Electrical Resistance Explained

Prospective fault current (PFC) is the highest electric current that can flow in a system during a short circuit. It helps determine equipment ratings, breaker capacity, and safety measures in electrical installations to prevent overheating, fire, or component failure.

What is the Meaning of Prospective Fault Current?

Prospective fault current refers to the maximum current expected during a short circuit at any point in an electrical system.

✅ Helps size circuit breakers and fuses for safe disconnection

✅ Ensures compliance with installation and safety codes

✅ Prevents equipment damage from excessive short-circuit current

Prospective fault current (PFC) is a key factor in the safety and design of electrical systems. It represents the maximum current that could flow in the event of a fault, such as a short circuit. Understanding PFC is essential for selecting protective devices that can handle fault conditions safely. This article explores what PFC is, how it is measured, and its importance for electrical installations, while addressing key questions. Understanding electrical short circuits is key to calculating prospective fault current and ensuring system safety.

When measuring prospective short circuit current in an electrical system, it’s essential to perform tests between L1 N CPC and L2 N CPC to assess the fault current across different phases and protective conductors. These measurements help identify the maximum prospective fault current present in the system, especially at points involving live conductors. Whether taking note of a single-phase supply or between line conductors on a three-phase supply, proper testing protocols must be followed. Technicians should always use insulated test leads rated for the expected voltage and current levels, and please refer to the test meter manufacturer’s instruction for safe and accurate operation. Reliable results ensure that the protective devices can safely interrupt fault conditions, preventing system damage and ensuring compliance with fault current protection standards.

Frequently Asked Questions

Why is it Important?

Prospective fault current refers to the maximum current that could pass through a system during a fault. The PFC helps determine the breaking capacity of fuses and circuit breakers, ensuring these protective devices can handle high currents safely. This is vital for protecting the electrical installation and those working near it.

Understanding PFC is critical for ensuring increased safety for employees and third parties. Protective devices must be selected to handle PFC; otherwise, they may fail to operate correctly, leading to severe consequences, such as fires or injuries. To fully grasp how PFC affects energy flow, it’s useful to review the concept of electrical resistance in a circuit.

How is Prospective Fault Current Measured or Calculated?

PFC can be measured or calculated using tools such as a multifunction tester, often during fault current testing. The instrument uses a single-phase supply or between line conductors on a three-phase supply to measure the maximum potential current at various points in the installation. Testing often involves checking currents between L1 N CPC, L2 N CPC, and L3 N CPC, which measure current between the lines to neutral in a three-phase system.

When performing these tests, technicians should follow regulation 612.11 of a single-phase supply or between line conductors on a three-phase supply, ensuring that simple and circuit protective conductors are all connected correctly. Accurate testing must also account for maximum current flow. Live testing requires extreme caution, and it is important to refer to the test meter manufacturer’s instructions to ensure proper usage and safety. In three-phase systems, 3-phase electricity significantly impacts how fault current behaves during a short circuit.

What is the difference between PFC and Short-Circuit Current?

Though often confused, prospective fault current and short-circuit current are distinct. Prospective fault current is the theoretical maximum current that could flow in a fault, used to predict the worst-case scenario for selecting protective devices. Short-circuit current refers to the actual current that flows during a fault, which depends on real-time conditions such as circuit impedance. Prospective fault current is one of the many concepts that form the foundation of electricity fundamentals.

How Does Prospective Fault Current Impact the Selection of Protective Devices?

The calculation of PFC plays a critical role in selecting the correct protective devices. Circuit breakers and fuses must have a breaking capacity that matches or exceeds the prospective fault current in the system. If the PFC exceeds the breaking capacity, the protective device may fail, leading to dangerous electrical hazards.

For instance, fault current testing using a multifunction tester between phases and neutral (L1, L2, L3) ensures that protective devices are rated to handle the highest potential fault current in the system. Proper circuit protection ensures that the system can interrupt faults safely, minimizing the risks to workers and equipment.

What Standards and Regulations Govern Prospective Fault Current Calculations?

Various standards, such as IEC 60909, govern how PFC is calculated and how protective devices are selected. These regulations ensure that electrical systems are designed to handle maximum fault conditions safely. Regulation 612.11 further specifies how live testing should be conducted using proper equipment and safety protocols.

It is essential to test PFC at relevant points in the system and follow testing standards to ensure compliance and safety. Devices selected based on PFC calculations help ensure that electrical systems can withstand faults and maintain reliable operation.

Prospective fault current is a crucial element in the safety and reliability of electrical installations. By calculating PFC, engineers can select protective devices that ensure safe operation in the event of a fault. Testing for fault currents at different points in the system and adhering to regulations are essential steps in preventing hazardous conditions.

By choosing protective devices with the appropriate breaking capacity and following safe testing practices, electrical installations can handle fault conditions and protect both workers and equipment from harm. Selecting protective devices that match the PFC is essential for reliable electric power systems design.

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Water electricity refers to electrical power generated using water, primarily through hydroelectric systems. It converts the kinetic energy of moving or falling water into electricity, providing a renewable and sustainable energy source with low carbon emissions and high efficiency.

What is: “Water Electricity?”

Water electricity, also known as hydroelectric power, is energy derived from moving water. It’s a leading source of renewable electricity worldwide.

✅ Converts water’s kinetic energy into usable electricity

✅ Offers renewable, low-emission electricity generation

✅ Used in dams, rivers, and tidal energy systems

 Most hydroelectric power is generated from the potential energy of dammed water, which drives a water turbine and generator. Less common variations utilize water's kinetic energy or undammed sources, such as tidal power. Hydroelectricity is a renewable energy source.

The energy extracted from water depends not only on the volume but on the difference in height between the source and the water's outflow. This height difference is referred to as the head. The amount of potential energy in water is directly proportional to the head. To obtain a very high head of water for a hydraulic turbine, water may be run through a large pipe called a penstock.

While many supply public power networks, some Water Electricity projects were created for private commercial purposes. For example, aluminum processing requires substantial amounts of power, and in Britain's Scottish Highlands, there are examples at Kinlochleven and Lochaber, designed and constructed during the early 20th century. Similarly, the 'van Blommestein' lake, dam and power station were constructed in Suriname to provide power for the Alcoa aluminum industry. In many parts of Canada (specifically, the provinces of British Columbia, Manitoba, Ontario, Quebec, and Newfoundland and Labrador), hydroelectricity is used so extensively that the word "hydro" is often used to refer to any power delivered by a power utility. The government-run power utilities in these provinces are called BC Hydro, Manitoba Hydro, Hydro One (formerly "Ontario Hydro"), Hydro-Québec and Newfoundland and Labrador Hydro, respectively. Hydro-Québec is the world's largest hydroelectric generating company, with a total installed capacity (2005) of 31,512 MW. For deeper insight into how to generate electricity from various sources, including water, visit our guide on hydroelectricity, which explains methods, systems, and real-world examples.

Importance

Water and Electricity power supplies 20% of the world's electricity. Norway produces virtually all of its energy from hydroelectric sources. In contrast, Iceland produces 83% of its requirements (as of 2004), and Austria generates 67% of all power in the country from hydroelectric sources (over 70% of its total requirements). Canada is the world's largest producer of Water Electricity, and produces over 70% of its electric power from hydroelectric sources.

Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it can be readily stored during off-peak hours (in fact, pumped-storage hydroelectric reservoirs are sometimes used to store power produced by thermal plants for use during peak hours). It is not a major option for the future in developed countries because most major sites in these countries that have the potential for harnessing gravity in this way are either already being exploited or are unavailable for other reasons, such as environmental considerations.

Regions where thermal plants provide the dominant supply of power utilize Water Electricity to provide the important functions of load following and regulation. This permits thermal plants to be operated closer to thermodynamically optimal points rather than varied continuously, which reduces efficiency and potentially increases pollutant emissions. Concurrently, hydro plants are then utilized to provide for hour-to-hour adjustments and to respond to changes in system frequency and voltage (regulation), with no additional economic or environmental effect.

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Electrical units measure various aspects of electricity, such as voltage (volts), current (amperes), resistance (ohms), and power (watts). These standard units are crucial in electrical engineering, circuit design, energy monitoring, and ensuring the safe operation of electrical systems.

What are Electrical Units?

Electrical units are standardized measures used to quantify electrical properties in circuits and systems.

✅ Measure voltage, current, resistance, power, and energy

✅ Used in electrical engineering, testing, and design

✅ Support safe and efficient electrical system operations

Electrical units are standardized measurements that describe various aspects of electricity, such as current, voltage, resistance, and power. These units, like amperes for current and volts for voltage, help quantify the behavior and interaction of systems. By understanding electrical units, professionals can assess performance, design circuits, and ensure safety across different applications. These electrical units play a crucial role in the functioning of everything from household appliances to industrial machinery, making them fundamental in engineering and everyday technology.

In common electricity systems, various electrical units of measure, such as magnetic field, are used to describe how electricity flows in the circuit. For example, the unit of resistance is the ohm, while the unit of time is the second. These measurements, often based on SI units, help define the phase angle, which describes the phase difference between current and voltage in AC circuits. Understanding these electrical units is critical for accurately analyzing performance in both residential and industrial applications, ensuring proper function and safety.

Ampere

The ampere is the unit of electric current in the SI, used by both scientists and technologists. Since 1948, the ampere has been defined as the constant current that, if maintained in two straight, parallel conductors of infinite length and negligible circular cross-section, and placed one meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10^7 newtons per meter of length. Named for the 19th-century French physicist André-Marie Ampere, it represents a flow of one coulomb of electricity per second. A flow of one ampere is produced in a resistance of one ohm by a potential difference of one volt. The ampere is the standard unit of electric current, playing a central role in the flow of electricity through electrical circuits.

Coulomb

The coulomb is the unit of electric charge in the metre-kilogram—second-ampere system, the basis of the SI system of physical electrical units. The coulomb is defined as the quantity of electricity transported in one second by a current of one ampere. Named for the I8th—I9th-century French physicist.

Electron Volt

A unit of energy commonly used in atomic and nuclear physics, the electron volt is equal to the energy gained by an electron (a charged particle carrying one unit of electronic charge when the potential at the electron increases by one volt. The electron volt equals 1.602 x IO2 erg. The abbreviation MeV indicates 10 to the 6th (1,000,000) electron volts, and GeV, 10 to the 9th (1,000,000,000). For those managing voltage drop in long circuits, we provide a helpful voltage drop calculator and related formulas to ensure system efficiency.

Faraday

The Faraday (also known as the Faraday constant) is used in the study of electrochemical reactions and represents the amount of electric charge that liberates one gram equivalent of any ion from an electrolytic solution. It was named in honour of the 19th-century English scientist Michael Faraday and equals 6.02214179 × 10^23 coulombs, or 1.60217662 × 10^-19 electrons.

Henry

The henry is a unit of either self-inductance or mutual inductance, abbreviated h (or hy), and named for the American physicist Joseph Henry. One henry is the value of self-inductance in a closed circuit or coil in which one volt is produced by a variation of the inducing current of one ampere per second. One henry is also the value of the mutual inductance of two coils arranged such that an electromotive force of one volt is induced in one if the current in the other is changing at a rate of one ampere per second.

Ohm

The unit of resistance in the metre-kilogram-second system is the ohm, named in honour of the 19th-century German physicist Georg Simon Ohm. It is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere (1 ohm = 1 V/A); or, the resistance in which one watt of power is dissipated when one ampere flows through it. Ohm's law states that resistance equals the ratio of the potential difference to current, and the ohm, volt, and ampere are the respective fundamental electrical units used universally for expressing quantities. Impedance, the apparent resistance to an alternating current, and reactance, the part of impedance resulting from capacitance or inductance, are circuit characteristics that are measured in ohms. The acoustic ohm and the mechanical ohm are analogous units sometimes used in the study of acoustic and mechanical systems, respectively. Resistance, measured in ohms, determines how much a circuit resists current, as explained in our page on Ohm’s Law.

Siemens

The siemens (S) is the unit of conductance. In the case of direct current (DC), the conductance in siemens is the reciprocal of the resistance in ohms (S = amperes per volt); in the case of alternating current (AC), it is the reciprocal of the impedance in ohms. A former term for the reciprocal of the ohm is the mho (ohm spelled backward). It is disputed whether Siemens was named after the German-born engineer-inventor Sir William Siemens(1823-83) or his brother, the engineer Werner von Siemens (1816-92).

Volt

The unit of electrical potential, potential difference, and electromotive force in the metre—kilogram—second system (SI) is the volt; it is equal to the difference in potential between two points in a conductor carrying one ampere of current when the power dissipated between the points is one watt. An equivalent is the potential difference across a resistance of one ohm when one ampere of current flows through it. The volt is named in honour of the I8th—I9th-century Italian physicist Alessandro Volta. Ohm's law defines these electrical units, where resistance equals the ratio of potential to current, and the respective units of ohm, volt, and ampere are used universally for expressing electrical quantities. Energy consumption is measured in kWh, or kilowatt-hours. Explore how devices like ammeters and voltmeters are used to measure current and voltage across components. To better understand how voltage is measured and expressed in volts, see our guide on what is voltage.

Watt

The watt is the unit of power in the SI equal to one joule of work performed per second, or to 1/746 horsepower. An equivalent is the power dissipated in a conductor carrying one ampere of current between points at a one-volt potential difference. It is named in honour of James Watt, British engineer and inventor. One thousand watts equals one kilowatt. Most electrical devices are rated in watts. Learn how a watt defines power in electrical systems and its relationship to volts and amperes through Watts' Law.

Weber

The weber is the unit of magnetic flux in the SI, defined as the amount of flux that, linking a circuit of one turn (one loop of wire), produces in it an electromotive force of one volt as the flux is reduced to zero at a uniform rate in one second. It was named in honour of the 19th-century German physicist Wilhelm Eduard Weber and equals 10 to the 8th maxwells, the unit used in the centimetre—gram—second system.

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Three phase electricity delivers power using three alternating currents that are offset in phase. It provides consistent and efficient energy for industrial, commercial, and high-load applications, improving stability and reducing conductor size.

What is Three Phase Electricity?

Three phase electricity is a power system that uses three alternating currents, each offset by 120 degrees, to deliver constant power flow.

✅ Delivers more efficient and stable power than single-phase systems

✅ Ideal for large motors, commercial buildings, and industrial equipment

✅ Reduces conductor material and energy loss over long distances

Three phase voltage, frequency and number of wires

Three phase electricity is the dominant method of electrical power generation, transmission, and distribution across the industrialized world. Unlike single-phase systems, which rely on a single alternating current, three-phase systems use three separate currents, each 120 degrees out of phase with the others. This setup provides a consistent and balanced power flow, making it significantly more efficient for high-demand applications, such as motors, transformers, and large-scale infrastructure. Understanding the difference between alternating current and direct current is essential to grasp how three-phase systems deliver constant power using offset waveforms.

Understanding the Concept

At the heart of a three-phase system are three sinusoidal waveforms, evenly spaced to maintain a continuous flow of energy. When one phase reaches its peak, the others are in different parts of their cycle, ensuring that at any given moment, some power is being delivered. This creates what is known as constant power transfer, a major advantage over single-phase systems that experience power dips between cycles. Since three-phase systems rely heavily on accurate current flow measurement, it’s important to know what ammeters measure and how they help monitor system balance.

For industrial and commercial operations, this stability translates to increased energy efficiency, extended equipment lifespan, and reduced operating costs. Large electric motors, for example, run more smoothly on three-phase power, which avoids the surging and vibration commonly associated with single-phase inputs.

A Brief History

Three phase electricity wasn’t invented by a single person but emerged through the contributions of several pioneers in the late 19th century. Galileo Ferraris in Italy, Nikola Tesla in the United States, and Mikhail Dolivo-Dobrovolsky in Europe all played crucial roles in developing and refining the idea of three-phase alternating current. Tesla’s patents and Dolivo-Dobrovolsky’s practical systems laid the groundwork for what is now a global standard. Engineers use wattmeters to accurately measure real power in three-phase systems, while Watts Law helps calculate the relationships between voltage, current, and power.

Wye and Delta Configurations

Three-phase systems are typically wired in one of two configurations: the Wye (Y) or Delta (Δ) configuration. Each has specific advantages depending on the application:

• In a Wye connection, each phase is tied to a central neutral point, allowing for multiple voltage levels within the same system. This is common in both commercial and residential applications, where both high and low voltages are required.

• A Delta connection utilizes a closed loop with no neutral, a configuration commonly found in industrial setups. It delivers the same voltage between all phases and is ideal for running large motors without needing a neutral return path.

One of the most important relationships in these configurations is the √3 ratio between line voltage and phase voltage, a fundamental aspect that engineers use in calculating load, cable sizing, and protective device coordination.

Technical Benefits

Three-phase systems have built-in advantages that go beyond stability. Because the sum of the three phase currents is zero in a balanced load, a neutral wire is often unnecessary. This reduces the amount of conductor material needed, lowering costs and simplifying design. Additionally, three-phase motors naturally create a rotating magnetic field, eliminating the need for external circuitry to start or maintain rotation.

Another major benefit is that power output remains consistent. In single-phase systems, power drops to zero twice per cycle, but three-phase systems deliver non-pulsating power, which is especially important in sensitive or precision equipment. The function of a busbar is especially important in three-phase distribution panels, helping to manage multiple circuit connections efficiently.

Where and Why It’s Used

While most homes use single-phase electricity, three-phase is the standard in virtually all commercial and industrial environments. Factories, data centers, hospitals, and office buildings rely on it to power everything from HVAC systems and elevators to conveyor belts and industrial machines.

Three-phase is also common in electric vehicle (EV) charging stations and renewable energy systems, where efficient, high-capacity delivery is essential. If you're working with three-phase motors or transformers, knowing the role of a conductor and how electrical resistance affects current flow is fundamental to efficient design.

For sites that only have access to single-phase power, phase converters—whether rotary or digital—can simulate three-phase conditions, enabling them to operate three-phase equipment. This flexibility has made three-phase solutions accessible even in remote or rural areas. Three-phase systems often operate at medium voltage, especially in commercial settings, and their stability can reduce the risks of ground faults.

Voltage Levels and Color Codes

Depending on the region, the standard line and phase voltages vary. In North America, typical voltage values include 120/208 volts and 277/480 volts, whereas in Europe and much of Asia, 230/400 volts is more common. Wiring color codes also differ: red/yellow/blue in Europe, black/red/blue in North America, and other variations depending on the country's electrical code. These standards ensure safety, compatibility, and ease of troubleshooting.

The Global Standard for Power

Three-phase electricity is not just a technical solution; it is the foundation of modern electrical infrastructure. Its ability to deliver large amounts of power efficiently, safely, and reliably has made it the system of choice for more than a century. From powering the machines that build our world to the systems that keep us connected, three-phase electricity remains indispensable.

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Electricity supplier delivers power via transmission and distribution networks, managing tariffs, load forecasting, SCADA, and power quality, integrating renewables, smart meters, and demand response to ensure grid reliability, compliance, and efficient kWh billing.

What Is an Electricity Supplier?

An electricity supplier procures and delivers power, manages tariffs, and ensures grid reliability and power quality.

✅ Energy procurement and wholesale market participation

✅ Distribution coordination, SCADA monitoring, and outage management

✅ Tariff design, metering, billing, and power quality compliance

Who is my electricity supplier?

In Canada and the United States, it’s easy to learn which energy provider serves your property or residence. It depends on whether you are trying to find your electricity and natural gas supplier. Sometimes, homes and businesses have the power to choose their energy service utility company and the products and service they provide. Customers are free to choose. It's a competitive energy marketplace. For a plain-language primer on infrastructure, see the electricity supply overview to understand typical delivery steps.

If you use both services, your property might have the same local distribution company for both fuels, – commonly known as a "duel fuel supplier". But if your utility records are stored separately, you might need to more research to learn who supplies your natural gas services and your electricity services separately. If you are curious where the power originates, this guide to how electricity is generated explains common fuel sources and grid integration.

Here is a list of accredited Electricity Suppliers in Canada

https://www.ic.gc.ca/eic/site/mc-mc.nsf/eng/lm00525.html

Market dynamics can vary by province, and recent electricity demand trends in Canada help explain seasonal shifts in offers.

Here is a list of accredited Electricity Suppliers in the United States.

https://www.electricchoice.com/blog/25-top-providers-part-1/

When comparing providers, consult current electricity price benchmarks to contextualize quoted rates.

Who can supply electricity?

All electricity supplier companies must have a licence from the Office of Gas and Electricity Markets (Ofgem). One of the licence conditions is that a  company must produce codes of practice on:

Although the codes of practice are not legally enforceable, they will be useful when negotiating with a company and any breach should be reported to governmental regulatory authorities. Understanding the basics in this introduction to what electricity is can make those obligations clearer.

Choosing an electricity supplier

You can change your company if you wish. If you are considering changing your company you should be aware that the pricing structures, services offered and policies will differ between the different companies.You should carefully check the information and contracts of the competing electricity suppliers, and compare these to your current terms, to make sure that you choose the best deal for your needs. A lot depends on your gas bills or electricity bill, and your location by postal code. You can also estimate bill impacts by applying tips from this guide on how to save electricity while comparing plans.

Dual fuel offers

Dual fuel is the supply of gas and electricity by the same company. Some gas and electricity suppliers are licensed separately by Ofgem to supply customers with both fuels. Some companies will supply both fuels under one contract, while others will give one contract for gas and another for electricity. For households with high usage, reviewing your typical electricity power consumption patterns can reveal whether dual fuel makes financial sense.

Electricity Suppliers who make dual offers will often give a discount off the total bill as they can make administrative savings by issuing combined bills and collecting combined payments. However, this does not necessarily mean that the cost of gas and electricity

• procedures for complaints
• payment of bills, arrangements for dealing with arrears and prepayment meters
• site access procedures
• energy efficiency advice
• services for older, disabled and chronically sick people. As part of this code of practice, the company must keep a register of these people and provide services to help those who are blind and partially sighted or deaf or hearing impaired. Especially during a power outage, it is important that these customers are restored asap.The customer may have to pay for some of these services.
  • how you will be charged for your electricity supply. Some companies may make a fixed standing charge and then a unit charge for the amount of electricity used; an company may not make a standing charge, but charge a higher unit price
  • whether different charges apply to different periods during the day
  • if cheaper prices are offered for particular payment methods, for example, if you pay by direct debit
  • what service standards each company is offering, for example, for repairs, extra help for older or disabled customers.All electricity suppliers must keep a list of their customers who ask to be identified as pensioners, chronically sick or disabled.The electricity supplier must tell all its customers that it keeps such a list and give information on how customers can be added to the list
  • the company's policies, for example, on debt and disconnection.

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Electricity cost reflects kWh rates, tariffs, demand charges, power factor penalties, and TOU peak/off-peak pricing, driven by load profiles, utility billing, transmission and distribution fees, and efficiency measures in industrial, commercial, and residential systems.

What Is Electricity Cost?

Electricity cost is the total price per kWh including energy, demand, and network charges under applicable tariffs.

✅ Includes energy (kWh), demand (kW), and fixed charges

✅ Varies by TOU tariffs, peak/off-peak, and seasons

✅ Affected by power factor, load profile, and efficiency

Electricity Cost principles involve looking at how much electricity consumption and we have to understand how it's measured.

At its core, understanding power use starts with grasping what electricity is and how it behaves in circuits.

It is determined at any moment and is measured in watts consumed. For example: if you want to determine the bill energy or bill electricity rate: For a refresher, see what a watt represents to relate device ratings to instantaneous power.

• A 100-watt light bulb uses 100 watts.
• A typical desktop computer uses 65 watts.
• A central air conditioner uses about 3500 watts.

These device ratings illustrate electric load in practical terms as each appliance contributes to total demand.

If you want to know how to rate electricity pricing, you want to know how much energy you're using. When you use 1000 watts for an hour, that's a kilowatt-hour. For example:

• Ten 100-watt light bulbs on for an hour, is 1 kWh
• Ten 100-watt light bulbs on for 1/2 an hour, is 0.5 kWh
• Ten 50-watt light bulbs on for an hour, is 0.5 kWh
• One 60-watt light bulb on for an hour, is 0.06 kWh (60/1000)
• Running a 3500-watt air conditioner for an hour is 3.5 kWh.

The average U.S. household used 10,654 kWh a year in 2001, or 888 kWh/mo. (Dept. of Energy) The U.S. as a whole used 3,883 billion kWh in 2003, or 13,868 kwH per person based on a population of 300 million. (Dept. of Energy)

Watt-hours

For smaller items we use the term watt-hours instead of kilowatt-hours. For example, we say a 60-watt light bulb uses 60 watt-hours of electricity billed, not 0.060 kWh. If you're unsure, this overview of what a watt-hour means clarifies the relationship between power and time.

Note that the "-hours" part is important. Without it we'd have no idea what period of time we were talking about.

If you ever see a reference without the amount of time specified, it's almost certainly per hour.

If your device lists amps instead of watts, then just multiply the amps times the voltage to get the watts. For example:

2.5 amps x 120 volts = 300 watts

Trivia: On a peak day in 2009, California used 50,743 megawatt-hours of electricity, or 50,743,000,000 watt-hours.

How much does electricity cost?

Electricity Cost

It depends on where you live (like Ontario), how much you use, and possibly when you use it. There are also fixed charges that you pay every month no matter how much electricity you use. For example, I pay $6/mo. for the privilege of being a customer of the electric company, no matter how much energy I use. Local infrastructure and electricity supply conditions can also influence pricing tiers.

Check your utility bill for the rates in your area. If it's not on your bill then look it up on the utility's website. National summaries of electricity prices help you compare trends across regions.

The electric company measures how much electricity you use in kilowatt-hours. The abbreviation for killowatt-hour is kWh. Note that on your bill there can be multiple charges per kWh (e.g., one for the "base rate", another for "fuel") and you have to add them all up to get the total cost per kWh. This measurement is recorded by a watt-hour meter that cumulatively tracks energy over time.

Most utility companies charge a higher rate when you use more than a certain amount of energy, and they also charge more during summer months when electric use is higher. As an example, here are the residential rates prices electricity for Austin, Texas (as of 11-03):

First 500 kilowatts5.8¢ per kilowatt hour (kWh)

Additional kilowatts (May-Oct.)10¢ per kilowatt hour

Additonal kilowatts (Nov.-Apr.)8.3¢ per kilowatt hour

These figures include a fuel charge of 2.265¢ per kWh.

The average cost of residential electricity was 9.86¢/kWh in the U.S. in March 2006. The average household used 888 kWh/mo. in 2001 and would pay $87.56 for it based on the March 2006 average rate. (Dept. of Energy)

The cost of electricity varies by region. In 2003 the price ranged from 5.81¢ in Tennessee to 12¢ in California, 14.314¢ in New York, and 16.734¢ in Hawaii. In Summer 2001, electricity was a whopping 20¢/kWh in parts of California.

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