My rough calculations agreed with yours- about 50% of what I should expect.

Would it suffice to turn it up full and measure the Voltage across it?
(pardon my ignorance)


This is raising 20C, if you're going from 10C to 50C, it'll be 3.5l/min.
To hit 2.5l/min, you need to be raising the water temp 60C or so.
Unlikely.
Sounds like something is wrong.

Can you measure if it's using the right amount of electricity?