The three quantities that run everything electrical
Almost every electrical question — sizing a breaker, choosing a generator, wiring an appliance, planning a solar system — comes down to three quantities and the relationships between them: volts, amps, and watts. Once you understand what each one measures and the single formula that ties them together, converting between them stops feeling like a mystery.
A useful analogy is water flowing through a pipe:
- Voltage (volts, V) is the pressure pushing the water.
- Current (amps, A) is how much water actually flows.
- Power (watts, W) is the total work being done — the result of pressure and flow together.
The one formula to remember
That is the whole foundation. Rearranged, it gives you every basic conversion you will ever need:
- To find watts: multiply volts by amps.
- To find amps: divide watts by volts.
- To find volts: divide watts by amps.
Worked example: amps to watts
A device draws 5 amps on a standard 120-volt US circuit. How many watts does it use?
Watts = 120 V × 5 A = 600 W
On a 230-volt circuit (UK, Australia, EU), the same 5 amps would be 230 × 5 = 1,150 W. This is why voltage always matters: the same current delivers far more power at a higher voltage. You can run these numbers instantly with our Amps to Watts Calculator.
Worked example: watts to amps
You want to run a 1,500-watt space heater on a 120-volt circuit. How much current will it pull?
Amps = 1,500 W ÷ 120 V = 12.5 A
That matters because a standard US household circuit is often rated for 15 amps. A 12.5-amp heater leaves little headroom, which is exactly why running a heater and other devices on the same circuit trips the breaker. Check any appliance with the Watts to Amps Calculator.
Where AC power gets slightly more complex: power factor
The simple formula above is exact for DC and for purely resistive AC loads (heaters, incandescent bulbs, kettles). For motors, air conditioners, and many electronic devices, alternating current introduces a wrinkle called the power factor — a number between 0 and 1 that describes how efficiently current is converted into useful work.
This is where kVA and kW come in:
- kVA (kilovolt-amperes) is apparent power — the total the system must supply.
- kW (kilowatts) is real power — the useful work actually delivered.
A generator rated at 10 kVA with a 0.8 power factor delivers 10 × 0.8 = 8 kW of usable power. This is critical for generator and UPS sizing, and you can convert either direction with the kVA to kW Calculator and kW to kVA Calculator.
Three-phase power
Industrial and commercial equipment often runs on three-phase power, which adds a factor of the square root of 3 (about 1.732) to the formula:
If you are sizing three-phase equipment, use a calculator that lets you select single- or three-phase, such as our Amps to kW Calculator, rather than doing this by hand.
Energy over time: kWh and your electricity bill
Watts measure power at an instant. Your electricity bill charges for energy — power used over time — measured in kilowatt-hours (kWh).
A 1,500-watt heater running for 4 hours uses 1,500 × 4 ÷ 1,000 = 6 kWh. Multiply by your electricity rate to get the cost. Our kWh Calculator and Energy Cost Calculator turn appliance wattage into a real dollar figure.
Quick reference: which calculator for which job
- Sizing a breaker or wire? Convert the load's watts to amps with the Watts to Amps Calculator.
- Sizing a generator or UPS? Work in kVA and kW with the kVA to kW Calculator.
- Estimating running cost? Use the kWh Calculator then the Energy Cost Calculator.
- Working with current and voltage directly? The Amps to Volts Calculator and Volts to Watts Calculator cover the rest.
Safety note
These formulas are for planning and education. Electrical work carries real risk of fire and electrocution, and wiring, breaker, and installation decisions should follow local electrical codes and, in most cases, be carried out or checked by a licensed electrician. When in doubt, consult a qualified professional.