GBM wrote:
What is the best way of measuring the power drawn by a boat's
refrigeration unit?

Is there a way to do this without any expensive instruments?

I was thinking of installing an ammeter in the line to the compressor
to determine the current draw, but manufacturer does not recommend
this. Even with that, I would need an hour meter or some such device
to know how often and for how long the compressor runs.

I would like to test existing installation, then add some "temporary"
insulation to the inside of the box and then test again to see what
improvement I might get if I added permanent insulation.

GBM

Connect it to an isolated, fully-charged cheap auto battery of known
capacity, and see how long it takes to discharge the battery. I assume that
is what you want the data for in the first place. Not very accurate, but
hellish useful!