What is the power specification for SATA?
As @bitshift says, you'll likely need to get the actual spec to be sure.
1.5A may well be a real supply requirement, though. The cables used are actually rated to 4.5A per supply rail. The connectors contain 3.3V, 5V, and 12V rails.
A regular (magnetic 3.5&quot;) hard drive can dissipate almost 10W. This energy is probably split across the three available power rails. Since I have no idea what the split may be, the worst case numbers look something like this :
All 10W @ 3.3V - 3.33A
All 10W @ 5V   - 2A
All 10W @ 12V  - 0.83A

The actual spec could help you understand how the load is allowed to be spread over the three supplies and what the effective ratings needed are.
If you don't care about it being SATA compliant, you could assume the load is lower by a factor of 2 or so since you're only using SSDs.