This mod comes with Tier 4/5/6 capacitors by default, including their recipes. You can also use the mod without editing anything (although that's not the intended purpose).
Add custom capacitors to EnderIO via KubeJS or Config.
Config Method
Your config file already contains an example as shown below.
[[capacitor]]
id = "tier4_capacitor"
name = "Tier 4 Capacitor"
level = 4
glow = false
color = "4488FF"
You can change the values and add as many capacitors as you want. Adding capacitors from the config file only supports color tinting in terms of textures. The hex code you enter will be applied to a base texture, and the texture's color will change.
Kubejs offers full custom texture capabilities.
Kubejs Method
It goes to startup_scripts.