You need to buy a bms that can handle at least a 4s battery pack. Thats the more important thing, as far as amps, if you only plan on using 40 amps or less, you need a bms that can handle 40 amps, if you plan on using 300 amps, you need a bms that can handle 300 amps.
Example I have a small 4s 10ah battery pack, since I use it to jump start cars, I use a bms that can handle 450 amps. Another example I had a 4s 220ah battery pack, and I only used a 30 amp bms since I never use more then 15 amps. You choose the bms by how many amps you plan to use not by the size of the battery amp hours.
Since you will be using solar I recommend the chargery bms8 (cost about 95 dollars). It uses contactors/relays to stop the charge/discharge, this bms can handle as much amps as your contactor/relay can handle.
The reason to use contactors is when the bms decides to stop the charge, a regular bms with mosfets, will cause voltage surges which damage equipment. With contactors, you place the contactor between the solar panel and the controller, when the bms activates it just disconnect the solar panel, and you get no voltage surges.
With the chargery you have to buy the contactors seperate, I use a 4 dollar 30 amp automotive relay, you can find large contactors/relays/solenoids for less then 50 dollars that can handle 300 or more amps. You need 2 one for charge and one for discharge, you can use a small one for charging since you charge at less amps then discharging. The chargery is a good solid bms, I been using it for the past year and proven very reliable. I used many bms on my solar system over the years but this is the best bms so far.
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