⚡ Electrical Power Calculator

Electrical power is the rate at which electrical energy is transferred by a circuit. In a DC circuit, power is simply P = V × I (voltage × current), measured in watts (W). In AC circuits with reactive components (inductors and capacitors), the situation is more complex: some energy oscillates between the source and the reactive components without doing useful work. This leads to three types of power: real power P (watts, the energy that does actual work), reactive power Q (VAR, volt-amperes reactive, the energy bouncing back and forth), and apparent power S (VA, the vector sum of P and Q).

The power factor (PF = cos φ) is the ratio of real power to apparent power: PF = P/S. A purely resistive load (heater, incandescent lamp) has PF = 1.0, meaning all apparent power becomes real power. An electric motor might have PF = 0.8, meaning 80% of the apparent power does useful work and 20% is reactive. Utilities charge industrial customers for low power factor because the reactive power still loads the grid even though it does no useful work.

For DC circuits: P = V × I, and P = I²R, and P = V²/R are all equivalent. For AC circuits with RMS values and phase angle φ between voltage and current:

The derived metric energy consumption over time is Energy = P × t, measured in watt-hours (Wh) or kilowatt-hours (kWh). A 100 W light bulb running for 10 hours consumes 1000 Wh = 1 kWh of energy.

1. Select circuit type: DC or AC.
2. Enter voltage (V) and current (I).
3. For AC, enter the power factor (PF = cos φ, between 0 and 1).
4. Click Calculate to get real power, apparent power, reactive power.
5. Review the energy consumption estimate for a given operating time.

What is the difference between watts (W) and volt-amperes (VA)?

Watts measure real power—the energy that does useful work like heating, lighting, or mechanical motion. VA measures apparent power—the total power the source must deliver, including reactive power that bounces back. For a resistive load (PF=1), W = VA. For a motor with PF=0.8, a 1000 VA load delivers only 800 W of useful work.

What is a good power factor?

PF = 1.0 (purely resistive) is ideal. Utilities typically require industrial customers to maintain PF ≥ 0.9. Below 0.85 is considered poor and may incur penalty charges. Capacitor banks or synchronous condensers are used to correct low power factor caused by inductive loads.

How do I calculate energy cost from power?

Energy (kWh) = Power (W) × Time (h) / 1000. Cost = Energy × Rate ($/kWh). A 1500 W heater running 4 hours/day for 30 days at $0.12/kWh costs 1500 × 4 × 30 / 1000 × $0.12 = $21.60.

What is reactive power and is it wasteful?

Reactive power (VAR) is the energy that oscillates between the source and inductive/capacitive components without doing useful work. It does not show up on your energy bill (which measures real power), but it loads the grid—thicker wires, larger transformers, and more generator capacity are needed to handle it. Utilities charge industrial customers for both energy (kWh) and demand/apparent power (kVA).

Can power factor be greater than 1?

No. Since PF = cos(φ), it ranges from 0 to 1. PF = 0 means purely reactive (no real power), PF = 1 means purely resistive (all apparent power is real). A negative power factor is possible if the load generates power (like a solar inverter feeding back to the grid).

How is three-phase power different from single-phase?

In a balanced three-phase system, total power = √3 × V_line × I_line × PF, where V_line and I_line are line-to-line voltage and line current. Three-phase delivers power more smoothly (less ripple) and uses less conductor material for the same power level, which is why it is standard for industrial applications.

Power factor correction is important for industrial electricity customers: adding parallel capacitors across inductive loads (motors, transformers) can raise PF from 0.7 to 0.95, reducing the apparent power and thus the utility's demand charge. UPS systems and generators are rated in VA, not watts, because they must supply the full apparent power regardless of power factor. A 1000 VA UPS at PF = 0.6 can only deliver 600 W of real power. When measuring power with a multimeter, note that cheap meters measure RMS voltage and current separately and compute apparent power (V×I)—only a true power meter (with phase measurement) can read real power. For three-phase systems, the total power is P = √3 × V_line × I_line × PF (for balanced loads). The electricity meter on your house integrates real power over time to compute kWh for billing.