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 · Jacob Willis, Net Zero Lead · Last reviewed July 2026

Every energy manager works with electricity, yet plenty of capable professionals reached their role without ever being taught what a volt actually is. This lesson fixes that from the ground up. No prior electrical knowledge is assumed: by the end you will know what voltage, current and resistance mean, how they connect through one simple law, and how they combine into power and cost, which is where energy management begins.

The water analogy

The easiest way into electricity is to picture water in a pipe.

Voltage is the pressure pushing the water along. It is measured in volts (V), and in the UK a standard socket supplies about 230 V. Higher voltage means a stronger push. On its own, though, pressure moves nothing: a closed tap has full pressure behind it and no flow.

Current is the flow itself: how much charge is actually moving. It is measured in amperes, always shortened to amps (A). A phone charger draws well under 1 A; a kettle draws about 13 A, which is why it gets the biggest fuse in the drawer.

Resistance is how much the pipe fights the flow: a narrow pipe passes less water at the same pressure. It is measured in ohms (Ω). Every cable, heating element and appliance has resistance, and it is resistance that turns electrical energy into heat, deliberately in a kettle, wastefully in an undersized cable.

Ohm's law: the one equation to remember

These three quantities are locked together by Ohm's law:

Current = Voltage ÷ Resistance, usually written I = V ÷ R

Push harder (more volts) and more current flows. Add resistance and less current flows. A kettle element of about 17.7 Ω on a 230 V supply passes 230 ÷ 17.7 ≈ 13 A. That is not a coincidence of design; the element's resistance is chosen to draw exactly the current the plug and circuit can safely deliver.

Generations of electricians have remembered the rearrangements with the same triangle. Tap the quantity you want to find and it shows you how to get it from the other two:

The Ohm's law triangle: cover the one you want
VIR
I = V ÷ R
The kettle: 230 V ÷ 17.7 Ω ≈ 13 A

Top over bottom: voltage divided by resistance gives the current that flows.

Cover the quantity you want with a thumb and the triangle shows how to get it from the other two. The worked numbers are the lesson's kettle: a 17.7 Ω element on a 230 V supply.

From volts and amps to power

Power is the rate at which energy is being used, and for simple loads it is voltage multiplied by current:

Power (watts) = Voltage × Current, or P = V × I

Worked example — the kettle, from first principles
Given
  • UK supply voltage: 230 V
  • Kettle current: 13 A
  • It runs for 4 minutes to boil; electricity costs £0.20 per kWh
Find
The kettle's power, and the cost of one boil.

If the kWh as a unit feels shaky, the units and conversions lesson covers it properly; the one-line version is that a kilowatt hour is the energy a 1 kW load uses in one hour, and it is the unit your bills are priced in.

Watts are the rate, kWh are the total

Confusing power (kW) with energy (kWh) is the most common electrical mistake in business, and it costs real money in misread bills and bad business cases. Power is how fast energy is being used right now, like the speedometer. Energy is how much has been used over time, like the odometer. A 2 kW heater is not "using 2 kWh"; it is using 2 kWh per hour it runs, and run hours are usually where the savings hide.

Why this matters for energy management

Nearly every electrical saving on a site is one of these three moves: reduce the power a load draws (a more efficient motor, an LED instead of a discharge lamp), reduce the hours it runs (controls, switching things off), or both. Volts and amps are how you measure what a load actually draws, rather than trusting a nameplate, and a simple clamp meter reading of current is often the first hard evidence in an investigation. The rest of this course builds on exactly these quantities: first why the grid uses alternating current, then why some current does no useful work at all.

Sources and further reading