Watt Calculator
Calculate electrical power in watts using common formulas from Ohm’s Law. Choose the values you already know, enter your numbers, and click calculate.
Use this when you know voltage and current.
Use this when you know voltage and resistance.
Use this when you know current and resistance.
Optional energy cost estimate
What Is a Watt?
A watt (W) is a unit of power. In simple terms, it tells you how fast electrical energy is being used at a specific moment. If a device draws 100 watts, it uses energy ten times faster than a 10-watt device while both are running.
Watts are essential for sizing power supplies, checking electrical loads, estimating battery drain, and calculating electricity costs. Whether you are designing electronics, troubleshooting home appliances, or trying to reduce your monthly bill, understanding wattage is foundational.
Core Formulas Used in This Calculator
This calculator supports three common ways to calculate wattage:
- P = V × I → Power equals voltage times current.
- P = V² ÷ R → Power from voltage and resistance.
- P = I² × R → Power from current and resistance.
Where:
- P = power in watts (W)
- V = voltage in volts (V)
- I = current in amps (A)
- R = resistance in ohms (Ω)
Why Watt Calculations Matter
1) Electrical Safety
Circuits, wires, fuses, and breakers all have limits. If total wattage exceeds a circuit’s safe capacity, components can overheat. Quick watt calculations help prevent overloads and reduce risk.
2) Equipment Selection
If you are choosing a power adapter, inverter, or generator, wattage tells you the minimum rating you need. A device requiring 300W should not be paired with a 200W supply.
3) Energy Cost Planning
Power (watts) plus runtime (hours) gives energy consumption. That energy is billed in kilowatt-hours (kWh), so converting watts to kWh helps predict cost before your bill arrives.
Watts vs Watt-Hours vs Kilowatt-Hours
People often mix these up:
- Watts (W): Instantaneous power draw.
- Watt-hours (Wh): Energy used over time.
- Kilowatt-hours (kWh): 1,000 Wh, the unit most utilities bill.
Example: A 200W heater running for 3 hours uses 600Wh, or 0.6kWh.
Practical Examples
Example A: From Volts and Amps
If a device runs at 120V and draws 2A:
P = 120 × 2 = 240W
Example B: From Volts and Ohms
A resistor with 12V across 6Ω:
P = 12² ÷ 6 = 24W
Example C: From Amps and Ohms
If current is 3A through 4Ω:
P = 3² × 4 = 36W
Typical Household Wattage Ranges
- LED bulb: 6W to 15W
- Laptop charger: 45W to 100W
- Microwave: 700W to 1,200W
- Hair dryer: 1,000W to 1,800W
- Space heater: 1,200W to 1,500W
Use these values as rough estimates. Nameplate ratings and real-world loads can differ.
How to Use the Calculator Effectively
- Select the formula that matches your known values.
- Enter positive values only (resistance must be greater than zero).
- Use the optional hours/day and cost/kWh fields for monthly cost estimates.
- Double-check units before calculating.
Common Mistakes to Avoid
- Confusing A (amps) with mA (milliamps).
- Entering kV when the field expects V.
- Using zero ohms in formulas that divide by resistance.
- Treating a device’s “max” rating as always-on power draw.
Final Thoughts
A reliable watt calculator is one of the most useful tools for anyone working with electricity, from students and hobbyists to homeowners and professionals. With a few known values, you can quickly estimate power demand, select safer equipment, and better understand your energy costs.
Bookmark this page and use it whenever you need a quick watts calculation from volts, amps, or ohms.