The V4-275R Stirling engines which are on the Gotland and Södermanland Class subs (Sweden) and the Soryu Class subs (Japanese). Each of these submariens mount four of the Sterling Engines. Each Sterling Engine produces approximately 75 Kw of power, for a total of 300 Kw. (That 75Kw figure makes ou wonder if the Chinese are using the Sterling engine, or a derivitive thereof.)That's the case with all AIP equipped submarines. Modern SSKs don't emphasize speed but rather stealth, while modern SSNs should emphasize both, along with endurance.
An SSK equipped with AIP allows for stealth and endurance, but naturally low speed compared to an SSN. In a defensive position, low speed shouldn't be as much of an issue.
A 2400 ton submarine would need as much as 200 kW electrical power for AIP propulsion (at something like 5 knots) and for all the other power requirements of the sub operating in that mode. So, four engines provides some theoretical reserve...however, reports indicate that the engines are actually produceing 5-10 Kw less than the 75 Kw advertised. If they produce 65 Kw, then four of them will produce 260 Kw, which leaves very little reserve.
Just the same, SSKs operating the Sterling engines are said to be able to remain submerged for several weeks operating at 5 knots.
The German Type 212 submarines which displace 1,800 tons submerged use a different technology, employing 2 HDW/Siemens Polymer-Electrolyte-Membrane (PEM) fuel cells. These PEM fuel cells make an efficient conversion of the chemical energy stored as hydrogen and oxygen into electricity for power. Each of these fuel cells is said to generate 120 Kw, for a total of 240 Kw.
For low speed, stealthy operations, some comparative studies of fuel cells and other air independent propulsion (AIP) systems such as the Stirling engines already discussed, have shown a better efficiency and other advantages of the low temperature PEM fuel cells to combustion-based solutions. This is why the newer German Type 212 submarines use the PEM technology...of course it does not hurt that this technology has been developed in Germany and is available through the German Siemens Corporation. Again, however, as with the Sterling engines, speeds are low when operating in AIP mode because of the low power availability.
Just the same, these vessels, if they can locate themselves in a position to wait for an oncoming adversary combatant, are very dangerous. This usually means in straits, or on the path of a Task force that is known due to intelligence, and in the littorals.
After conducting exercises in such conditions with Swedish Gotland subs and those subs being able to photograph US Navy aircraft carriers at relatively close range, the US leased the use of one Swedish Gotland class sub and its crew for two years. During that time the US Navy developed defenses against such technology, and developed doctrine regarding how to prosecute them. The results are highly classified.