About Electricity Conversion
Electrical unit conversions cover the quantities used to describe how electricity flows and is measured: current in amperes, potential difference in volts, power in watts, and electrical charge in coulombs or amp-hours. These units relate to each other through fundamental physical laws, most notably Ohm's law (voltage equals current times resistance) and the power law (power equals voltage times current), which means several electrical quantities can be derived from others rather than measured independently.
Because electrical systems range from tiny microelectronics to household wiring to industrial power grids, the same base units get scaled with metric prefixes constantly: milliamps for small electronic currents, kilowatts for household power, and megawatts for power plant output. This converter covers the everyday and engineering electrical units, amps, volts, watts, and their common multiples, along with amp-hours used for battery capacity, using standard SI definitions so a converted battery rating, appliance power draw, or wiring specification stays accurate.
Understanding how amps, volts, and watts relate is useful well beyond a physics classroom: sizing an extension cord or circuit breaker safely, understanding an appliance's power draw, or comparing battery capacities across devices all depend on correctly relating these basic electrical quantities. Because electrical units scale from the tiny (milliamps in a microcontroller circuit) to the enormous (megawatts from a power plant), fluency in moving between prefixed units and calculating one quantity from the other two is a practical skill for anyone working with electronics, appliances, or home wiring.
When working through an electrical conversion or calculation, it helps to write out the known values (voltage, current, power, or resistance) explicitly before applying a formula, since mixing up which of the interrelated quantities is being solved for is a common source of error, especially when a problem provides more information than is strictly needed to find the answer.
Common Electricity Measurements
The ampere (A) is the SI base unit of electric current; the volt (V) and watt (W) are SI derived units for potential and power.
| Unit | Symbol | Equal to |
|---|
| Milliampere | mA | 0.001 A |
| Ampere | A | 1 A (SI base unit) |
| Volt | V | 1 V (SI derived unit) |
| Watt | W | 1 V × 1 A |
| Kilowatt | kW | 1,000 W |
| Amp-hour | Ah | 3,600 coulombs of charge |
Conversion Formulas
- Power (W) = Voltage (V) × Current (A)
- Current (A) = Power (W) ÷ Voltage (V)
- Voltage (V) = Current (A) × Resistance (Ω), by Ohm's law
- Milliamps to amps: divide by 1,000
- Watts to kilowatts: divide by 1,000
Practical Examples
- A device drawing 2 A at 120 V uses 240 W, since 2 × 120 = 240.
- A 1,200 W hair dryer on a 120 V circuit draws 10 A, since 1,200 ÷ 120 = 10.
- A phone battery rated 3,000 mAh holds 3 Ah of charge.
- A circuit with 12 V and 4 Ω resistance carries 3 A, since 12 ÷ 4 = 3.
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Frequently asked questions
How are watts, volts, and amps related?
They're linked by the power law: watts equal volts multiplied by amps (P = V × I). This means if you know any two of the three quantities for a simple resistive circuit, you can calculate the third.
What is an amp-hour and why is it used for batteries?
An amp-hour (Ah) measures electrical charge capacity, how much current a battery can supply over time. A 2 Ah battery can theoretically supply 2 amps for one hour, or 1 amp for two hours, which makes it a practical way to compare battery capacity independent of voltage.
Why do household circuits in the US use 120V while Europe uses 230V?
The difference is historical: US electrical infrastructure standardized around 120V (and 110V earlier) in the early 20th century, while European countries standardized around 220-240V. Higher voltage systems can deliver the same power with less current, reducing wiring losses, which is one reason many countries chose the higher standard.
How do I convert milliamps to amps?
Divide the milliamp value by 1,000. For example, a phone charger rated at 2,000 mA supplies 2 A.
What does Ohm's law say about voltage, current, and resistance?
Ohm's law states that voltage equals current multiplied by resistance (V = I × R). It's the foundational relationship for basic circuit calculations, alongside the power law relating watts to volts and amps.
History
Electrical units are named after 19th-century scientists whose experiments defined them: the volt after Alessandro Volta, inventor of the first chemical battery; the ampere after André-Marie Ampère, an early researcher of electromagnetism; and the ohm after Georg Ohm, who formalized the relationship between voltage, current, and resistance in 1827. These units were formally standardized internationally in the late 1800s as electrical engineering became a practical industry rather than a laboratory curiosity, driven by the need for interoperable equipment across the rapidly expanding telegraph and electrical power networks. The ampere later became one of the seven SI base units, with volts and ohms defined in terms of it, cementing a coherent system that still underlies every electrical specification today.
Why Convert Electricity Units?
Electrical specifications often need converting between units when comparing international appliance ratings, wiring codes, or component datasheets. Since power and energy usage are closely related, see our Power converters and our Energy converters.