What does this calculator do?
Electrical power tells you how fast a circuit uses or delivers energy. Knowing it lets you choose power supplies, resistors, converters and wires with a safe thermal margin.
P = V × I
For a DC load, multiply voltage by current. In AC, with non-sinusoidal waveforms or a phase shift, real power also depends on RMS values and the power factor.
Worked example
A load supplied with 12 V that draws 0.5 A uses 6 W.
Practical tips
- Don’t confuse power with energy: energy also includes time.
- Leave headroom when choosing a power supply.
- Avoid running resistors continuously at their rated power.
Common mistake to avoid
Don’t pick a part rated exactly at the calculated value: real dissipation changes with temperature, tolerances and airflow.
Complete electrical power formulas
Power is energy transferred per unit of time and is measured in watts (W): 1 W is 1 joule per second. Combining P = V · I with Ohm’s law (V = I · R) gives three equivalent forms; use the one that matches the data you have:
- From voltage and current: P = V · I
- From current and resistance: P = I² · R (handy when you know the current through a resistor)
- From voltage and resistance: P = V² / R (handy when you know the voltage across a resistive load)
- Current from power: I = P / V
- Energy used: E = P · t; in kWh, E = P(W) × hours / 1000
These expressions are exact for DC and for purely resistive AC loads using RMS values. If the load is a motor, a switch-mode power supply or a ballast, AC calculations need the power factor: use the single-phase AC power calculator or the three-phase power calculator.
Typical values
| Voltage | Current | Power | Example |
|---|---|---|---|
| 5 V | 0.5 A | 2.5 W | USB 2.0 port |
| 5 V | 3 A | 15 W | USB-C charger, 5 V / 3 A |
| 12 V | 0.5 A | 6 W | Fan or small 12 V load |
| 12 V | 5 A | 60 W | 5 m LED strip on a 12 V supply |
| 24 V | 2.5 A | 60 W | Industrial DIN-rail power supply |
| 120 V | 12.5 A | 1500 W | Space heater (PF ≈ 1) |
| 230 V | 8.7 A | ≈ 2000 W | Electric kettle or water heater (PF ≈ 1) |
Values come from P = V · I. A power supply’s rating is the maximum it can deliver, not what the load always draws.
Worked example: power in a resistor and monthly energy use
Case 1. A 100 Ω resistor is connected to 12 V. The current is I = 12 / 100 = 0.12 A and the power is P = V² / R = 144 / 100 = 1.44 W. A 1/4 W resistor would burn out. As a rule of thumb, choose at least twice the calculated power, so a 3 W resistor is appropriate (or 5 W inside a closed enclosure).
Case 2. The 60 W LED strip from the table runs 5 hours a day for 30 days: E = 60 × 5 × 30 / 1000 = 9 kWh per month. Multiply by your electricity rate to estimate the cost.
Common mistakes
- Confusing power and energy: watts describe a rate; kWh describe the total consumed over a period.
- Forgetting to convert milliamps: 250 mA is 0.25 A; 12 V × 250 gives 3000 W instead of 3 W.
- Using V · I for AC motors: if the power factor is below 1, the result is apparent power (VA), not active power.
- Sizing at the limit: a resistor or supply running at 100 % of its rating heats up and ages quickly; leave a margin.
- Ignoring wire resistance: in 12 V systems with high currents, the I · R drop in the cable reduces the power that reaches the load.
Frequently asked questions
How many amps does a 1000 W appliance draw?
If it is resistive, I = P / V: about 4.35 A at 230 V and 8.33 A at 120 V. Motors and electronic power supplies draw somewhat more because their power factor is below 1.
What is the difference between W and VA?
W (watts) is active power, the part actually converted into heat, light or work. VA (volt-amperes) is apparent power, V × I without considering the phase shift. They are equal only for resistive loads; see the power factor calculator.
How do I choose a resistor’s power rating?
Calculate P = I² · R or P = V² / R with the real circuit values and choose a resistor rated at least twice that. To find the current, use the Ohm’s law calculator.
Want to see where these formulas come from, with more examples? Read the electrical power guide.
Educational tool. Check tolerances, datasheets, standards and real operating conditions before using the results in critical applications.