<i>20V is not a 'stock' DC/DC output voltage, so we're going to look for something roughly 24V and find something that has an adjustable sense line for reducing the output to 20V.</i><p>Note that the absolute voltage limit is likely due to 25V electrolytic capacitors in the input circuitry (find a schematic for a model that uses the "20V nominal" PSU and inspect it to be sure), so 24V should be OK as long as you're aware of how far the tolerance of the output of the PSU you're using can be. I suspect the fact that laptop PSUs seem to come in a wide variety of very-close voltages around 20V (I've seen 18.5, 19, 19.5, 20, 21, even 18.75 or 19.25) is more for vendor-lock-in reasons than anything else --- 19V is only 5% less than 20V, and a PSU rated at 20V may even end up at 19V or less under full load from resistive losses. Due to using DC-DC converters, a slightly higher voltage may even increase efficiency as there's less I^2R losses with the lower current that results.<p><i>I did check the stock laptop power supply and it came in at 20.4V, which was somewhat curious.</i><p>That's only 2% high, and if it was measured unloaded, I wouldn't be surprised.<p>I explained the reasons for the ~20V standard for laptops in this comment from almost a decade ago, and not that much has changed since: <a href="https://news.ycombinator.com/item?id=7594383" rel="nofollow">https://news.ycombinator.com/item?id=7594383</a>