One ongoing issue with robotics is reducing the size and weight of power systems. This is no easy feat - but would eventually allow for smaller and more advanced robots.
The basic solution is conventional batteries - usually referred to as primary batteries. These are seriously flawed. Firstly - they represent a large amount of weight. They will need to be replaced- which means removing them from the robot, and fitting new ones. They then represent a significant waste, and cost issue. They also take up more volume than most other components.
There are a number of types of rechargeable batteries. It is worth seriously considering the power your robot will need, and safely draw. Equally - some robots may need to be modified to deal with these. As some of these can deliver higher currents - a stalled motor may cause transistor and/or battery damage where a conventional battery would be okay - hence the warnings on Cybot. On the whole I recommend Metal Hydride for most things. Ni-Cad are becoming less common - and for good reason as they are fairly harmful to the environment.
An Internal Combustion Engine may be useful for large flying bots - but is generally not really controllable enough for a robot, big, cumbersome and polluting. But yet there are alternatives springing up like methanol based engines that fit in 1cm square with virtually no pollutants. We will be watching that technology as it may proove quite fruitful. There are people who consider ICE days numebred by EV technology like http://www.goingreen.com.
Solar Panels are very idealistic. Yes they do not need to be changed or refueled, but they do not generate a great deal of power- especially in the levels we get in the uk. They are fine for BEAM Robots - but things much bigger need greater surface areas. They can be still quite expensive, and a lot of people point out that they use a lot of very harmful hemicals in their construction. on the whole - for smaller robots - I prefer them until something better comes along.
Mains DC converters (Wall-Warts) are useful for prototyping, and initial testing - if your robot is noty mobile - this is a very good solution. If it needs to be mobile - try going for a large cap or battery, and a mains docking station. An umbilical could be used during testing - but be sure to include the weight of the batteries as ballast when testing and in your specifications. When you buy one, ensure that its power output can match your needs - it would not be good to overload one of these. If I requrie a fairly stable 12V DC, I use a PC power supply - note that you should also not remove the housings of these and tinker with the innards unless you really know what you are doing - they have big capacitors which can deliver fatal shocks from their stored charge.
The next big thing in power storage and generation is the Fuel Cell. There are already busses in London operating on the technology. In Japan - Toshiba have developed one such cell for use in mobile phones.
One reason I like electric vehicals so much is that they will aid, along with mobile gear, the development of mobile power sources by a long way. The recent advertising for the Toyota Prius being able to escape the C-Charge caught my attention as one of the big companies finally waking up to the potential of EV. It is one of the few cars that might get Orion to get a license!
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.