Energy Academy
Level 1/Electrical Science Refresher
AC Fundamentals1 / 10

Voltage, Current & Power

Ohm's law, relationships between V, I, R, and how power is calculated.

10 min read

Electricity is everywhere in energy systems — powering motors, lighting, heaters, and controls. Before you can manage electrical energy, you need to understand the fundamentals: voltage, current, and power, and how they relate through Ohm's law.

Voltage, current, and resistance

Voltage (V) is the electrical pressure that pushes current around a circuit. Measured in volts.

Current (I) is the flow of electrons. Measured in amperes (amps, A).

Resistance (R) is how much a component opposes the flow of current. Measured in ohms (Ω).

The relationship between them is Ohm's law:

V = I × R

Example: If a heating element has 10 Ω of resistance and 20 amps of current flow through it, the voltage across it is 20 × 10 = 200 V.

Ohm's law is foundational

Understanding V = I × R unlocks nearly all electrical calculations. If you know any two, you can find the third.

Power in electrical systems

Power (P) is the rate at which electrical energy is used. Measured in watts (W) or kilowatts (kW).

Power is calculated as:

P = V × I

or equivalently:

P = I² × R (from substituting Ohm's law)

Example: A device with 230 V across it drawing 5 amps uses P = 230 × 5 = 1,150 W (or 1.15 kW).

This is the rate of energy consumption right now. If it runs for 1 hour, it uses 1.15 kWh.

Single-phase AC: the sine wave

Household and office electricity is single-phase AC — the voltage and current oscillate sinusoidally at 50 Hz in the UK (50 cycles per second).

The instantaneous voltage varies: V(t) = V_peak × sin(2πft)

But what matters for practical power calculations is the RMS value (root mean square), which is what meters show. For a 230 V outlet, that's the RMS voltage; the peak is actually about 325 V.

Why RMS? It's the equivalent DC voltage that would deliver the same power. A 230 V RMS AC source delivers the same power as a 230 V DC source of the same current.

Tip

When you see "230 V" on a meter or spec sheet, that's always the RMS value, not the peak. You can use it directly in P = V × I without worrying about the sine wave.

Resistive loads (heaters, lights, etc.)

For a resistive load — one where current is in phase with voltage (a heating element, incandescent light) — the power is simply:

P = V × I = I² × R = V² / R

All three forms are equivalent.

Example: An electric heater with 10 Ω resistance in a 230 V circuit draws I = V / R = 230 / 10 = 23 A, so P = 230 × 23 = 5,290 W (5.29 kW).

In energy management, you'll often see:

  • Heating elements, lighting (incandescent or LED), and resistive loads behave this way
  • Voltage is what the circuit provides (230 V single-phase, or a higher voltage in three-phase)
  • Current and power are what you measure; use them to calculate energy use

Practical implications for energy management

  • Higher voltage = lower current for the same power (P = V × I constant). This is why distribution systems use high voltages — it reduces current, which reduces losses in wires (P_loss = I² × R).
  • Higher current = more heat in wires. An undersized wire carrying high current gets hot and wastes energy. Proper cable sizing is crucial.
  • Power (watts) is what you pay for. Your electricity bill charges per kWh, which is power × time.