I was interested in cooling to brew beer, it seemed a bit extreme to use a whole fridge or freezer just to drop the temperature a few degrees, using the Peltier seemed the best option as only needed for first 4 days and only a few degrees. During the hunt I found there is a formula to work out the standard usage and it is from this formula that percentage efficiency is worked out. The F to A and now A+++ rating.
However there are different standards for upright and chest, and temperature they are designed to work in, if built in or not, and if auto defrost or not, so impossible to compare a chest freezer with upright auto defrost. However since forced to publish the average annual usage you can compare those figures, and using those figures you can also work out if insulation has failed, so if 365 kWh/annum then if it uses around 1 kWh/day then working as designed, but at 1.5 kWh/day likely insulation failed.
I worked out the actual power used with my own units and they mostly used a little more than the spec except for the chest freezer which used a little less, one very old fridge/freezer was over the rating and it did have known insulation problem, mothers little freezer used well over, and it was found the thermostat had failed and temperature was -22°C not -18°C about the only use of an energy meter.
My idea was fit a water tank in a small Peltier fridge and a pump switched using a temperature switch and a coil of plastic pipe in the fermenter to cool it when it went over 20°C however when I found how enefficant the Peltier was, and I had a fridge/freezer which was faulty with insulation failure I bought new one for house and old one used to brew with.
http://www.ericmark.talktalk.net/Appliance-power.html']This was the java script page I made to work it out. However I failed to link to government website where I down loaded the pdf that I used to construct it from. Likely still on lap top but that is waiting repair.