DC power
| P = V · I | Watts = Volts × Amps |
|---|---|
| P = I² · R | Current squared × resistance |
| P = V² / R | Voltage squared / resistance |
| Energy | W = P · t (watt-seconds = joules) |
| kWh | P(kW) × t(hours) — utility billing unit |
AC single-phase
| Real power P | = V · I · cos φ (watts) |
|---|---|
| Apparent power S | = V · I (VA) |
| Reactive power Q | = V · I · sin φ (VAR) |
| Power factor | PF = cos φ = P / S |
| Always use RMS | V and I in AC formulas are RMS values |
AC three-phase
| Real power | = √3 · V_line · I_line · cos φ |
|---|---|
| Apparent power | = √3 · V_line · I_line |
| Line vs phase | V_line = √3 · V_phase (wye); I_line = √3 · I_phase (delta) |
Mechanical power
| Rotational | P = τ · ω (torque × angular velocity in rad/s) |
|---|---|
| From RPM | P(W) = τ(N·m) · RPM · 2π / 60 |
| Horsepower | 1 hp = 746 W (mechanical); 1 hp = 745.7 W (metric) |
| Linear | P = F · v (force × velocity) |
Unit conversions
| Unit | Equals |
|---|---|
| 1 W | 1 J/s |
| 1 kW | 1.341 hp |
| 1 hp | 745.7 W |
| 1 Btu/h | 0.293 W |
| 1 ton (AC) | 3.517 kW of cooling |
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