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Ohm's Law and Power Calculator

Solve DC, single-phase, three-phase, and energy-cost calculations with power factor and power triangle outputs for field-ready electrical analysis.

Most basic calculators miss AC reality. This tool keeps DC equations, then adds single-phase and three-phase power calculations with power factor, apparent power, reactive power, and operating cost outputs for practical electrical troubleshooting.

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Ohm's Law and Power Formulas — Quick Reference

DC / Ohm's Law

V = I × RI = V / RR = V / IP = V × IP = I² × RP = V² / R

AC Single-Phase

kW = V × I × PF / 1000kVA = V × I / 1000kVAR = kVA × sin(θ)PF = kW / kVAI = kVA × 1000 / VI = kW × 1000 / (V × PF)

AC Three-Phase

kW = √3 × V × I × PF / 1000kVA = √3 × V × I / 1000√3 = 1.732I = kVA × 1000 / (√3 × V)I = kW × 1000 / (√3 × V × PF)V(L-N) = V(L-L) / √3

Energy and Cost

kWh = kW × hoursCost = kWh × rateDaily = kW × hrs/day × rateMonthly ≈ Daily × 30Annual = Daily × 365kW = kVA × PF

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Ohm's Law for Electricians: What Basic Tools Miss

DC formulas are straightforward, but AC troubleshooting requires power factor awareness. Once inductive loads enter the picture, apparent power and reactive power become critical for conductor and equipment decisions.

kW vs kVA vs kVAR

kW is real work, kVA is total current demand, and kVAR represents reactive exchange. Use apparent power for circuit loading decisions; use real power for energy cost and work output understanding.

Field TipFor three-phase checks, ensure you use line-to-line voltage with the √3 factor. Using single-phase equations on three-phase circuits gives incorrect power values.

Common Field Calculations

Current from known watts

Single-phase current estimate: I = P / (V × PF). Three-phase estimate: I = P / (1.732 × V × PF).

Load operating cost

Daily cost = kW × hours × rate. Monthly and annual projections help compare equipment options and identify high-cost loads worth optimizing.

Frequently Asked Questions

What is power factor and why does it matter?

Power factor is the ratio between real power and apparent power. Lower PF means more current is required for the same useful output, affecting conductor and equipment loading.

What is the difference between kW and kVA?

kW is real work. kVA is total electrical demand. They are equal only when PF is 1.0.

Why does three-phase use √3?

Balanced three-phase systems use vector relationships between phases; √3 is required when using line-to-line voltage in power equations.

How do I convert amps to watts?

For DC or resistive AC: W = V × A. For AC with PF: W = V × A × PF. For three-phase: W = 1.732 × V × A × PF.

What is reactive power (kVAR)?

Reactive power is non-working power exchanged by inductive/capacitive elements. It increases current demand without equivalent useful output.

How do I estimate three-phase motor current?

Use I = kW × 1000 / (1.732 × V × PF), then validate against measured field current and applicable NEC motor rules.

Ohm's Law and Power Calculator

Ohm's Law and Power Formulas — Quick Reference