Wattage Calculator
Need to know exactly how much power a device draws? Whether you are sizing a backup generator, choosing an uninterruptible power supply (UPS), or estimating your monthly electricity cost, you need a reliable way to translate volts and amps into watts. The wattage calculator below gives you that answer instantly – for both DC and AC circuits.
Understanding Watts, Volts, and Amps
Electrical power starts with three basic units:
- Volts (V) – the “pressure” that pushes electricity through a conductor. In North America, a standard wall outlet delivers 120 volts; many European outlets supply 230 volts.
- Amps (A) – the flow of electrical current. A 10‑amp circuit can carry more electricity than a 5‑amp circuit, much like a wider pipe allows more water through.
- Watts (W) – the actual rate of energy use. A device that draws 1,200 watts transforms that much electrical energy into heat, light, or motion every second.
A wattage calculator simply multiplies voltage by current to show you real power consumption in watts. When you understand the relationship, you can check whether a circuit is overloaded or estimate how long a battery will last.
The Wattage Formula and How to Use It
The core formula is straightforward:
DC and purely resistive AC loads
Watts = Volts × Amps
Single-phase AC with inductive loads
Watts = Volts × Amps × Power Factor (PF)
Three-phase AC
Watts = √3 × Volts × Amps × PF
Power Factor accounts for the phase difference between voltage and current. A resistive load like an incandescent bulb has a PF of 1.0; a motor or compressor often has a PF between 0.7 and 0.95. If you don’t know the PF, use 0.8 for motors and 1.0 for electronics – but remember that the result will be an estimate.
Example: A 120‑volt appliance draws 8.5 amps. Without any PF correction, its wattage is 120 × 8.5 = 1,020 watts. If the nameplate shows a PF of 0.9, the real power is 120 × 8.5 × 0.9 = 918 watts.
AC vs. DC Power Calculations
Direct current (DC) flows one way, so the calculation never changes: watts = volts × amps. Most battery-powered devices and solar panels use DC.
Alternating current (AC) switches direction, and inductive or capacitive loads store and release energy. The formula must include the power factor for accurate real‑power figures. Off‑the‑shelf wattage calculators often let you toggle between DC, single‑phase AC, and three‑phase AC to handle all three scenarios without manual math.
For three‑phase systems, the multiplier is √3 (approximately 1.732). A 400‑volt, 15‑amp three‑phase motor with a PF of 0.85 draws 1.732 × 400 × 15 × 0.85 = 8,829 watts.
Typical Wattage of Common Household Appliances
Below are typical running wattages gathered from manufacturer specifications and energy‑monitoring data. Use these figures as a starting point and always check the appliance’s own label.
| Appliance | Estimated Running Watts |
|---|---|
| LED light bulb (10 W equivalent) | 10 W |
| Laptop charger | 45–90 W |
| Ceiling fan | 50–90 W |
| Refrigerator (full size) | 150–400 W |
| 42‑inch LED TV | 80–120 W |
| Microwave oven | 700–1,200 W |
| Electric kettle | 1,200–1,800 W |
| Portable air conditioner | 1,000–1,500 W |
| Washing machine | 500–1,200 W |
| Central air conditioner (3‑ton) | 3,000–5,000 W |
| Clothes dryer (electric) | 2,000–4,500 W |
Surge or starting wattage can be 2–3 times higher for devices with compressors or motors. A 400‑watt refrigerator might need 1,000–1,200 watts for a few seconds when the compressor kicks in. Always account for that peak when choosing a generator or inverter.
How to Estimate Your Energy Consumption
A wattage calculator gives you instantaneous power, but your electricity bill charges you for energy over time – kilowatt‑hours (kWh). To estimate daily or monthly consumption:
- Find the running wattage of each device (use the calculator or the nameplate).
- Divide by 1,000 to convert watts to kilowatts.
- Multiply by the hours the device runs per day.
- Multiply by the number of days in the billing cycle.
Example: A 1,500‑watt space heater runs 6 hours a day. Daily consumption = 1.5 kW × 6 h = 9 kWh. Over a 30‑day month, that’s 270 kWh. At an average rate of $0.16 per kWh, the heater adds $43.20 to the monthly bill.
For devices that cycle on and off (refrigerators, AC), log the actual runtime with an energy meter for the most accurate number.
Practical Uses for a Wattage Calculator
Beyond curiosity, calculating watts helps in several real‑world situations:
- Generator sizing – add up all devices you plan to run simultaneously and include the largest starting surge.
- UPS backup selection – match the total running watts plus a 20–25% margin to ensure the UPS can handle the load during a power outage.
- Circuit load check – a 15‑amp residential circuit at 120 volts supports 1,800 watts maximum (continuous load should stay below 80%, or 1,440 watts).
- Solar panel setup – determine daily consumption in watt‑hours to size the battery bank and inverter appropriately.
- Energy‑efficiency audits – pinpoint which appliances consume the most power and replace them with higher‑efficiency models.
Factors That Affect Real‑World Wattage
Nameplate ratings are measured under ideal conditions. In practice, several variables influence actual power draw:
- Voltage fluctuations – a 10% drop in voltage reduces power for resistive loads but may increase current draw for motors.
- Temperature – motors and compressors work harder and draw more current in extreme heat.
- Aging equipment – older appliances often pull more watts as bearings wear and dust accumulates on coils.
- Simultaneous start‑ups – when multiple motorized devices start at the same instant, the peak spike can be far higher than the sum of their individual surge ratings.
For critical installations, consult a licensed electrician who can measure load with a true‑RMS meter and verify wiring, breaker, and panel capacity.
Disclaimer: This calculator provides estimates for informational purposes only. Always consult a qualified electrician for wiring, safety, and code‑compliance decisions.