Ok thanks, I understand.
My bed is a mk2a heated bed with a 1.8 ohms resistance.
Wiki said: "The MK2 PCB heatbed heats up to 110C when powered through the heated bed connection on Sanguinolou, your power supply should be 300W and ensure your wires from your power supply to the Sanguinololu should be capable of handling the total draw of 20A+. Using a 300W ATX power supply with the 4wire ATX dual 12V connector is working well for me."
My PSU could devliver 350W ( 24v-14.6a ). I have another 350W PSU 12V but the current could reach 29A !