I have a small wireless IR video camera and receiver (for bird nest watching), both of which work fine off small 9v batteries, but don't want to work very well with off-the-shelf all-things-to-all-people multivoltage transformers (the batteries don't last much over 4-5 hours, unfortunately). I know that I have the (+) and (-) the right way around, BTW! I think that these cheap transformers are probably quite noisy, and I think I need a clean, regulated 9v power supply. I think I need a transformer-rectifier-smoother-regulator, but I'm not sure if those 4 components alone will do the trick.
If you use a solar panel, you will require a re-chargeable battery, 12V sealed lead acid [SLA]. Many solar panels are wired to give a charge output suitable for a 12Vbty.
The power rating of the solar panel and battery will be determined by the power consumption of the camera system. I expect you only run the camera during daylight ? [indoors or outdoors?]
The reason why I am suggesting a SLA battery powered system is avoid running long lengths of cable and the safety aspect regarding mains power.
You could have a low drop out 9V regulator located within a box housing the 12V battery, this would give the very 'clean' supply you are looking for.
The solar panel would keep the 12Vbty topped up, subject to the rating requirement of the system.
It would be possible to build a small mains driven battery charger and also mount that within the battery box, for those times when the sunlight is limited.
PS. I'm also considering using solar cells (4-5 mini-panels each providing 2v and 150mA) to run the camera (output power 200mW at 8~9v) -- is this likely to be a goer? or a waste of time?