If you have an outdoor light, you could screw-in a device into the lamp receptacle that provides an outlet. Most hardware stores carry these in the electrical department.
As far as power consumption. The modern twinkle lights use very little power. These are the ones where you pull one bulb out, and half the string goes out.
You can use a rough calculation for power consumption, or measure amperage with an ampmeter. A string of 100 lights draws about 0.33 amps and string of 150 lights draws about 0.50 amps. Therefore, 300 lights draws 1.0 amps (about the same as a 100 watt light bulb).