A solar panel that produces 10 kilowatts in Earth orbit might only produce 1-2 kilowatts out at Jupiter and even less at Saturn. The solar intensity drops off that much. Building a solar array capable of producing 10 kilowatts at Jupiter is possible but it would be enormous, unwieldy, and expensive. As an example, according to NASA, the most efficient solar arrays currently available would need to have an area of more than 500 square meters to provide the 500 to 600 watts of electricity that Cassini needs at Saturn. VASIMR is a promising technology but it depends on a lot of electricity to function. Solar arrays capable of powering a VASIMR engine at Saturn would be somewhere on the order of 10,000 square meters. That's roughly 2.4 acres. Concentrating solar arrays like those used on DS-1 could probably reduce that quite a bit but they'd still be large, heavy, and expensive.

The best solution for VASIMR propulsion, especially for deep space missions, is nuclear power. I'm not talking about SNAP-type radioisotope units but small reactors. For a SNAP unit to generate 10 kilowatts, you'd need a large amount of radioactive material. A true reactor like the Russian Topaz would be a better solution. The Russians orbited quite a few reactors in the 1970s and 1980s to power that RORSAT satellites. The US purchased an unfueled Topaz reactor in the early 1990s. The last I heard, it was transferred to the Defense Nuclear Agency in 1994.

Here's an

online lecture on nuclear power in space. Unfortunately with all of the anti-nuclear Luddites out there, it's also the most difficult to sell politically.