The Pyrothermic Generator (PTG) is the first craftable power generator available to the player. The power per second (PPS) it generates is determined by the fuel it is burning and by the depth it is placed. Place the fuel in an adjacent hopper and the Pyrothermic Generator will supply power to any adjacent machine that consumes power. It will only start burning fuel once its internal buffer drops below half, and takes several seconds to ramp up. Each unit of fuel will burn for 30 seconds. The following table lists the power generated per second for each fuel type (on 'Plentiful' power setting).
The generator becomes less efficient the further below the surface it is placed because of a lack of air. From the surface till -25m it will work at 100% efficiency. Each tile lower result in -3% efficiency. The lowest depth it can work at is -55m at an efficiency of 10%. Below that, it doesn't work at all.
On Scarce, Infused Coal gives ~17 pps for a total of 504 power.