Sizing your system (USA)

I want to capture usage from all of my circuits, but the system capacity isn’t that big. How does one decide how to best configure their IoTaWatt? Should I look at combining some of the smaller circuits to be able to get a better picture?

This comes up all the time with prospective new users. When you measure the mains, you capture the usage of all of the circuits. You then measure some number of relatively high usage circuits individually, and yes collectively by combining smaller circuits.

You can define an output that is the difference between your mains and the sum of the circuits that are measured, which is like having an extra input with a CT that includes all of the unmonitored circuits. Consider a circuit for something like a garbage disposal. It runs 3-4 times / day for 30 seconds. Even though it may draw 1000 Watts, it amounts to less than 30 Wh (.03 kWh) per day. By comparison a 2W nightlight that averages 12 hours/day will use 60Wh and is considered insignificant. Another example is a washing machine. They really don’t use much energy unless you have three kids in diapers.

When you graph the usage of individual circuits, its usually possible to identify the various devices on that circuit. Here’s my basement today:

It’s pretty easy to see the freezer cycling with a duty cycle of about 40% running about once an hour. Around 8am you can see something switching on and off then go to about 80W pretty steady. That’s the pump on my solar thermal hot-water. Aroung 12:30 my dehumidifier came on for about 5 hours. So with one CT on the basement circuits, I can see pretty much everything that’s going on and relative power.

My advice is to list what you might want to monitor in decreasing priority, and then assess what you might be missing if you stop at number 14.


Thank you! That was extremely useful. Sorry for asking a repeated question.