Amps, volts, or ohms are still useful here, in order to describe the optimal load. For a completely overexaggerated example, if the battery has an internal resistance of 1 teraohm, it means that, for practical output voltages, it acts as a constant current source of 0.026 microamps, no matter the voltage, and the majority of energy is wasted. To get the claimed 170 microwatts, you would need to allow the output terminals to charge to 13 KV (which would then reduce the output current by half), something completely impractical to work with.
In other words, I don't know how to convert 13 KV and 0.013 microamps DC into 1.3 volts and a bit less than 130 microamps DC.