Knowing the power and energy consumed by a device can be important when determining the system temperature increase or other thermal considerations when designing electronic circuits. This calculator may also be useful when finding the energy consumption of a circuit to determine the battery life or choosing the correct size battery for a device.

**Figure 1.** Ohms law for a simple, resistive circuit.

**Figure 1.**Ohms law for a simple, resistive circuit.

### Electrical Energy Equation:

We can use any of the following equations to find the power dissipated by a resistive circuit element.

$$P = VI = I^2R = frac{V^2}{R}$$

Where:

- P = Power dissipated in a resistive element in watts
- V = The voltage drop across the resistive element in volts
- I = The current through the resistive element in amps

### Joule’s Law Equation for Energy Consumption

From here, we can find the total energy consumed by the circuit by using the following equation (also known as Joule’s Law):

$$E = Pt$$

Where:

- E = Energy dissipated in a resistive element in joules
- P = Power dissipated in a resistive element in watts
- t = The duration of time the energy was dissipated in seconds

By definition 1 watt = 1 joule per second (1 W = 1 J/s). Similarly, you can flip that equation around and it becomes 1 joule = 1 watt for 1 second (1 J = 1 w-s)

### Converting Watt-hours to Joules

It’s also important to note that energy is also commonly expressed in terms of watt-hours which, as the name suggests, is the energy equivalent of one watt of power consumed continuously for one hour. One watt-hour is equal to 3600 Joules (3.6 kJ). Below is a quick conversion equation.

$$Wh = frac{E}{3600}$$

Where:

- Wh = Energy consumed for a period of time in watt-hours
- E = Energy dissipated in a resistive element in joules

### Further Reading

Textbook—Voltage and Current

Textbook—DC Circuit Equations and Laws

Worksheet—Energy, Work, and Power

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