1,200
W
10
A
120
V
1.2
kW
1.2
kWh
288
kWh/month
1,200
W
10
A
120
V
1.2
kW
1.2
kWh
288
kWh/month
Electricity is governed by three fundamental quantities — watts (power), amps (current), and volts (voltage) — and the simple relationships between them underpin every electrical calculation from household appliance sizing to industrial power system design. The Watts, Amps & Volts Calculator gives you instant access to all three interconversions simultaneously, so you can solve the relationship from any two known quantities to find the third.
The core formula is deceptively simple: P = V × I, or equivalently W = V × A. Power in watts equals voltage in volts multiplied by current in amperes. Rearranging: A = W ÷ V (current equals power divided by voltage) and V = W ÷ A (voltage equals power divided by current). These three forms of the same equation cover virtually every everyday electrical question.
When will you use these formulas? The scenarios are endless. An appliance draws 1500 W from a 120 V outlet — how many amps does it pull? (Answer: 1500 ÷ 120 = 12.5 A, which means you need a 15 A circuit at minimum, and a 20 A circuit for safety margin.) A circuit breaker is rated 20 A at 240 V — what is the maximum wattage of load you can connect? (Answer: 20 × 240 = 4800 W, but NEC requires loading to not exceed 80%, so 3840 W.) Your solar inverter produces 5000 W at 48 V DC — what current must the battery cables carry? (Answer: 5000 ÷ 48 ≈ 104.2 A — requiring very heavy gauge cable.)
This calculator also outputs the entered wattage in kilowatts (kW), energy consumed in one hour (kWh), and estimated monthly energy consumption assuming 8 hours of daily operation — the most common usage scenario for appliances like air conditioners, water heaters, and pool pumps. These outputs turn a simple power calculation into a practical energy audit tool.
For DC circuits and purely resistive AC loads, the watt-amp-volt relationship is exact. For AC circuits with inductive or capacitive loads (motors, transformers, fluorescent ballasts), the actual real power is reduced by the power factor: P_real = V × I × PF. If you need to account for power factor, use our ElectriCalc Pro calculator. This Watts-Amps-Volts calculator uses PF = 1.0 (unity) and is ideal for resistive loads, DC systems, and quick estimates.
Understanding the watts-amps-volts triangle is a foundational skill for homeowners, electricians, solar installers, electronics hobbyists, and anyone who works with electrical systems. Whether you are planning a home generator, wiring an EV charger, specifying a UPS system, or simply checking whether a new appliance will overload a circuit, this calculator provides immediate, reliable answers.
Three parallel formulas are computed simultaneously from three inputs (V, A, W):
Additional outputs: kW = W ÷ 1000 converts watts to kilowatts. Energy (kWh) = kW × 1 hour gives one-hour energy consumption. Monthly kWh = kW × 8 h/day × 30 days estimates monthly consumption at 8 hours of daily use — adjustable manually by multiplying kW by your actual daily hours and days per month.
W = V × A: Check this against the appliance's nameplate wattage to verify consistency. Discrepancies may indicate a non-unity power factor load.
A = W ÷ V: Compare this to your circuit breaker rating. NEC requires continuous loads to not exceed 80% of breaker rating (NEC 210.20). For a 15 A breaker, maximum continuous load current is 12 A.
V = W ÷ A: Useful when verifying that your supply voltage matches a device's requirement, or when sizing battery banks for DC loads.
Monthly kWh: Multiply by your electricity rate ($/kWh) to estimate monthly running cost. Example: 2.4 kWh/month × $0.13/kWh = $0.31/month for an LED bulb running 8 h/day.
Inputs
Results
A 1500W space heater draws 12.5 A on a 120V circuit. This exceeds the 80% continuous load limit of a 15 A breaker (12 A max continuous), so a dedicated 20 A circuit is recommended. Running 8 hours/day for 30 days consumes 360 kWh — at $0.13/kWh, that's $46.80/month.
Inputs
Results
A Level 2 EV charger at 7200W draws 30 A on 240V, requiring a dedicated 40 A circuit (30 A ÷ 0.8 = 37.5 A, round up to 40 A breaker per NEC). Charging 8 hours nightly adds ~57.6 kWh/day or ~1728 kWh/month to your electricity consumption.
In DC circuits and purely resistive AC circuits, watts and volt-amperes are identical. In AC circuits with inductive or capacitive loads (motors, transformers, reactors), volt-amperes (VA) represents apparent power while watts represents real (active) power. The ratio of watts to VA is the power factor (PF). This calculator assumes PF = 1 (purely resistive / DC). Use ElectriCalc Pro for power-factor-corrected calculations.
Use A = W ÷ V to find the appliance's current draw. Compare to your circuit breaker rating, applying the 80% continuous load rule: maximum safe continuous current = breaker rating × 0.8. Add this to the existing load on the circuit. If the total exceeds 80% of the breaker rating, you need a dedicated circuit or a higher-rated circuit (requires a licensed electrician).
NEC 210.20(A) requires that branch circuits serving continuous loads (loads energized for 3+ hours) be sized so the load does not exceed 80% of the circuit's ampacity. This provides a thermal safety margin preventing overheating in conductors and breakers during sustained operation. Non-continuous loads (e.g., a microwave used briefly) can use up to 100% of circuit rating.
The circuit breaker should trip, interrupting the circuit. If the breaker is faulty or oversized for the wire, continued overloading causes conductor overheating, insulation degradation, and fire risk. This is why proper circuit sizing and correct breaker selection per NEC Article 240 is critical. Never replace a tripped breaker with a higher-rated one without verifying that the wiring can handle the increased current.
Use A = W ÷ V with V = 240. Example: a 3600W electric dryer draws 3600 ÷ 240 = 15 A. For a 240V, 30 A dryer circuit, maximum continuous load is 30 × 0.8 = 24 A, supporting up to 24 × 240 = 5760 W of continuous load. The typical 5500W dryer heating element is within this limit.
The monthly estimate assumes 8 hours of operation per day for 30 days. If your actual usage differs, multiply the kW output directly: kWh/month = kW × daily_hours × days_per_month. For appliances with duty cycles (refrigerators, HVAC compressors), actual consumption is lower than the nameplate wattage implies because the compressor cycles on and off.
Yes. For battery sizing: if you need to run a W-watt load for H hours, required battery capacity in amp-hours = (W × H) ÷ V. For solar panel sizing: a panel rated at I amps at V volts produces W = V × I watts under standard test conditions (STC). Real-world output is typically 75–85% of STC due to temperature, shading, and irradiance variation.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
