comes from your outlets. That measurement is referred to as volts. One outlet can usually produce up to 120 volts.
WHAT ARE THE DIFFERENT TYPES OF VOLTAGE CURRENTS?
Direct Current (DC): Electricity flows in one direction. This is the type of current that most of your digital electronics will use.
Alternating Current (AC): Electricity will change the direction of its flow periodically. Most houses are wired for AC, and so your home most likely is built for it too.
HOW MANY VOLTS ARE COMING OUT OF MY OUTLET?
Again, make sure you know your circuit’s amperage number. Then, check the device you’re plugging into the outlet for how many watts it uses up. All you have to do after that is divide the number of watts by your circuit’s amperage number. The resulting number is the number of volts coming out of your outlet to help support your device.
We’ve discussed amps and volts above, but there’s still one more to consider—watts. A watt is a measurement of electricity or one unit of power.
HOW CAN YOU CALCULATE THE NUMBER OF WATTS YOUR CIRCUIT CAN HANDLE?
All you need to know is two things. As discussed in previous calculations, you’ll need to know your circuit’s amperage. You’ll also need to know how many volts your outlet can produce. Then, multiply the amperage by the number of volts. This is the maximum amount of watts your circuit can support at one time. If you exceed that amount, it’s quite possible an electrical blowout will occur.
CONTACT JP ELECTRICAL FOR SUPPORT
If your circuit breaker ever trips or you’re experiencing any other electrical issues in your home, give us a call. You can also count on us to do the math for all your home’s electrical needs, so you can prevent a blowout before it ever occurs. We provide various residential and commercial services and are especially proficient in wiring, lighting, and electrical panels. We can also provide generators!
Both watts (W) and volt-amperes (VA) are units of measurement for electrical power. Watts refer to “real power,” while volt-amperes refer to “apparent power.” Usually, electronic products show one or both of these values to provide information about how much energy they will consume or how much current they will draw. Each of these values can be used for various purposes.
What Are Watts?
The real power in watts is the power that performs work or generates heat. Power in watts is the rate at which energy is consumed (or generated). One watt is one joule (energy) per second (1 W = 1 J/s). You pay your utility company for watts expressed as energy, which is power consumed for a time period, typically shown by your utility company in kilowatt-hours. For example, a 100-W light bulb left on for 10 hours consumes 1 kW-hour of energy (100 W x 10 hours = 1000 W-hours = 1 kW-hour).
How Are Watts Calculated?
Real power for dc circuits is simply the voltage (Vdc) times the current (Idc):
The concept for calculating the real power for ac circuits is straightforward, though performing the calculation is much more difficult. To get the power in watts, you need to know the instantaneous voltage with time, v(t), and the instantaneous current with time, i(t). When you multiply these together, you get the instantaneous power with time, p(t).
Since this instantaneous power is changing over time, we need to get an average value, so we integrate the power over a period of time and divide by the time period to get the average. That gives us the watts dissipated by the device in a circuit with voltage v(t) across it and current i(t) through it for the period of time evaluated. Assuming that the voltage and current are both periodic waveforms of period T, the strict mathematical way to express the power calculation for a periodic waveform of period T is:
So while this may be easy to visualize, it is not easy to calculate. Even the measurement of real power in watts for ac circuits requires specialized equipment (a wattmeter) because the voltage and current waveforms must be measured over a precise period of time, the measurements must be simultaneous, and the average must be calculated over the measurement time period. A standard multimeter can’t make this type of power measurement.
What Are Watts Used For?
These ratings are useful if you have to get rid of the heat generated by the device consuming the watts or if you want to know how much you will pay your utility company to use your device since you pay for kilowatt-hours (power used for a period of time). To combine the real power of multiple dc or ac devices, you can just add up the individual power ratings in watts of each device to get the total power (watts add linearly).
What Are Volt-Amperes?
The apparent power in VA is used to simplify power ratings, making it easier to calculate current draw. Since VA = RMS volts x RMS amps, you can divide the VA rating by your RMS voltage to get the RMS current the device will draw. Knowing the RMS current helps you properly size wires and circuit breakers or fuses that supply current to your device.
How Are Volt-Amperes Calculated?
The apparent power for dc circuits is simply the voltage (Vdc) times the current (Idc):
The apparent power for dc circuits is the same as real power for dc circuits (for dc, VA = W).
For ac circuits, VA are the product of the RMS voltage (VRMS) times the RMS current (IRMS):
VA = VRMS x IRMS (4)
You can calculate the apparent power in volt-amperes for ac circuits by multiplying the measured RMS voltage times the measured RMS current. A standard multimeter usually can make both of these RMS measurements.
What Are Volt-Amperes Used For?
Volt-amperes provide insight into the amount of current drawn by a product or circuit, assuming you know the voltage. For example, the standard residential voltage in the United States is 120 VRMS. If a product is rated for 300 VA (the rating implies this is the maximum VA the product will draw) and is powered from a 120-VRMS ac line voltage, you can calculate the expected maximum current as 300 VA/120 VRMS = 2.5 ARMS maximum (see the figure). Thus, you would want to ensure that the wires and associated circuitry supplying this product accommodate at least 2.5 ARMS.
Power factor = PF = W/VA (5)
Power factor is always a number between zero and one because the watts drawn by a device are always less than or equal to the volt-amperes. Note that it is possible for a circuit to have a large voltage across it and to draw substantial current, but consume no energy (dissipate zero watts).
While this seems counterintuitive, it is true if the circuit is purely reactive (a pure capacitor or pure inductor). The circuit will do no work and produce no heat, so it is drawing (and dissipating) zero watts. Yet it can draw substantial current, resulting in substantial VA.
In this case, the power factor is zero. This is possible because the phase relationship between the voltage and current waveforms is such that the circuit is alternately absorbing real power and giving that real power back, so the net real power consumption is zero.
W and VA are both units of measurement for power, but that’s where the similarity ends. Watts do work or generate heat, while volt-amperes simply provide you with information you need to size wires, fuses, or circuit breakers. Watts add linearly, while volt-amperes doe not. And to measure W, you need a special wattmeter. You can calculate VA by using a standard multimeter to measure VRMS and IRMS and finding the product (see the table).