Before we get started, I think you should know that this particle story pertains to tablet PCs in general, and their ability to save a considerable amount of electricity, and the iPad is just an example of one. Tablet PCs, in general, consume little power, so it isn’t that the iPad itself is particularly efficient.
The power consumption of desktop PCs is almost always less than 100 watts. Most of the time, however, cumulative energy usage (total energy used over a period of time) is a sum of the appliance’s time in use and wattage. Time used is just as important as wattage.
You will realize the importance of this in the computer calculations I make below.
In other words, cumulative power consumption/energy usage equals appliance wattage multiplied by time used (more detailed explanation).
Real World Example:
A common but low-power personal desktop computer with a dual core CPU (this is the predecessor of the faster i3 processors) consumes 40 watts on average with normal use and a few programs open, such as the Google Chrome browser, Skype, and a text editor.
If you haven’t already realized, the power consumption (in watts) of such a computer is equal to 40 times the time used. It is common for people to leave their personal computers on all day — in extreme (but still common) cases, computers are left on for 24 hours per day. 40 x 24 = 960 Wh (Watt-hours), or 0.960 kWh, assuming that all power-saving features are turned off. (I made this assumption for the sake of simplicity because people’s computer usage and power saving settings vary widely.)
After a month, this amounts to 28.8 kWh, which costs $2.88 USD, assuming an average electricity cost of $0.10 per kWh. The total annual cost is $34, which is about 25 times more than the tablet PC mentioned above. Despite the fact that tablet PCs have very limited capabilities compared to desktop PCs, and do not provide anywhere near the performance that desktops do for the money, they are portable, convenient, and energy efficient.
I cannot tell you what is best for you. Tablet PCs are not for everyone, so, please think carefully about whether a desktop, laptop, or tablet PC would suit you best.
Another thought about DC-powered tablet PCs: they are easier to power with solar! Solar panels generate DC current by default, which can charge tablet PC batteries, so only a solar panel and voltage regulator is needed to charge a tablet PC, rather than the larger and, hence, more expensive inverter, batteries, voltage regulator, and solar panel that would be required for a desktop PC.
If you do want a tablet PC, good choices are available, such as the Toshiba Thrive product line, as well as that of the Samsung Galaxy Tab, Asus Eee Pad Transformer, and the Blackberry Playbook.