There are about 1000 joules in a mars bar, and assuming 500v storage @50uf.... we get about 6-7joules (1/2 E^2xC).... so it has stored energy of 6 thousandths of a mars bar.... mars bar much more dangerous maybe.
But it is the impedance of the input winding of the power transformer that will dictate the power of the surge ( assuming perfect switching in the scr)...
This means very low impedance will give very very high voltage, as the pulse width will be very very small... so power will be huge ( hundreds of thousands of watts in fact).
If they had an inductor in series with the primary, and a capacitor ( 2uf??) across the primary, then they are both making the life of the SCR much easier, and widening the pulse, and lowering the power.
This yields two things, .... less radio noise, and better transmission characteristics ( less lossy wave, and less spiky). They need to keep within a tight time range to NOT cause electrocution... your heart pulse sending system cant see the pulse below a certain width, so cant cause heart stoppage/fibrillation etc.
Nervous system really sees it though, so it is not safe to cop too many contiguous pulses..... or some other parts of you may shut down.
So yes it will give a wallop, but the winding thickness and length, and the usual in line inductor will keep the voltage down below 10000v.
It won't kill from electrocution, but will kill an elephant if it gets trapped into a string of pulses.
Seen full sized bulls killed by these things. Don't know how long it takes... may only be minutes..... never never use barbed wire near the live wires, and it is illegal here to liven up barbed wires directly... for that reason.
I use 50uf motor run caps for this also