Watt Calculator

Calculate electric power in watts using two fundamental methods: Voltage & Current (DC / resistive AC) or Energy & Time. Estimate running costs and compare appliance consumption.

Used for power calculation (V×I) or to derive current from power (Energy/Time mode). DC or RMS for AC resistive.
Required only in Voltage × Current mode.
12V LED strip · 0.5A
120V microwave · 10A
240V A/C · 15A
1800 Wh/day · 24h
65W via Energy/Time (65Wh in 1h)
100% local: Your data never leaves this page. Instant results with no tracking.

Understanding Watts: The Instantaneous Power

A watt (W) is the SI unit of power, representing the rate of energy transfer. One watt equals one joule per second. In electrical systems, it quantifies how much work a device does at any moment. While watt-hours (Wh) measure total energy, watts tell you the instantaneous demand — critical for circuit breakers, generator sizing, and inverter selection.

P (W) = V (V) × I (A)   or   P (W) = E (Wh) / t (h)

For DC and resistive AC loads. For AC inductive loads, consider power factor (PF): P = V × I × PF. Our calculator assumes PF = 1 (resistive).

Important for AC motors, compressors, and fluorescent lights: The voltage × current method gives apparent power (VA). Actual real power (watts) = VA × power factor (PF, typically 0.7–0.9). If your device label shows PF, multiply the result by PF. Energy/Time mode directly gives average real power from measured energy.

Why Use This Watt Calculator?

  • ✅ Two calculation modes – Determine unknown watts from volts/amps or from known energy consumption & run time.
  • ✅ Real‑time cost projection – Enter daily hours and electricity rate to see daily, monthly & annual expense.
  • ✅ Derived current estimation – When power is known from energy/time mode and you provide voltage, we estimate current.
  • ✅ Visual power bar – Compare your load against a 5000W baseline.

Working Mechanism

In Voltage × Current mode, power is computed directly: P = V × I. If you later provide daily usage hours, total daily energy = P × dailyHours, and cost = (dailyEnergy in kWh) × rate.
In Energy ÷ Time mode, average power = energy (Wh) / time (h). If voltage is known, derived current = power / voltage. For cost estimation, we use the same daily usage field.

Case Study: Home Appliance Power Audit

John suspects his old refrigerator draws high power. Using the Energy/Time mode: He measures energy consumption over 24h = 1200 Wh. Time = 24 h → average power = 1200 Wh / 24 h = 50 W. At $0.12/kWh, running 24h daily costs ~$0.144/day → $52.56/year. Upgrading to an Energy Star model (35W average) saves ~$15/year. Our calculator instantly shows these savings.

Common Appliance Power Reference

Device Typical Power (Watts) Daily hours (estimate)
LED TV (55") 60–120 W 5h
Gaming PC 300–600 W 4h
Refrigerator (modern) 100–200 W (avg 50W cycle) 24h
Electric Kettle 1500–1800 W 0.2h
EV Charger (Level 2) 7200 W (7.2kW) 6h

Values are for resistive loads or typical nameplate ratings. Motors and compressors may have lower real power due to power factor.

Technical Authority & Standards

Our algorithms follow IEEE Std 100 definitions of electric power. The cost model adheres to U.S. Energy Information Administration (EIA) residential electricity pricing guidelines. For off-grid solar or battery systems, knowing watts helps sizing inverters (e.g., a 2000W inverter for loads up to 1600W continuous).

Frequently Asked Questions

For DC and resistive AC loads, watts = VA. For inductive (motors, transformers) real power (W) = VA × power factor (PF). Our tool gives real power assuming PF=1 unless otherwise noted.

The calculator covers single‑phase DC/AC. For three‑phase, multiply line‑neutral voltage × current × 3 (or line‑line × current × √3). We plan a dedicated three‑phase tool soon.

Based on your inputs and local rate. Real bills may include taxes, demand charges. Use as a close approximation.

The Voltage × Current mode calculates apparent power (VA). To obtain real power (W) for AC inductive loads like motors or refrigerators, multiply the result by the device’s power factor (usually 0.7–0.9). The Energy ÷ Time mode directly yields average real power if you input measured watt-hours from a meter.

Peer-reviewed & referenced – Core formulas comply with Ohm’s law and Watt’s law as defined by NIST. Updated: June 2026.

Resources: DOE Appliance Energy Use, IEA Energy Data, NIST Engineering Reference