Need a contact closure based on power draw from outlet


Active Member
So, I have a drainback solar HW system to which I'm attaching a whole bunch of temperature sensors to track usage and efficiencies. I'm using a differential controller (no, not an ELK, at least not just yet!) which is attached to two 10k ohm resistors. One on the roof that measures array, one at the tank that measures the tank. When the panel > tank, the differential controller powers the pumps.

Getting the temperature sensors to data log the various water temperatures is pretty easy.

What I need is a way to know when the pumps kick on. This tells me the period of time that the temperature sensors have "meaning." Essentially, I'm looking for a plug in contract-closure that closes when a certain amount of wattage is drawn from the outlet. Any ideas?


So if I can re-phrase to make sure I am understanding this correctly.

You have "some" current draw, then this current increases by a large amount when the pump kicks on. You want to know when this pump kicks on and off.

I did a How-To on appliance monitoring using a CR magnetics current sensor that "trips" when the current draw reaches two amps (will not trip under two amps). If you can use something like this and match the needed threshold with this type of sensor, this may work.

Take a look at THIS How-To.

Also, can you possibly provide some additional details on the current draw scenarios?


Active Member
I have three pumps that are plugged in.... two to get the water to the roof (one 1.3amp pump that provides 35' of head, and then a booster pump that provides another 15' of head. The booster pump is wired to an on-delay timer -- eg: when power is connected it runs for 1 minute, then stops), and one 1.3 amp pump to circulate storage tank water through a heat exchanger.

My goal on the next sunny day is to hookup a Kill A Watt and determine draw during the various states. I'm reasonably sure that the pumps don't draw a constant 1.3amps- that's probably only upon startup.

The CR method isn't a bad option, but it seems like there should be an easier option. Measuring current to something that's plugged in is essentially what a kill-a-watt does, the only difference is I'd like an output instead of a screen.


I don't know of another way, maybe someone else will have some ideas. Are all the pumps plugged into one outlet?

I think a solution of having a current donut measuring the "actual" current (it provides a DC voltage proportional to the current measured) may be the needed solution, with an analog to digital converter reading the output. Look at the Analog to Digital Converters guide in the How-To section for details.

Another option would be to use a current donut that provides a "pulse rate" proportional to the current measured, but you would need a device to "register" these pulses. There are some home power monitoring threads here and on the HomeSeer board which describe these methods.

What systems do you have for general home automation? An Ocelot, one-wire setup, etc... There may be a solution that can use our existing infrastructure.


Senior Member
Any time I have looked at current detection for monitoring on/off actions it always turns out to be easier to wire an outlet in parallel with the motor or heater or whatever that I want to monitor. I plug a wall wart into the outlet, connect a relay to the wall wart, and then have a dry contact to connect to any system I want.