I recently posted a quick explanation here: Understanding units - #2 by Robert.Wall
What we do in emonLib is sample the whole waveform - for me in the UK, there’s about 55 sample pairs (voltage & current) per cycle, but rather fewer for your 60 Hz mains. We can then accurately calculate the average power (if you also supply the voltage waveform) or the true rms average current.
What you’re doing, when you just pick off the peak value, is you are relying on the shape of your current wave staying the same, and knowing the shape of the current wave so that you can calculate (or fix by trial and error) the rms value knowing the ratio of that to the peak value.
There are two further sources of error: your voltage won’t always stay at exactly 120 V, and the power factor of your pumps will be significantly less than unity, which means that the “power” - actually apparent power - you calculate will be greater than the real power you pay for.
Provided that all these things remain constant, and they are likely to remain reasonably so in your particular circumstances, then you should be able to adjust the calibration to correct your readings. And it would appear that you have succeeded in doing that:
But in general, the power factor of the load - say your whole house - will change according to the types of appliances you have operational at any time, and the voltage won’t remain at 120 V exactly.
That is most likely due to the c.t. you’re using - its accuracy is only specified down to 10 A - and you’re probably also adding some electrical noise picked up and measured along with the current you’re trying to measure.
So the answer to that is - generality. If you change the pump motor for a different size or make, or use your monitor on something other than that particular pump motor, you will need to re-calibrate it because the assumptions inside CALIBRATION_CONSTANT will be wrong.