Ohms law, Those heated beds have two resistors for all intense and purpose. if wired for 12v or 24v does not matter, they will put out nearly the same wattage. What I did to my own bed is I run it at 14v. Dropped the heat up time to to about 7 minutes to 105c. I am planing on running it around 30v and wiring it up for 24v to ease the load on my DC-DC upconverter as its basically limited to ~10A. At this point its putting out more than 10A on the bed. With that said, I run an unusually large power domain on my printer, ~700w at 110v or 1300w @ 220V. That gives me enough room to run everything else including a heater for my chamber and my web cam and a bunch of other accessories.

To put the math to work.

if your bed is like mike a MK3 heated bed which has a resistance of 1.4-1.6ohm at 12v or 5.0 - 5.4ohm at 24v

Here are the calculations
12.3v - 1.4ohm = 108W = ~8amps on the rail
14.0v - 1.4ohm = 140W = 10Amps on the rail
24.0v - 5.0ohm = 115.2W = 4.8Amps on the rail
24.0v - 1.4ohm = 411.4W = 17.1Amps on the rail
Running a mild boost should bring up the wattage while keeping the wires from going up in smoke.
35.5V - 5.0ohm = 252.05W = 7.1Amps on the rail.

It is much more friendly to be able to control the voltage on this kind of resistive load, as a bed that was meant to put out ~120w of heat running over 400w should have a very short life cycle. Even the mild boost I stated above would be taxing. I will probably set mine to run around 30v and keep the bed in the 180w-200w range.

As it stands my printer spikes to 475w out the wall socket when running full tilt and heating everything up.