So, if you're using T-45 right now, I'd say go for T-51. The difference between T-45f and T-51f is negligible in terms of Aluminum (0 for T-45f and 1 for T-51f), but with a pretty substantial defensive boost. Power Armor | Physical | Energy | Radiation | Full Repair Cost Here are all the fully upgraded power armor sets (excluding uniques), including defensive values and repair costs: VI, as this will result in a marked increase in defensive rating without any additional repair costs. The level your power armor is upgraded to does not influence the repair cost (in my experience, wiki confirms), so if you haven't already done so, get your Armorer and Science built up so you can get to F or Mk.