Volts, Amps, Watts, and Ohms Explained

6 min read•June 9, 2026•Reviewed: June 9, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 9, 2026

Every electrical spec you encounter — a phone charger, a circuit breaker, an EV charging station — is described by four quantities: voltage (volts), current (amperes), resistance (ohms), and power (watts). They measure different things, but they are tightly linked: knowing any two usually lets you calculate the others. That relationship, Ohm's Law, is the single most useful piece of electrical knowledge for a non-engineer.

Because electrical values span an enormous range — from millivolt sensor signals to megawatt power plants — they are almost always written with SI prefixes. Converting milliamps to amps or watts to kilowatts is a matter of moving the decimal point three places, but knowing which direction to move it (and what the numbers mean) is what this guide covers.

What Each Unit Measures

Voltage (volts, V) is electrical pressure — the force pushing charge through a circuit. A AA battery supplies 1.5 V, USB ports deliver 5 V (up to 20 V with USB Power Delivery), US wall outlets provide 120 V, and European outlets 230 V. Current (amperes, A) is the flow rate of charge: a phone charging draws around 1–3 A, while a whole house panel is rated for 100–200 A.

Resistance (ohms, Ω) is how strongly a material opposes current flow — a 100-watt incandescent bulb has a filament resistance of roughly 144 Ω when hot. Power (watts, W) is the rate of energy use, the product of pressure and flow. It is the number most people actually care about, because it is what your utility bills you for (as kilowatt-hours, power × time).

Ohm's Law and the Power Formula

V = I × R   (volts = amps × ohms)
P = V × I   (watts = volts × amps)

Derived forms:
P = I² × R
P = V² ÷ R

SI prefix conversions:
1 kW = 1,000 W      1 MW = 1,000 kW
1 A  = 1,000 mA     1 V  = 1,000 mV

Converting Between Prefixes

All within-quantity electrical conversions are powers of ten: milli (m) means one-thousandth, kilo (k) means one thousand, mega (M) means one million. So 1,500 W is 1.5 kW, 250 mA is 0.25 A, and 3.3 MW is 3,300 kW. Moving up a prefix (W → kW) divides by 1,000; moving down (A → mA) multiplies by 1,000.

What you cannot do with a simple factor is convert across quantities — watts to volts, or amps to ohms — because those are different physical things. That conversion requires Ohm's Law and a known third value. A 1,200 W space heater on a 120 V circuit draws 1,200 ÷ 120 = 10 A; the same heater on a European 230 V supply would draw only about 5.2 A.

Real-World Reference Points

Phone charger: 5–20 W. Laptop charger: 45–140 W. Microwave: 800–1,200 W. Space heater: 1,500 W (the practical maximum for a US 15 A household circuit). Central air conditioning: 3–5 kW. Level 2 EV charger: 7–19 kW. DC fast charger: up to 350 kW. A typical US home averages about 1.2 kW of continuous draw, or roughly 10,500 kWh per year.

Circuit safety follows directly from these numbers: a 15 A breaker on a 120 V circuit can deliver 1,800 W, which is why running a space heater and a microwave on the same circuit trips it. Electricians size wiring by current, not power — current is what heats wires.

Quick Tips

✓
Watts = volts × amps. If a device lists only volts and amps, multiply to get its power draw.
✓
To convert W to kW, divide by 1,000 — a 1,500 W heater is 1.5 kW.
✓
Wire and breaker ratings use amps; energy bills use kilowatt-hours; device labels usually use watts.
✓
US outlets: 120 V at 60 Hz. Most of Europe, Asia, Africa: 220–240 V at 50 Hz. Check both voltage and frequency before using appliances abroad.
✓
kW measures rate; kWh measures total energy — a 2 kW device running for 3 hours uses 6 kWh.

Frequently Asked Questions

Can I convert watts directly to amps?

Only if you know the voltage. Amps = watts ÷ volts. A 600 W appliance draws 5 A at 120 V but just 2.6 A at 230 V. Without the voltage, watts and amps aren't convertible — they measure different things.

Why do the US and Europe use different voltages?

Historical infrastructure choices. Early US grids standardized on 110–120 V (Edison's distribution voltage), while Europe later adopted 220–240 V, which delivers the same power with half the current and thinner wires. Neither is inherently better; devices are simply designed for their regional standard.

How many watts can a normal household circuit handle?

A US 15 A circuit at 120 V supports 1,800 W peak, but continuous loads should stay below 80% of that — 1,440 W. That's why portable space heaters max out at 1,500 W and shouldn't share a circuit with other large loads.

What's the difference between a milliamp and an amp?

1 amp = 1,000 milliamps. Small electronics are rated in mA (a sleeping smartwatch may draw 5 mA) because writing 0.005 A is awkward. Divide mA by 1,000 to get amps.

Try the Electrical Converter

🔌 Open Electrical Converter →