What it is
Power Probe switches a device off, reads a whole-home power sensor, switches the device on, reads again, then switches it off and reads a last time. The device's draw is the on reading minus the mean of the two off readings around it, which cancels a house load that is slowly rising or falling.
It reports an uncertainty and a quality rating (good, fair, poor or inconclusive) and says what limited the result: drift between the two off readings, meter scatter, or disagreement between cycles. Powercalc's own measure tool needs a monitoring plug on the device; Power Probe covers loads that cannot sit behind one, such as lights on a wall dimmer or a fan on a wall switch.
My Setup
The whole-home reading comes from my Refoss EM16P's two mains, which report about every 16 seconds, a rate the Medium timing suits. The two runs below were short, single-cycle runs rated Poor. They are examples of how the tool behaves in a busy house, not validation of its accuracy, and neither estimate was a successful measurement.
The meter behind it
The Refoss total line counts only the two mains, so branch circuits are not counted twice. My review of the monitor covers its setup and how closely it tracked the utility meter's readings.
My Refoss EM16P reviewA floor fan, October 30, 2026
One single-cycle run gave 183.9 W, plus or minus 70 W, rated Poor; the two off readings differed by about 140 W because other loads in the house changed during the run. That was enough to show that Powercalc's fixed 60 W for the fan was implausibly low. A separate plug-in power monitor then read 146.5 W, and that plug reading, not Power Probe, is the reference I used to change the Powercalc value to 146 W.
Four 40 W incandescent bulbs, October 30, 2026
One single-cycle run gave 95.5 W, plus or minus 54 W, rated Poor. That is far below the bulbs' combined 160 W rating and carries substantial uncertainty: the two off readings differed by about 110 W because other loads changed during the run, and the light's brightness at the time was not recorded.
Why I use it this way
Many loads in my house cannot sit behind a monitoring plug, and a fixed Powercalc value with no measurement behind it can be badly wrong. Power Probe gives a rough check from the meter I already have, and its rating says how rough. In a house with loads that cycle, a short run is a sanity check, not a measurement.
What it can do
Off, on, off measurement
Settles after each switch, averages the meter over a window, and repeats on and off for up to five cycles to average out other loads.
Honest uncertainty
Counts distinct meter reports rather than how often it looks, flags windows with fewer than three reports, and rates each result with the factor that limited it.
Sidebar panel for admins
Pick the meter and device, watch each step and the live reading, cancel at any time, and copy a ready-made Powercalc snippet when the run finishes.
Actions, events and entities
Start or cancel a run from scripts and automations, get the result back as a response or a finished event, and keep the latest 50 results across restarts.
Puts the device back
Restores the device's original state by default, including after a cancelled or failed run, and recovers an interrupted run after a restart.
Install and connect
It switches devices off and on
Do not use it on anything that must stay running, such as a fridge, a freezer, medical equipment, a sump pump or anything related to life safety. A run keeps the device off for a few minutes in total.
Before you start
- A whole-home power sensor in watts or kilowatts that includes the circuit the device is on, such as a Refoss, Emporia Vue, Sense, Shelly EM or IoTaWatt meter.
- The device to test as a switchable entity in Home Assistant.
- Home Assistant 2026.10.2 or newer, and an admin account for the sidebar panel.
Add the repository by link
In Settings > Marketplace, select Add from link and enter the project's GitHub repository. Connect GitHub if asked, then open Power Probe and select Install. Before Home Assistant 2026.11, add it to HACS as a custom repository of type Integration, download it and restart.
Add the integration
In Settings > Devices & services, add Power Probe and choose the default whole-home sensor, a timing profile, the number of cycles and whether to restore the device afterwards.
Match the timing to the meter
Look at the meter's history graph for how often it changes. The settle time has to outlast its update interval, and each window needs at least three real updates. Slow cloud-polled meters need the Custom timing and long runs.
Start with a large, steady load
Run it when the house is quiet. Bigger loads give cleaner results; for a poor rating, run again with more cycles before changing anything else.
Limits and caveats
- One short run is good to roughly plus or minus 60 W in a house with loads that cycle, according to the project's own two checks. That suits loads of about 500 W and up.
- It measures one operating point and the difference between on and off, so standby draw and dimmer or speed settings need separate handling in Powercalc.
- On a net meter in a house with solar, changing sunlight counts as changing house load.
- A run switches the device, so it is not suitable for anything that must stay on.