I have to agree with Since87: that 1.4 factor could be off by 0.1, which means that you've got a 7% margin of error right there (all figures theoretical).
To measure the cold side, you'd need a thermal probe at the inlet and outlet, and if you measure those temps to within 0.1 deg C, which will be expensive, you'll still have a +/- 0.2 error margin, on a measurement that should turn out to be about 0.6 degrees, so an added ~10% margin of error on the total (all figures are approximate).
The heater is much simpler: measure the current and voltage, within 1% (or better), and you've got yourself a power measurement that's within 2%.
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