I took an entire desktop and checked the power requirements to have it run on battery power using a M4-ATX-HV 220W Intelligent Wide Input 6-34V Vehicle/Battery DC-DC Power Supply. The M4ATX accepts a variable voltage input and is able to supply a constant voltage to the motherboard - abilities that my original picoPSU lacked. A picoPSU needs a 12V constant input and steps that voltage up to usable levels for the board. This is problematic for battery usage because if the voltage drops below the usable 12V at any moment, the entire computer would shut off! While I could theoretically create an automatic voltage regulator to set the battery to the desired voltage, it wouldn't be worth the effort considering I needed to change out the picoPSU entirely, anyway - I didn't have the right connectors to go from my battery to the picoPSU, which is intended for a sort of "laptop charger" style cable.
Laptop charger cables have their own transformers built in, enabling a constant current input that can't easily be supplied with a battery. In the end, I used the M4ATX along with a 3300mAh battery pack rated for 60C at 22.2V, which is basically a high-powered RC car or drone battery. The 60C on the battery means that the pack is rated for up to 120 Amperes of current draw, while the 3300mAh indicates the capacity of the battery; 3.3 Amp-hours. This can be directly converted to watt-hours by multiplying together 22.2 * 3.3 = 73 Watt-hours. I have included a Watt Hour calculator below for your convenience.
My CPU took 77W max, and a Mini ITX motherboard (20W) + SSD (11W) + 32GB RAM (6W) = 37W, the total Wattage drain would be 114W, which means I could run my portable desktop on that battery for a little under one hour, as long as I limit CPU consumption. Since the required components (motherboard, etc) used 37W, that leaves me with 36W of battery power left solely for the cpu. Considering this, that means I could theoretically use the cpu at approximately 46% power draw for one hour and drain the battery completely.
In practice, you cannot drain a LiPo battery completely, as they tend to spontaneously combust. As a result, I drained my LiPo only partially before recharging, and only used the CPU for very low level machine code compiling (the software of which did not work on my normal computer, but that's another story) and was able to get about 45 minutes of near-idle use out of it before recharging. I drained my battery approximately from 37V total to 23V, which is a 14V difference or ~46 watts. This is ~63% of the total watt hours I could use, and implies my CPU was usually running at ~11% capacity.