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To understand battery energy, let's think about how much energy a battery can give before it runs out. This energy is often measured in Watt-hours (Wh), which combines power (Watt) and time (hour). A standard flashlight battery typically delivers about 2.0 Wh of energy. This means the battery can power a device using 2 Watts for 1 hour, or equivalently, 1 Watt for 2 hours. This is quite useful for small devices.
In the example exercise, knowing that a battery can deliver 2.0 Wh is key to determining how many batteries would be needed to power a larger device, like a 100 W lamp, for longer periods.

Power consumption refers to the amount of energy used by a device over time, and it's measured in Watts (W). When you know the power consumption and the time a device will be used, you can calculate the total energy required using the formula:

• Energy = Power × Time

In the flashlight example, a 100 W lamp running for 8 hours means that it uses a total of 800 Watt-hours (Wh) of energy. To find how much energy a device consumes is crucial when planning how to power it, especially if you're deciding between using batteries or plugging into an electrical outlet.
This calculation helps you ensure you have enough power supplies and can compare costs, as done in the problem.

Electrical efficiency is about using energy wisely and minimizing waste. It's significant when comparing how power is supplied to a device, like batteries versus direct electrical supply from the grid. One way to think about efficiency is considering how much it costs to provide the same amount of energy using different methods.
For instance, if you use batteries to power a device, you might end up paying more per unit of energy than if you used electricity from the grid. In the lamp example, using batteries to supply 800 Wh costs US$320, while the same energy from the grid only costs US$0.048. This vast difference illustrates how much more efficient it can be to use grid electricity for high power devices and long running times.
Understanding electrical efficiency helps make more informed choices about energy sources, contributing to both cost savings and environmental conservation.