Just received the heatbed.
Did measurments and IMHO the statement that
Resistance at 12 Volt setting is around 1,5 Ohm
is wrong leading to multiple issues (maybe this is already discussed somewhere above).
The resistance between 1-2 is 1.5 Ohms
The resistance between 1-3 is 1.5 Ohms
The resistance between 2-3 is 2.7 Ohms (and this is the configuration for 24V setup, circuit runs through 2-3)
However in 12VDC (15VDC) configuration those resistances work in parallel (1-2 is for "right" side of the bed, 1-3 is for the "left" side).
Thus the actual calculated reisistance is
1/Rt = 1/1.5 + 1/1.5
Rt = 0.75 Ohms
I did measure it in parallel (on 3 multimeters), it gave Rt = ~0.95 Ohms in reality (technically there is yet another resistance in serial, the trace from pin 1 going to the place where 2 and 3 branches divide off, which explains it), but I'll assume 0.75 Ohms in calculations below.
Thus current @15VDC will be 15/0.75=20A
And power (PSU) needed is 400W (taking 300W calculated @ 75% factor).
Sure, the resistance will increase when everything heats up, but 15V 320W PSU suggested in the heatbed-for-dummies-v3.pdf is too low for heatbed alone!
This is bad as I trusted the specs in the original post.
I already bought 350W PSU based on "1.5 Ohm" resistance, given that it will need 200W for heatbed (meaning it can provide 150W calculated (15*15/1.5) @75%) and 100W for existing stuff + extra 50W) but it is too low for actual calculated 400W (300W calculated @ 75% factor).
One way would be running 350W PSU @ 15VDC through 2-3 (24 VDC circuit) as it will give 15/2.7 = 5.5A load, which would be safe for the PSU, but of course everything will heat up soooo much slower.
Other way would be taking the risk (and it seems many have here), because I have measured that dual head mode only on peaks exceed 50W, on normal operation it is ~50W. As resistance empirically for "12V circuit" is ~0.9 Ohms, it gives that heatbed will eat up 15*15/0.9=250W (and it can be considered as a pure resistive load which helps in this case, although way how traces are laid out introduces extra inductance, it is negligible) thus 350W PSU could "just manage it", but in long term...
Thus beware and don't buy 320W as suggested in heatbed-for-dummies-v3.pdf or 350W as I have, go beefier.