Why the Lights Go Out: Volts, Amps, Watts, and Ohm's Law

Plug too many space heaters into one outlet and a breaker in your electrical panel will suddenly flip off, killing every light in the room. That's not bad luck, it's math. A standard US household circuit is rated for 15 or 20 amps at 120 volts, and once you cross that line, the breaker trips on purpose to keep the wires from overheating and starting a fire. Understanding volts, amps, watts, and the relationship between them, called Ohm's Law, is the difference between fixing a dead outlet yourself and accidentally burning your house down.

- What voltage, current (amps), and resistance actually measure - How Ohm's Law (V = I x R) connects all three - How to calculate watts and figure out if a circuit is overloaded - Why circuit breakers trip and what that protects you from

Electricians often compare electricity to water in a pipe. Voltage (measured in volts, V) is like water pressure, the force pushing electricity through a wire. Current (measured in amps, A) is like the actual flow rate of water, how much electric charge moves past a point per second. Resistance (measured in ohms, symbol Omega) is like a narrow section of pipe that fights against the flow. A standard US wall outlet supplies about 120 volts of pressure. A phone charger might draw 0.5 amps, while a hair dryer can draw 12-15 amps because its heating coil demands far more current.

German physicist Georg Ohm published the law that bears his name in 1827: V = I x R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. If you know any two values, you can always solve for the third. For example, a lamp with a 60-ohm bulb resistance plugged into a 120-volt outlet draws I = V / R = 120 / 60 = 2 amps. Electricians use this every day to check whether a wire or device is safe to use on a given circuit.

Power, measured in watts (W), tells you how much energy something uses, and it's calculated as P = V x I. A 1,500-watt space heater on a 120-volt circuit draws I = P / V = 1,500 / 120 = 12.5 amps. Plug in two of those heaters, 25 amps total, on a single 20-amp breaker, and the breaker will trip. This is exactly why running a microwave, toaster, and coffee maker on the same kitchen outlet strip can knock out power: their combined amp draw crosses the breaker's rated limit.