Volts x Amps = Watts

So you need to find out the max wattage of your charger and/or what packs you'll be charging and go slightly above that.

For example my MRC 989 is a 106w charger.

12v x 8.83a = 106w

So a 12v 10amp power supply would be enough. It comes with a 15v 10amp power supply.

Here are some examples of common lipo batteries and the max amperage my charger could charge them at.

2S 7.4v x 14.3a = 106w
3S 12.6v x 8.41a = 106w
4S 16.8v x 6.3a = 106w

However my 989 has a max charge rate of 10amps.

I hope this helps you out.