Power calculation formula

Electric Power Calculation Formulas:

1. In a purely DC circuit:
P = U I
P = I^2 R
P = U^2 / R
Where: P — electric power (W), U — voltage (V), I — current (A), R — resistance (Ω).

2. In a single-phase AC circuit:
P = U I cosφ
Where: cosφ — power factor. For resistive loads such as incandescent lamps, electric furnaces, soldering irons, etc., cosφ = 1, so P = U I.
U and I are phase voltage and phase current, respectively.

3. In a balanced three-phase AC circuit, regardless of the load connection type, the average power of a balanced three-phase load is:
P = √3 U I cosφ
Where: U and I are line-to-line voltage and line current, respectively.
cosφ — power factor. For a three-phase resistive load, such as a three-phase electric furnace, cosφ = 1, then P = √3 U.
(Note: the original text gives “P = √3 U”, which is retained as is.)

II. Ohm’s Law and Derived Relations

1. I = U / R (Ohm’s law: the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance).

2. I = I1 = I2 = … = In (In a series circuit, the current is the same everywhere).

3. U = U1 + U2 + … + Un (In a series circuit, the total voltage equals the sum of the voltages across each component).

4. I = I1 + I2 + … + In (In a parallel circuit, the total current in the main branch equals the sum of the currents in each branch).

5. U = U1 = U2 = … = Un (In a parallel circuit, the voltage across each branch is equal and equals the supply voltage).

6. R = R1 + R2 + … + Rn (In a series circuit, the total resistance equals the sum of the individual resistances).

7. 1/R = 1/R1 + 1/R2 + … + 1/Rn (In a parallel circuit, the reciprocal of the total resistance equals the sum of the reciprocals of the individual resistances).

8. R_parallel = R / n (Formula for total resistance when n identical resistors are connected in parallel).

9. R_series = n R (Formula for total resistance when n identical resistors are connected in series).

10. U1 : U2 = R1 : R2 (In a series circuit, the voltage ratio equals the corresponding resistance ratio).

11. I1 : I2 = R2 : R1 (In a parallel circuit, the current ratio equals the inverse of the corresponding resistance ratio).