Calculate Watts, Amps, or Volts
Use this calculator to solve for one missing electrical value. It works for basic DC loads and most AC estimates when you include a power factor.
What Is Wattage?
Wattage is a measure of electrical power. In plain language, it tells you how fast electricity is being used at a specific moment. A device rated at 60 watts uses power at a rate of 60 joules per second while it runs.
People often confuse watts and kilowatt-hours. Watts measure instantaneous power. Kilowatt-hours (kWh) measure energy consumed over time. That difference matters if you are trying to size a power supply, check a breaker load, or estimate your monthly electric bill.
Core Wattage Formulas
Basic formula (DC or simple resistive AC loads)
W = V × A
Where W is watts, V is volts, and A is amps. This is the most common relationship for quick calculations.
AC loads with power factor
W = V × A × PF
For many AC appliances, especially motors and compressors, true power depends on power factor. If PF is unknown, using 1.0 gives a rough estimate. For more accurate sizing, check the nameplate or manual.
Rearranged formulas
- A = W ÷ (V × PF) when solving for current
- V = W ÷ (A × PF) when solving for voltage
- PF = W ÷ (V × A) when measuring real power and apparent power
How to Use This Wattage Calculator
- Select whether you want to calculate watts, amps, or volts.
- Enter the two known values.
- Set power factor if needed (or leave blank to use 1.0).
- Click Calculate to get the result instantly.
- Optionally add daily usage hours and utility rate to estimate energy use and cost.
This approach helps with everything from choosing a generator to planning a battery bank or checking whether a circuit is overloaded.
Common Household Wattage Ranges
- LED bulb: 6–12 W
- Laptop charger: 45–100 W
- Desktop computer: 150–500 W
- Microwave: 800–1,500 W
- Hair dryer: 1,200–1,875 W
- Portable space heater: 1,500 W
- Refrigerator (running): 100–800 W, with higher startup surge
- Window AC: 500–1,500+ W depending on size and efficiency
From Wattage to Energy Cost
To estimate usage cost, convert watts to kilowatts and multiply by time:
kWh = (W ÷ 1000) × hours
If a 1,000 W appliance runs for 3 hours daily, it uses 3 kWh/day. At $0.15/kWh, that is about $0.45 per day or roughly $13.50 per 30-day month.
This is why lowering runtime often saves more than chasing tiny differences in nameplate wattage.
Safety and Circuit Sizing Tips
Follow the 80% rule for continuous loads
For loads running 3+ hours continuously, electricians typically size to 80% of breaker rating. A 15A circuit is usually treated as 12A continuous.
Account for startup surges
Motors, compressors, and pumps can draw several times their running current at startup. A device that runs at 700 W may require much more for a short moment when turning on.
Use proper wire gauge and protection
Power calculations are only one part of electrical safety. Correct conductor size, breaker type, environment, and code compliance are equally important.
Quick FAQ
Is higher wattage always better?
Not necessarily. Higher wattage means more power consumption. “Better” depends on performance needs and efficiency.
Can I use this calculator for solar setups?
Yes, for basic planning. You can estimate panel output, inverter loads, and battery draw. For final system design, include peak sun hours, inverter efficiency, battery chemistry, and safety margins.
What power factor should I use if I do not know it?
Use 1.0 for resistive loads like heaters. For motors or inductive equipment, a rough assumption might be 0.8 to 0.95, but nameplate data is best.
Bottom Line
A wattage calculator is one of the simplest tools for smarter energy decisions. Whether you are managing utility costs, selecting equipment, or checking electrical load limits, understanding the relationship between volts, amps, and watts gives you immediate practical advantage.
Use the calculator above as your quick daily reference, and pair it with safe installation practices whenever real wiring or panel work is involved.