Wattage Calculator
Calculate wattage from voltage and current for any device. Enter circuit values to calculate the selected electrical quantity.
Device Parameters
Electronics guide · Electrical power, energy, and wiring
About Wattage Calculator
Calculate electrical power from the voltage, current, resistance, or other supported values entered in the selected mode. The result is only as useful as the input units and circuit assumptions, especially for AC loads.
Power and energy calculations relate quantities such as voltage, current, resistance, time, and load. AC and DC systems can require different assumptions, and a calculated estimate is not a substitute for checking source capability, wire ratings, protection, efficiency, and equipment specifications.
How to Use This Tool
- 1.Select the formula or known-value mode that fits the information you have.
- 2.Enter values with their units and confirm any AC, DC, or power-factor option.
- 3.Calculate wattage and compare it with the rated limits of the source, wire, and load.
Technical Details to Check
1.Check AC or DC assumptions
In AC calculations, voltage and current may refer to RMS values, and phase relationships can affect real power. Do not apply a DC relationship to an AC system unless the tool's stated assumptions fit.
2.Power is not the same as energy
Watts describe a rate of energy use; watt-hours and joules describe an amount of energy over time. A runtime or consumption estimate needs both a load and a duration or duty profile.
3.Real systems lose energy
Inverters, batteries, wiring, motors, and power supplies are not perfectly efficient. Confirm whether the selected calculation includes efficiency, power factor, voltage drop, or only ideal values.
4.Ratings and protection are essential
An estimated current or power value does not determine a safe wire size, fuse, battery connection, or supply. Follow applicable standards, product ratings, and qualified design guidance.
How the Calculation Works
For DC or a resistive load, power can be found with P = VI, P = I²R, or P = V²/R. For AC real power, voltage and current may need an additional power-factor term. The page uses the mode selected in its interface.
Worked Example
A 5 A current through a 10 Ω resistor gives P = I²R = 25 × 10 = 250 W, assuming the resistance remains stable.
Where This Tool Can Help
Check a simple component power estimate or understand how voltage and current combine into watts.
Estimating a load
Use known voltage and current or power figures to understand a device's approximate demand before checking its label and startup behavior.
Planning energy use
Combine a power estimate with realistic operating hours to compare possible consumption, while accounting separately for standby use and efficiency.
Checking a wiring estimate
Use current or voltage-drop results as an early check, then verify conductor size and protection with the correct installation rules and environment.
Comparing supply options
Compare candidate sources on the same voltage, current, power, and duration basis, and include headroom where the equipment specification requires it.
Tips for More Reliable Results
- Confirm whether voltage and current are RMS, peak, nominal, or measured under load.
- Keep watts, watt-hours, joules, and time units distinct when entering values.
- Include efficiency or power factor only when the tool provides that input and you have a suitable value.
- Check conductor, connector, battery, and supply ratings before using an estimate in a real installation.
Limitations and Assumptions
This does not include all AC waveform, transient, thermal, or installation effects. Resistors and wires can overheat; follow ratings, safety requirements, and qualified electrical guidance.
Electrical power tools estimate values from entered figures and stated assumptions. They do not measure a circuit, model every transient or loss, or certify wiring and equipment as safe. Confirm important designs with manufacturer data and applicable electrical requirements.
Frequently Asked Questions
What is a watt?
A unit of power, or energy transfer per unit time.
Why can AC calculations need power factor?
Voltage and current may not be in phase, so apparent power differs from real power.
Does the result prove a component is safe?
No. Compare it with ratings and real operating conditions.
Are watts and watt-hours interchangeable?
No. Watts are a rate; watt-hours measure energy used over time.
Does a calculated value make a circuit safe to build?
No. Safety depends on ratings, protection, installation conditions, and applicable standards as well as the arithmetic.