Electrical Power Calculator

P = IV — compute power from voltage and current, or from voltage and resistance, or current and resistance.

Inputs

Result

Loading calculator…
—

General calculation reads

Amazon affiliate
How Not to Be Wrong
Jordan Ellenberg - math thinking
Shop on Amazon
The Joy of X
Steven Strogatz - accessible math
Shop on Amazon
Scientific calculator (Casio fx-991EX)
Shop on Amazon

As an Amazon Associate we may earn from qualifying purchases. This does not add cost for you.

How to use this calculator

  • Pick which two of V/I/R you have.
  • Enter both values.
  • Read power in watts and kW.

About this calculator

Power in an electrical circuit is the rate of energy transfer. P = IV in DC; for AC with phase difference φ, P = IV cos φ (real power). Energy used over time: E = P × t (kWh = kW × hours). A 100 W bulb running for 10 hours uses 1 kWh.

Frequently asked

Watts vs volt-amperes (VA)?+
In DC and resistive AC, watts = VA. In AC with reactive load, real power (W) = VA × power factor (cos φ).
How much does a 1500W heater cost per hour?+
At $0.15/kWh: 1.5 kW × 1 hr × $0.15 = $0.225/hr. Use the kwh-cost-per-month calculator for monthly estimates.
Why is electrical power important?+
It determines wire size, fuse rating, breaker size, and energy bill. Undersized wiring on high-wattage circuits causes fire hazard.
How does P = I²R show up in real life?+
Power loss in transmission lines is I²R, which is why grids step up to high voltage (lower I) over long distances.
kW vs kWh?+
kW is rate (instantaneous power). kWh is energy (kW × hours). Bills are in kWh.

Related calculators

Ohm's Law Calculator (Extended)
V = IR, P = IV = I²R = V²/R. Pick which two values you know and solve for the others.
Parallel Resistance Calculator
1/R = 1/R₁ + 1/R₂ + … — find equivalent resistance of resistors in parallel.
Series Resistance Calculator
R_total = R₁ + R₂ + R₃ + … — find equivalent resistance of resistors in series.
kWh Cost Per Month
Cost = W × hr/day × 30 / 1000 × $/kWh.
Capacitance Calculator
C = Q/V or parallel-plate C = ε₀A/d. Compute capacitance, charge stored, or energy.
RC Time Constant Calculator
τ = RC. Time for an RC circuit to charge to ~63% (or discharge to ~37%) of final value.

More tools you might like

Hand-picked tools that pair well with this one — same audience, same intent.

Capacitance Calculator
Calc

C = Q/V or parallel-plate C = ε₀A/d. Compute capacitance, charge stored, or energy.

Parallel Resistance Calculator
Calc

1/R = 1/R₁ + 1/R₂ + … — find equivalent resistance of resistors in parallel.

Series Resistance Calculator
Calc

R_total = R₁ + R₂ + R₃ + … — find equivalent resistance of resistors in series.

Ohm's Law Calculator (Extended)
Calc

V = IR, P = IV = I²R = V²/R. Pick which two values you know and solve for the others.

RC Time Constant Calculator
Calc

τ = RC. Time for an RC circuit to charge to ~63% (or discharge to ~37%) of final value.

Power of Attorney Request Worksheet (Pre-Attorney)
Docs

A planning worksheet to organize a power of attorney before you see an attorney or complete a state form — who your agent is, the type of POA, which powers to grant, when it takes effect, durability, and limits.