Electrical Power (DC) Calculator
Solve P = V·I for power, voltage, or current in a DC circuit — with an Ohm's law cross-check, an energy & electricity-cost helper, a P-vs-V chart, and reference tables.
Voltage — linear contribution
At a fixed current, power scales directly with voltage: doubling V doubles P.
Current — also linear, but ties to R²
At a fixed resistance, power scales with the square of current (P = I²R) — current changes matter more than they first appear.
Resistance links V and I
Via Ohm's law V = IR, any two of V, I, R fix the third, and all three equivalent power formulas (VI, I²R, V²/R) must agree.
DC only, as written
P = V·I gives real power directly for DC. For AC it only holds for a purely resistive load or with RMS values and unity power factor — otherwise P = V·I·cos φ.
Ohm's Law Cross-Check & Energy/Cost Helper
Add a resistance value to cross-check all three power formulas, and estimate energy use and electricity cost over time.
Ohm's law cross-check (uses V, I, P above)
Energy & electricity cost (uses P above)
Common DC Voltage Standards
| Voltage | Typical use |
|---|---|
| 1.5 V | Single alkaline/NiMH cell |
| 3.3 V | Digital logic, microcontrollers |
| 3.7 V | Single Li-ion/LiPo cell (nominal) |
| 5 V | USB power |
| 12 V | Automotive, many DC power supplies |
| 24 V | Industrial control, some solar systems |
| 48 V | Telecom equipment, server power |
Power Formula Triangle
| Solve for | Formula |
|---|---|
| P | P = V·I = I²R = V²/R |
| V | V = P/I = I·R = √(P·R) |
| I | I = P/V = V/R = √(P/R) |
| R | R = V/I = V²/P = P/I² |
How to Use This Calculator
The power equation links three quantities: voltage, current, and power.
Choose what to solve for
Pick P, V, or I — the formula rearranges automatically.
Enter values or use presets
Common DC voltages and currents are one click away.
Check the chart
See how power scales with voltage at a few representative currents.
Cross-check with Ohm's law
The companion panel shows the implied resistance and confirms all three power formulas agree.
Estimate energy and cost
Enter hours of use per day and your electricity rate to see daily and monthly cost.
Go Advanced
Adds sensitivity and the full Ohm's law companion panel.
What This Tool Offers
Solve any variable
Compute power, voltage, or current.
Power vs. voltage chart
Compare your current against two reference currents.
Ohm's law cross-check
See the implied resistance and verify P = VI = I²R = V²/R.
Energy & cost estimator
Turn power and hours of use into kWh and estimated cost.
Reference tables
Common DC voltages and the full power formula triangle.
Sensitivity check
See how a 1% change in an input moves the result.
What Does P = V·I Measure?
Electrical power is the rate at which energy is delivered to a load. In a DC circuit, multiplying the voltage across the load by the current through it gives that rate directly, in watts.
V = P / I
I = P / V
Frequently Asked Questions
Only for purely resistive AC loads, or when using RMS values with a power factor of 1. Reactive AC loads (motors, transformers, many inductive/capacitive devices) need the full P = V·I·cos φ relationship instead.
Power (watts) is a rate — how fast energy is being used. Energy (watt-hours or kilowatt-hours) is power multiplied by time, which is what electricity bills actually charge for.
Once you know any two of V, I, and P for the load, Ohm's law (R = V/I) and the power formulas (R = V²/P = P/I²) all give the same resistance — the cross-check panel computes this automatically.
Because at fixed resistance, voltage itself scales with current (V = IR), so substituting into P = V·I gives P = I·(IR) = I²R — current affects power both directly and through the voltage it produces.
This calculator only computes the power relationship — it doesn't account for wire ampacity, derating, or local electrical codes. Always follow your jurisdiction's electrical code and manufacturer ratings for actual component sizing.
References
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| P | Power | Watts |
| V | Voltage | Volts |
| I | Current | Amperes |
What it means
In a DC circuit, the electrical power delivered to a component is the product of the voltage across it and the current through it: P = V * I. This power represents the rate of energy conversion, measured in watts (joules per second). For a resistor, power can also be expressed as I²R or V²/R using Ohm’s law. This formula is fundamental for sizing power supplies, selecting components with adequate power ratings, and calculating energy consumption. In DC motors, the electrical input power is converted to mechanical power (minus losses). In batteries, the power output depends on the terminal voltage and load current. Understanding this relationship is vital for efficiency calculations, thermal design, and battery life estimation. The formula also applies to instantaneous power in AC circuits, but for AC, average power is V_rms * I_rms * cosθ. The power rating of resistors, lamps, and heaters is typically specified in watts. For safety, components must be operated below their maximum power rating. Example: A 12V motor drawing 2A consumes 12 * 2 = 24 watts of power.
Worked example
DC Power – Practical Example
Real‑World| Parameter | Value |
|---|---|
| V | 12 V |
| I | 1.5 A |
| Formula | P = V · I |
Common mistakes
- Units: V in volts, I in amperes → P in watts.
- RMS values: For AC, use RMS voltage and current to get average power.
- Instantaneous power: p(t) = v(t)·i(t) – this formula gives average power for DC.
- Sign: Positive power means power consumed (passive sign convention).
- Alternatives: P = I²R = V²/R – useful when only two quantities are known.
Applications
Electrical power in a DC circuit is the product of voltage and current, representing the rate of energy conversion or transfer, measured in watts. This is the fundamental power equation, used to determine the power consumption of loads, to size wires and overcurrent protection, and to calculate energy costs. Engineers use P = VI to design power supplies, to select appropriate components, and to ensure that devices operate within their rated power limits. In renewable energy systems, it helps estimate the output of solar panels and wind turbines. By understanding this relationship, professionals can optimise efficiency and avoid overheating. This simple formula is the basis for all power calculations in electrical engineering.
- Power consumption estimation of appliances and loads
- Sizing of conductors, fuses, and circuit breakers
- Design of power supplies and battery systems
- Energy efficiency analysis and cost calculation
- Educational foundation for power systems
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