Super Capacitor to log power outtage

Whenever we lose power, the Sense seems to lose power (duh). However, we have our network on a UPS and also have a generator. Is there a way to allow the Sense to live long enough to

A Report/record the power outtage (not as a critical hardware event)
B. Report the return of power after a complete outtage (again, not as the result of a critical hardware failure)
C. Report the total time of the outtage

Bonus points would be a way for the sense to also monitor the generator feeds.


You can almost so this with the solar kit.

If you have a generator, you probably have a transfer switch. Power the Sense from a circuit fed from the transfer switch. (Don’t power the Sense from a UPS).

Use the main current sensors on the utility power feed, and the “solar” current sensors on the feed from your generator.

When your house is on Utility power, Sense will work normally, and show zero “solar usage”.

If utility power goes away, the sense powers off, but will restart when the generator kicks in. The app will show that all power is coming from “Solar”.

Other than a short outage during the transition, this works very well if all loads are on the transfer switch.

If only some loads are protected, you will have some reporting issues if the utility power comes back, and the transfer switch waits a few minutes before returning the protected loads to utility power. During this period, some loads are on utility power, and some are on generator power. It is probably that the two are not in perfect phase sync with each other, and that the phase difference will be drifting. Sense doesn’t deal with the two sources being out of sync and will show slowly varying values for utility power usage that range from correct, to completely wrong. If the actual utility usage is 1KW, Sense will show values that range from 1KW to -1KW (negative one KW).

When the AC power stops for a brief power outage the sense monitor loses power too and when the power returns the monitor has to reboot. Sometimes the power will drop again in a few seconds and on my little computers (raspberry pi) the flash memory cards often get corrupted. I don’t know if the sense is subject to power failures but having a super capacitor to keep the sense running for several seconds and provide a safe shutdown when the power fail exceeds the energy available in the super cap.

The question in my mind is: does the Sense device use its ‘power circuit’ connection for ‘sensing’ or does it solely use the input from the “clamps” (and solar, if one has it)?

If that ‘circuit’ is just 240 power, could it be on a UPS device? (such as one might have for networking devices.)

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This is a great suggestion, @keithmclaurin. I believe this would be what our hardware team refers to as a “giant honking” capacitor, which may be challenging to fit inside the current small form factor of Sense, but they’re always up for a challenge, so I’ll make sure they hear about it!

And you’re right, @Dan.C.

Sense does monitor voltage on both legs through the 240v service, and I believe a UPS could inadvertently smooth and shift some of the small variations in magnitude, phase, and frequency that Sense depends on for device detection, but I’ll ask the engineering team to be sure!

The sense uses the AC voltage of each mains circuit and the current from the clamp sensors to determine the power. If you connect the sense to a voltage source that is different from the actual voltage on the main lines then an error in the power calculation will result. The sense I have reports Mains power levels and the corresponding voltage. When I have a 1500 watt load on one main circuit the sense sees the voltage drop from 120.1 Volts to 118.8 Volts. On a recent power outage I had to connect a backup generator to the house and the sense could see the frequency was 61.2 Hz and when the load increased to about 80% the generator frequency dropped to 59 Hz and the line voltage drop from 122V to 115V. This is a very capable sensor system.