According to NASA engineers who spoke at the recent National Atomic Testing Museum in Las Vegas, the Kilipower Project began testing the current reactor iteration in November 2017. It’s a small-scale reactor designed to produce power in the 1-10 kilowatt range, as the name implies. One kilowatt is about what you’d need to power a toaster or a few laptops, and the test design should reliably produce that much. That’s not enough to power an entire Mars habitat, though. NASA estimates you would need 40-50kW of power, so it may send several small KRUSTY devices if it cannot develop a single reactor that can reach the necessary power levels. The Curiosity rover, by comparison, uses about 200W (0.2 kW).
The system is between five and six feet tall, but the uranium-235 nuclear fission core is about the size of a paper towel roll. Heat from the rector is distributed by a series of sodium heat pipes. The heat generates power via a high-efficiency Stirling engine, which drives a mechanical flywheel and piston via the repeated expansion of gases. By coupling the engine to an alternator, the system produces power.