Notebook Computers

Though notebook computers are designed to be energy economical compared to desktops, the energy consumption of a laptop depends on the usage pattern. To reduce energy consumption, it is necessary to have a usage pattern that results in optimal energy consumption. Let's examine the power consumption of a laptop's different components and discuss how each component's power consumption could be reduced or optimized.

Notebook computers are generally powered by a rechargeable battery, which when in good condition powers the device for about 3-5 hours. The battery is recharged with the help of a charger, which can charge the battery irrespective of whether the device is running or is in a powered-down state. The lifetime of a battery is generally short: They last for only about 300-600 charge cycles. This results in the need to replace the battery a few times during the life of a laptop, thereby contributing to e-waste. A few companies are also coming out with notebook computers which run on solar power.

Notebook computers are also characterized by their liquid crystal display (LCD) or LED monitors, which are integrated into the device. The monitors and CPU consume the largest share of power that is utilized by the laptop (Mahesri and Vardhan, 2005; Roberson et al., 2012). The larger the monitors, the greater the laptop's power consumption (Albert, 2009); and the more intensive the operations performed by the processor, the higher its power consumption.

Energy can be conserved in a laptop in several ways. According to various studies, quite a lot of the energy wasted in a laptop happens when the laptop is being charged (Snyder, 2009). The chargers convert the AC to DC and steps down the voltage. This happens as long as the charger is connected to the power socket, irrespective of whether the laptop is connected to the charger or not, thus resulting in wasted energy. Hence it is important to switch off the power supply and unplug the charger from the power socket when it is not in use. This will reduce power wastage. Of late there have also been green chargers (such as the iGo Green Charger) available which can detect whether a charger is connected to a notebook computer or any other device, and reduce the power consumption when a charger is not connected to a device.

Monitors consume about 20-30% (Mahesri and Vardhan, 2005) of the total energy used by a laptop; hence it is important to reduce their power consumption. Strategies that help in reducing their power consumption include the following:

1. Reduce the brightness of the monitor to an appropriate level. A brighter screen consumes more energy.

2. When some background task is running on the computer and there is no need to use the monitor during this time, switch off the monitor instead of using screen savers as screen savers also consume some energy.

3. Most computer operating systems provide power-saving profiles which, when enabled, reduce the amount of energy consumed by the computer. For example, when the laptop is starting up or shutting down, these applications reduce screen brightness to a minimum.


Laptop processors also consume a lot of power particularly when carrying out computationally intensive tasks such as encryption, analytics, computer games and image and video processing. Many aspects of applications that run on a system impact power consumption. The following guidelines will help in optimizing energy consumption:

1. The background processes and other applications which are not being used are kept running to keep the processor active, thus resulting in energy wastage. So when an application is not in use, close the application and also stop the background processes that are not being used.

2. Multitasking is a trade-off between CPU time spent executing tasks and inefficiencies brought in by context switching. When a task starts running slower than it would normally run, one of the likely causes is due to inefficiencies due to context switching. It is advisable to reduce the number of tasks when such slowing down is noticed.

3. The more processes that are set to start at start-up, the longer amount of time the laptop needs to boot up, thus resulting in more unproductive time for the processor and the monitor. So keep the number of processes at start-up to a minimum. The required applications can be started when needed.

4. When playing games or other multimedia applications, the greater the level of detail, the greater the processing power consumed. So it is better to keep the level of detail to the required level; most games support this feature.


The higher the speed at which a processor runs, the more energy is consumed, so advanced users could run the processor at a lower frequency (i.e. 'under-clocking' it) to save power. This can be done by going to the basic input-output system (BIOS) of the laptop and setting the appropriate processor speed. In most computers going to the BIOS is easy, though it depends on the computer configuration. In contrast, if a computer is perceived to be slow and you are contemplating replacing the processor, a better option would be to try over-clocking the processor to see whether the increased speed meets the need. Over-clocking a processor can harm the computer, so practice caution and have a computer expert do it. In addition multicore processors consume relatively less energy for the same amount of processing done by multiple single-core processors. This is primarily because of the decreased power required to drive signals external to the chip.

The other major component of the laptop that consumes a lot of power is the hard disk. Since hard disks are physical devices, they consume relatively more energy than many other components. Hence it is important to keep hard disk access to a minimum and use it only when necessary. The spinning of the hard disk is one of the important parts that results in higher energy usage. Lessening the spin of the hard disk results in lower energy consumption. The spin of the disk is greater when the files are fragmented and scattered all over the hard disk. Defragmenting the hard disk reduces the spin of the hard disk and will result in less energy usage. Solid-state drives (SSDs) are known to consume less energy than comparable hard disk drives (HDDs). In addition, many power-saving options are available on computers that switch off the hard disk when it is not in use.

Peripheral devices connected to a computer also consume energy, even though they might not be in active use. For example, devices connected to USB ports in a computer draw power even when the device is not in use. So when these external devices are not in use, it is better to unplug them.

Most notebook computers can operate in at least two idle power modes - standby and hibernate. These modes help keep the laptop's energy consumption to a minimum whilst still retaining the state of the computer. In standby mode, the laptop's internal devices and optical drives are powered off, but the power is still maintained to the RAM, where the state of the device is preserved to facilitate instant resumption. Though this state requires considerably less power compared to when the computer is running, it still requires some power, which needs to be supplied by the battery or an external power source, or else the system will shut down immediately, thus losing the state. Retaining the state is very helpful when we are away from the computer for a short period of time.

The hibernate mode makes the laptop completely shut down whilst still retaining the powered-on state, thus reducing the start-up time. When a laptop goes to hibernate mode, the current state of the machine is stored in the hard disk, and all the devices including the RAM are powered off. When the computer is resumed from this state, the state information from the hard disk is reloaded into RAM. It takes more time to resume from hibernate mode than from standby mode, but it is more energy efficient. This mode can be used when the state of the computer needs to be retained for a longer period of time.

Most laptops today are provided with power management features and software. Power management software can help in regulating the use of the bat
tery and electric power. It monitors the load on the hard disk, the activities on the laptop and the ambient brightness, and changes various computer settings to make optimal use of power. This software also allows users to set timings when the computers can automatically go to standby or hibernate mode. For example, notebook computers can be set to go to standby mode after 20 minutes of inactivity or when the laptop lid is closed. This helps in automatically managing the power consumption of the laptops.

Taken from : Harnessing Green IT: Principles and Practices