Look at any appliance or electrical device and you'll see three numbers: volts, amps, watts. They're related but distinct. Once you understand the analogy with water flowing through pipes, the relationship is intuitive.

The water analogy

  • Voltage (volts, V): water pressure. The "push" behind the flow.
  • Current (amps, A): water flow rate. How much water moves per second.
  • Resistance (ohms, Ω): pipe narrowness. Restricts flow for a given pressure.
  • Power (watts, W): total energy delivered. Pressure × flow rate.

This works almost perfectly for electricity. Replace water with electrons and you have it.

Voltage: the "push"

Voltage is the electrical potential difference between two points. It's measured between two points (always relative — you can't have absolute voltage at one point).

  • 1.5 V: AA battery
  • 3 V: two AA in series
  • 9 V: 9-volt battery
  • 12 V: car battery, household low-voltage lighting
  • 120 V: U.S. household outlet
  • 240 V: European household, U.S. dryer/oven outlets
  • 480 V: commercial three-phase
  • 10,000 V+: transmission lines
  • 765,000 V: long-distance high-voltage transmission

Higher voltage = more "push." For the same wire, higher voltage = more current.

Current: the "flow"

Current is the rate of electron flow. Measured in amperes (amps, A).

  • 0.001 A (1 mA): below feel threshold for skin
  • 0.01 A (10 mA): noticeable tingle
  • 0.05 A (50 mA): can stop your heart (lethal threshold)
  • 0.5 A: a small fan or LED bulb
  • 1 A: a 100W bulb on 120V
  • 10 A: a typical hair dryer
  • 15 A: standard U.S. home outlet capacity
  • 30 A: dryer outlet
  • 200 A: total home service

Power: the "energy delivered per second"

Power = voltage × current. Watts = volts × amps.

Same example: a 100W bulb on 120V draws 100/120 = 0.83 A. Same bulb on 240V would draw half that (50 W = wait, that's wrong).

Actually: a "100W bulb" is rated for a specific voltage. On 240V instead of 120V, it would burn out (twice the voltage = twice the current = 4× the power, way over its rating).

Ohm's law

The fundamental electrical relationship: V = I × R.

  • V = voltage in volts
  • I = current in amps
  • R = resistance in ohms

Worked example: A 60 W bulb on 120 V draws 0.5 A current. Resistance = V/I = 120/0.5 = 240 Ω.

A 1500 W hair dryer on 120 V draws 12.5 A. Resistance: 120/12.5 = 9.6 Ω.

Why current matters more than voltage for safety

Common misconception: high voltage kills. Reality: current kills.

  • 9V battery touched to skin: voltage is 9V but current through wet skin is too low to feel.
  • 120V outlet contact: hand-to-hand at 1500 Ω (typical wet skin) = 80 mA. Lethal threshold.
  • 240V same circuit: 160 mA. Even more lethal.

What matters is the combination: voltage drives current through your body's resistance. Higher voltage = higher current for the same resistance, hence more dangerous.

That said, even high voltage with very low current (like a Van de Graaff generator demonstration) can deliver a static shock without harm. The available current is the killer.

Why power lines are high voltage

Long-distance electrical transmission uses very high voltage (765 kV common, up to 1.1 MV experimental). The reason: power loss on a wire is I²R (current squared × resistance). Halving the current quarters the power loss.

P = V × I. Same power, half the current means double the voltage. So transmission uses very high voltage to push power across hundreds of miles with minimal loss.

Substations step the voltage down (120 V) for residential use. Wall outlets are 120V because that's a safer voltage for humans, even though it's less efficient for transmission.

AC vs DC

AC (Alternating Current): household power. Direction reverses 60 times per second (60 Hz in U.S., 50 Hz in Europe). Easy to transmit and step up/down with transformers.

DC (Direct Current): batteries, USB ports, electronic circuits. Constant direction.

The "War of the Currents" (Edison vs Tesla, 1880s) was about which to use for transmission. Tesla's AC won because it's cheaper to transmit at high voltage.

Real-world examples

Phone charger: 100–240V AC input → 5V DC at 2A output. Power: 10W.

Laptop charger (65W): 100–240V AC input → 19V DC at 3.4A. Power: 65W.

Typical kitchen at peak: microwave (1000W) + toaster (1500W) + coffee maker (1000W) = 3500W = 29A on 120V. Easily trips a 30A breaker if all run simultaneously.

Tesla Model S charging: ~10 kW = 41 A on 240V Level 2. Or 250 kW peak on Supercharger DC fast charging.

The unit conversions

Common units:

  • 1 kW = 1000 W
  • 1 MW = 1,000,000 W
  • 1 mA = 0.001 A
  • 1 kV = 1000 V

For energy (power × time): 1 kWh = 1000 watts for 1 hour. Your electric bill is in kWh.

Calculate it

Our Ohm's law calculator handles V = IR and P = VI. Enter any two of voltage, current, resistance and get all four values plus power. Useful for circuit design, troubleshooting, or just understanding what's on the box of any electrical device.