Volts, Amps, Watts and Ohms: What Each One Actually Measures

Four units cover most practical electrical work, and confusing them causes both blown equipment and bad purchasing decisions. Each measures something genuinely different.

Voltage: electrical pressure

A volt measures potential difference — the energy available per unit of charge. One volt is one joule per coulomb.

Voltage is always between two points. "The voltage at this wire" is incomplete unless a reference is implied, usually ground. This is why a bird on a high-voltage line is unharmed: both feet are at nearly the same potential, so there is negligible difference across the bird.

Current: rate of flow

An ampere measures charge flow: one coulomb per second, roughly 6.24 × 10¹⁸ electrons past a point each second.

Current is what harms you. Roughly 1 mA is perceptible, 10 mA can cause muscle contraction strong enough to prevent letting go, and 100 mA through the chest can cause fibrillation. Voltage matters only because it drives current through your body's resistance.

Resistance: opposition to flow

An ohm measures how much a material resists current. One ohm allows one ampere to flow under one volt.

Ohm's law ties the three together:

V = I × R

Any two give the third. This single relationship covers a large fraction of practical electrical work.

Power: rate of energy use

A watt is one joule per second. For electrical circuits:

P = V × I

Combined with Ohm's law this yields P = I²R and P = V²/R, both constantly useful.

Where the water analogy breaks

Voltage as pressure, current as flow rate, resistance as pipe narrowness — it is a reasonable starting point. But it fails in ways worth knowing:

• Electricity needs a complete circuit; water can spill.
• Electrons drift remarkably slowly, on the order of millimetres per second. The signal travels near light speed, because the field propagates rather than the electrons themselves.
• The analogy has no equivalent for inductance, capacitance or AC phase relationships.

AC complications

Alternating current adds distinctions that matter for anything larger than a lightbulb.

RMS versus peak. Mains "230 V" is an RMS value — the equivalent DC voltage for heating purposes. The actual peak is about 325 V. Insulation must be rated for the peak, not the RMS.

Power factor. In AC circuits with motors or transformers, current and voltage can fall out of phase, so not all apparent power does useful work:

• Real power (W) — does actual work
• Apparent power (VA) — voltage × current
• Power factor — real ÷ apparent

This is why UPS units are rated in VA rather than watts, and why a 1000 VA UPS may only support 600 W of load. Industrial customers are often billed penalties for poor power factor, because the utility must supply the current regardless of whether it does useful work.

Practical calculations

Will this run on my circuit? A 1500 W heater on 120 V draws 12.5 A. On a 15 A circuit that is 83% — above the 80% continuous-load guideline, so it should have a dedicated circuit.

Voltage drop in long runs. A 100-foot run of 14 AWG carrying 15 A at 120 V drops around 3 V, or 2.5%. Beyond about 3% you should step up wire size, or motors run hot and lights dim.

Why transmission uses high voltage. Losses are I²R. Raising voltage tenfold for the same power cuts current tenfold, which cuts losses a hundredfold. That is the entire reason for high-voltage transmission lines.

Battery capacity

Batteries are usually rated in amp-hours, which is charge rather than energy. Comparing a 5000 mAh phone battery with a 100 Ah car battery is meaningless without voltage.

Multiply by voltage to get watt-hours: a 5000 mAh battery at 3.7 V holds 18.5 Wh; a 100 Ah battery at 12 V holds 1200 Wh. The car battery stores about 65 times more energy.

Our electrical converters cover voltage, current, resistance, charge, capacitance and inductance.