I'm not looking for specific PS suggestions, I'll decide once I know how to. Rather, how does one figure how much PS they need? TIA

I can't speak for anyone else but I look at the charger I have or want and look at what it requires, then go with the biggest PS I can afford that will give the charger enough juice to provide me with full current.

For example, the PL8 can take 24+v and will only apparently give full power at 24 volts, so it makes little sense to invest in that charger and power it with 13v.

If you do a bit of searching about building power supplies you can find other sites where you will learn what server supplies to purchase that will give you more than enough power for any charger.

Number of lipos being charged x number of cells x 4.2 per cell full voltage x charge amps / charger efficiency x power supply headroom, equals the power supply required in watts.

For example:

2 lipos x 4S x 4.2V x 5 amps / 80% x 130% = 273 watts.

Assumptions:
- Lipos
- 80% charger efficiency, generally a slightly conservative estimate
- Extra 30% power supply headroom for reliable operation, recommended by many manufacturers
- Switch mode based charger. Most if not all are these days.


If you need power supply amps divide the watts by the power supply voltage. Many bench supplies are 13.8 volts, computer derived are around 12 volts.

Example:

273 watts / 12 volts = 22.75 amps


Note many of the big chargers require 24V supplies or higher for full output, or at least higher than 12V.