They probably have battery systems on board for the sudden total energy it needs when it is time to do some science stuff with all the instruments at once.
Then it goes to sleep and recharges them.
http://news.stanford.edu/news/2010/august/sun-082310.html
Stanford Report, August 23, 2010
The strange case of solar flares and radioactive elements
When researchers found an unusual linkage between solar flares and the inner life of radioactive elements on Earth, it touched off a scientific detective investigation that could end up protecting the lives of space-walking astronauts and maybe rewriting some of the assumptions of physics.
BY DAN STOBER
It’s a mystery that presented itself unexpectedly: The radioactive decay of some elements sitting quietly in laboratories on Earth seemed to be influenced by activities inside the sun, 93 million miles away...........
Hibernation for New Horizons means the science instruments are turned off, the high gain antenna is pointed toward Earth, the spacecraft computer system is put in a low power mode, and the thrusters are fired to put the spacecraft into a gentle spin. The instruments are turned off to save wear and tear on the electronics. The spin helps the spacecraft maintain course.
MD, who worked on the Alice instrument on New Horizons