Watt-minutes (Wm) to Milliwatt-hours (mWh) conversion

What is watt-minutes?

Watt-minutes is a unit of energy. Below is a breakdown of what watt-minutes are, how they're formed, and some examples.

Understanding Watt-Minutes

Watt-minutes (W⋅min) is a unit of energy derived from the more standard unit, the watt-hour (W⋅h), and ultimately from the SI unit of energy, the joule (J). It represents the amount of energy consumed or produced when one watt of power is used for one minute.

Formation and Calculation

Watt-minutes are formed by multiplying power in watts by time in minutes.

• Power (Watts): The rate at which energy is used or generated.
• Time (Minutes): The duration for which the power is applied.

The formula to calculate energy in watt-minutes is:

$E = P \cdot t$

Where:

• $E$ is the energy in watt-minutes (W⋅min)
• $P$ is the power in watts (W)
• $t$ is the time in minutes (min)

Conversion to Other Units

• To convert watt-minutes to watt-hours, divide by 60: $Wh = \frac{W \cdot min}{60}$

• To convert watt-minutes to joules, multiply by 60: $J = W \cdot min \cdot 60$

Practical Examples

1. Small Electronic Devices: A small LED light bulb might consume 5 watts. If it's used for 20 minutes, the energy consumed is $5 W \cdot 20 min = 100$ watt-minutes.
2. Charging a Smartphone: If a phone charger delivers 10 watts of power and the phone charges for 30 minutes, it consumes $10 W \cdot 30 min = 300$ watt-minutes.
3. Microwave Oven: A microwave oven operating at 1000 watts for 2 minutes uses $1000 W \cdot 2 min = 2000$ watt-minutes.

Relation to James Watt

James Watt was a Scottish inventor and mechanical engineer whose improvements to the steam engine were fundamental to the Industrial Revolution. The watt, the unit of power, is named in his honor. While Watt didn't directly work with "watt-minutes" as a standalone unit, his work on power and energy laid the groundwork for understanding these concepts.

Usefulness and Relevance

Watt-minutes are a convenient unit when dealing with energy consumption or production over relatively short periods. They offer a more granular view compared to watt-hours, especially when analyzing the energy usage of devices or processes with durations shorter than an hour.

What is Milliwatt-hours?

Milliwatt-hours (mWh) are a unit of energy, commonly used to express the capacity of small batteries and the energy consumption of low-power electronic devices. Understanding mWh helps in comparing battery life and energy efficiency.

Understanding Milliwatt-hours

A watt-hour (Wh) is a unit of energy equivalent to one watt of power sustained for one hour. A milliwatt-hour (mWh) is simply one-thousandth of a watt-hour. Mathematically:

$$1 \text{ mWh} = \frac{1}{1000} \text{ Wh} = 0.001 \text{ Wh}$$

Since a Watt is defined as Joules per second ($J/s$) and an hour is 3600 seconds, 1 Wh can also be represented in joules ($J$).

$$1 \text{ Wh} = 1 \frac{J}{s} \cdot 3600 s = 3600 \text{ J}$$

Therefore, converting mWh to Joules ($J$)

$$1 \text{ mWh} = 3.6 \text{ J}$$

Formation of Milliwatt-hours

Milliwatt-hours represent the amount of energy a device consumes or a battery provides over a period of time. It is derived by multiplying power (in milliwatts) by time (in hours):

$$\text{Energy (mWh)} = \text{Power (mW)} \times \text{Time (h)}$$

For example, a device consuming 10 mW of power for 5 hours uses 50 mWh of energy.

Real-World Examples and Applications

• Coin Cell Batteries: Small electronic devices like watches, calculators, and some sensors use coin cell batteries with capacities often measured in mWh. For example, a CR2032 battery might have a capacity of around 200-240 mWh.
• Wearable Devices: Smartwatches and fitness trackers use batteries with capacities expressed in mWh. A typical smartwatch battery might be rated at 300-500 mWh.
• Hearing Aids: These devices require very small, energy-efficient batteries. Their capacity is naturally expressed in mWh.
• IoT (Internet of Things) Devices: Many IoT sensors and devices designed for long-term deployment and low power consumption rely on batteries measured in mWh to operate for extended periods.
• Low-Power Integrated Circuits: Evaluating the power consumption of a chip in milliWatts gives engineers an idea about heat dissipation, battery life and other metrics.

Milliwatt-hours and Battery Capacity

Battery capacity is often specified in mWh. A higher mWh rating indicates that the battery can supply more energy and power a device for a longer time, assuming a constant power draw. When comparing different batteries for the same device, look for the higher mWh rating for longer usage. For an in-depth understanding, refer to resources on battery capacity.