Electrical sizing mistakes are expensive: undersized wire, a transformer that saturates, or a motor starter that trips on kVA instead of kW. You do not need a bench full of meters to check the first-pass math.
Ohm’s law (V = IR) and power (P = VI) are the foundation. If you know voltage and load resistance, current and wattage follow immediately: thetoolss.com/tool/ohms-law-power-calculator
Three-phase real power uses the √3 factor on line-to-line voltage: P = √3 × VL × IL × PF. Apparent power (kVA) ignores power factor; reactive power (kVAR) is the triangle remainder. That split matters when you size inverters, generators, and power-factor correction: thetoolss.com/tool/three-phase-power-calculator
Transformers scale voltage with turns and current inversely. A 230 V to 12 V winding with 460 primary turns needs about 24 secondary turns, and secondary current rises by the same ratio: thetoolss.com/tool/transformer-turns-ratio-calculator
Capacitors store ½CV² joules. The RC time constant τ = R×C is the time to ~63% charge; five time constants is a practical “fully charged” mark: thetoolss.com/tool/capacitor-energy-rc-time-calculator
Inductive reactance XL = 2πfL rises with frequency. On 50/60 Hz mains that number is modest; on PWM motor drives it is not: thetoolss.com/tool/inductor-reactance-calculator
These calculators run in the browser. Treat results as planning estimates and confirm with manufacturer tables before you buy or energize equipment.