03 // Bidirectional STEM Calculators // PHYSICSP = V × I

Electrical Power (Watt's Law) Calculator

Watt's Law defines instantaneous DC electrical power transfer P as the product of potential difference V and current I: P = V × I.

P = V × I

Pick which value you want to calculate (or type into any field):Solving for: P — Electrical Power

Electrical PowerCalculated

Rate of electrical energy transfer in Watts

150,000.00

Supply Voltage

RMS or DC circuit potential in Volts

11,000.00

Current Draw

Electrical current in Amperes

0.05200

Calculated Result · Electrical Power (P)

1,800.00W

Converted units:

Step-by-Step Calculation

01Canonical Law: P = V × I

02Given Inputs (Base Units): V = 120 V, I = 15 A

03Solve for P (Electrical Power): P = 1,800 W

Watt's Law defines instantaneous DC electrical power transfer P as the product of potential difference V and current I: P = V × I.

US NEMA 5-15 Branch Circuit120V × 15A = 1,800 W

1,440 W continuous load limit (80% NEC rule)

EU Schuko Type F Circuit230V × 16A = 3,680 W

Standard European residential branch capacity

Level 2 EV Charger (48A)240V × 48A = 11.52 kW

Requires 60A dedicated breaker

Electrical Power (P) · Supply Voltage (V)

Compare across:

Electrical Power (Watt's Law) Calculator — Supply Voltage → Electrical Power
#	Supply Voltage (V)	Electrical Power (W)	Δ %
01	7.6125 V	114.1875 W	-93.66%
02	20.7572 V	311.3586 W	-82.70%
03	33.902 V	508.5296 W	-71.75%
04	47.0467 V	705.7007 W	-60.79%
05	60.1914 V	902.8717 W	-49.84%
06	73.3362 V	1,100.04 W	-38.89%
07	86.4809 V	1,297.21 W	-27.93%
08	99.6257 V	1,494.38 W	-16.98%
09	112.7704 V	1,691.56 W	-6.02%
10	120 VCurrent	1,800.00 W	0.00%
11	139.0599 V	2,085.90 W	+15.88%
12	152.2046 V	2,283.07 W	+26.84%
13	165.3493 V	2,480.24 W	+37.79%
14	178.4941 V	2,677.41 W	+48.75%
15	191.6388 V	2,874.58 W	+59.70%
16	204.7836 V	3,071.75 W	+70.65%
17	217.9283 V	3,268.92 W	+81.61%
18	231.073 V	3,466.10 W	+92.56%
19	244.2178 V	3,663.27 W	+103.51%
20	257.3625 V	3,860.44 W	+114.47%

Was this tool helpful?

How do I size a circuit breaker for a known wattage appliance?+

Click SOLVE FOR on I (Current Draw), enter appliance wattage P and line voltage V, then multiply by 1.25 for continuous loads per NEC guidelines.

Why do high-power transmission lines use high voltage?+

For a fixed power P, raising voltage V proportionally reduces current I, cutting resistive line losses (I²R) quadratically.

Can I view power in Kilowatts (kW) or Horsepower (hp)?+

Yes. Switch the inline unit dropdown on P to kW or hp to convert automatically.