Personally,
I would go with a 24v,
because a 24v will provide power for anything up to 24 volts making it more future proof,
you never know what your future needs might be.
More importantly, is the A, or, VA, rating,
So,
seeing that you measured 18.5 volts from your trans.
you most likely have a,
16v 10a, or, va "witch is known as a standard contractors grade transformer"
So, I would look for a,
24v 30a, or, va "where "a", and, "va" mean the same thing, they both hint at the "wattage""
Where as a,
16v 30va
would work wonders,
the price difference is a negligible amount for the jump to a,
24v 30va