Ohm’s Law and Power, in Plain English

Every electrician learns Ohm’s law early, and we still use it nearly every day. It is the quickest way to answer questions like “why does this breaker keep tripping?” or “can this circuit handle one more appliance?” You don’t need to be an electrician to follow it.

The four numbers

A common way to picture electricity is water in a hose:

• Voltage (V, volts) is the pressure, the push behind the electricity. Most U.S. homes have 120 V and 240 V circuits.
• Current (I, amps) is how much electricity is flowing, like gallons per minute.
• Resistance (R, ohms, Ω) is what restricts the flow. A heater element has fairly low resistance; a nightlight has much more.
• Power (P, watts) is the rate the energy is used, the number printed on bulbs and appliances.

The hose picture isn’t perfect, but it gets the relationships right: more push gives more flow, and more restriction gives less.

Ohm’s law

For a simple resistive load, current equals voltage divided by resistance. Written the usual way: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. The triangle above is a memory trick: cover the letter you want and the other two show the math.

Example: a 120 V circuit feeding a heater element of 9.6 Ω draws 120 ÷ 9.6 = 12.5 A.

The power formula

Power ties the other numbers to the label on your appliance: P = V × I. That same 12.5 A heater on 120 V uses 120 × 12.5 = 1,500 W, a very common space heater size. Combine the two formulas and you also get P = I² × R and P = V² ÷ R.

Energy on your utility bill is power over time: 1,500 W for one hour is 1.5 kilowatt-hours (kWh).

Try it

Ohm’s law calculatorEnter any two values. We fill in the other two.

Volts (V)Amps (I)Ohms (R)Watts (P)Clear

For resistive loads. Motors and electronics have power factor; real-world readings differ.

Why it matters at home

• Breakers and wire are rated in amps. A common 15 A, 120 V circuit can carry about 1,800 W in total (120 × 15). Two 1,500 W heaters on that circuit add up to about 25 A, which is why the breaker trips.
• 240 V moves the same power with half the current. A 4,800 W water heater draws 40 A at 120 V but only 20 A at 240 V. Less current means smaller wire and less heat, which is why big appliances use 240 V.
• Heat in a bad connection is I²R. A loose or corroded connection adds resistance right where current flows through it, and the heat climbs with the square of the current. That is how a loose lug can scorch a panel even when the breaker never trips.

Where the simple math stops

Ohm’s law describes heaters, incandescent bulbs and other purely resistive loads very well. Motors, LED drivers and electronics also have power factor and inrush current, so the watts on the label and the amps you measure don’t always line up neatly. We cover that in AC power: single, split and three-phase.