Watts Calculator
Use this calculator to find electrical power (watts) from common known values like volts, amps, and resistance.
What is a watt?
A watt (W) is the standard unit of power. In electricity, power tells you how quickly energy is being used or delivered. If you are comparing appliances, designing a circuit, estimating battery life, or checking an electrical bill, watts give you a practical way to quantify load.
At a high level, more watts means more power consumed (or produced) at a given moment. A 10 W LED bulb uses much less power than a 1500 W space heater, which is why the heater draws significantly more current and costs more to run.
Core watts formulas
1) From voltage and current (DC)
For direct current (DC) systems, power is straightforward:
- P = V × I
- P is power in watts
- V is voltage in volts
- I is current in amps
2) From voltage and current (AC)
For alternating current (AC), real power depends on power factor (PF):
- Single-phase: P = V × I × PF
- Three-phase: P = √3 × V × I × PF
Power factor captures phase difference and reactive effects. If PF is low, real watts are lower than apparent volt-amps.
3) From voltage and resistance
If you know voltage and resistance:
- P = V² / R
This is common in heater elements, resistors, and basic load checks.
4) From current and resistance
If current and resistance are known:
- P = I² × R
This form is useful when you are already measuring current in a branch and know the load resistance.
How to use this calculator
- Select the method that matches your known values.
- Enter the required numbers (voltage/current/resistance/PF).
- Click Calculate Watts to get power instantly.
- Optionally add runtime hours and utility rate to estimate energy usage and cost.
The result includes watts, kilowatts, and—if optional fields are filled—watt-hours, kilowatt-hours, and estimated cost.
Worked examples
Example A: DC fan
A 12 V fan draws 1.8 A. Power is:
P = 12 × 1.8 = 21.6 W
Example B: AC appliance (single-phase)
An appliance at 230 V draws 4 A with PF = 0.92:
P = 230 × 4 × 0.92 = 846.4 W
Example C: Heating element from V and R
A load at 120 V has resistance 24 Ω:
P = 120² / 24 = 600 W
Common mistakes to avoid
- Mixing AC and DC equations.
- Ignoring power factor in AC real-power calculations.
- Using milliamps without converting to amps first (1000 mA = 1 A).
- Entering line-to-line voltage incorrectly in three-phase scenarios.
- Confusing power (W) with energy (Wh or kWh).
Watts vs. kW vs. kWh
- Watt (W): instantaneous power.
- Kilowatt (kW): 1000 watts.
- Kilowatt-hour (kWh): energy used over time.
To estimate energy: kWh = (Watts × Hours) / 1000. This is the number utilities typically bill.
Final takeaway
A reliable watts calculator helps with circuit planning, appliance comparison, battery calculations, and cost estimation. Start with the right formula, use accurate units, and include runtime if you need energy/cost outputs. With those basics, power calculations become quick and dependable.